Springer Finance
Editorial Board M. Avellaneda G. Barone-Adesi M. Broadie M.H.A. Davis E. Derman C. Klüppelberg W. Sch...

Author:
Giovanni Cesari | John Aquilina | Niels Charpillon | Zlatko Filipovic | Gordon Lee | Ion Manda

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Springer Finance

Editorial Board M. Avellaneda G. Barone-Adesi M. Broadie M.H.A. Davis E. Derman C. Klüppelberg W. Schachermayer

Springer Finance Springer Finance is a programme of books addressing students, academics and practitioners working on increasingly technical approaches to the analysis of financial markets. It aims to cover a variety of topics, not only mathematical finance but foreign exchanges, term structure, risk management, portfolio theory, equity derivatives, and financial economics. Ammann M., Credit Risk Valuation: Methods, Models, and Application (2001) Back K., A Course in Derivative Securities: Introduction to Theory and Computation (2005) Barucci E., Financial Markets Theory. Equilibrium, Efficiency and Information (2003) Bielecki T.R. and Rutkowski M., Credit Risk: Modeling, Valuation and Hedging (2002) Bingham N.H. and Kiesel R., Risk-Neutral Valuation: Pricing and Hedging of Financial Derivatives (1998, 2nd ed. 2004) Brigo D. and Mercurio F., Interest Rate Models: Theory and Practice (2001, 2nd ed. 2006) Buff R., Uncertain Volatility Models – Theory and Application (2002) Carmona R.A. and Tehranchi M.R., Interest Rate Models: An Infinite Dimensional Stochastic Analysis Perspective (2006) Dana R.-A. and Jeanblanc M., Financial Markets in Continuous Time (2003) Deboeck G. and Kohonen T. (Editors), Visual Explorations in Finance with Self-Organizing Maps (1998) Delbaen F. and Schachermayer W., The Mathematics of Arbitrage (2005) Elliott R.J. and Kopp P.E., Mathematics of Financial Markets (1999, 2nd ed. 2005) Fengler M.R., Semiparametric Modeling of Implied Volatility (2005) Filipovi´c D., Term-Structure Models (2009) Fusai G. and Roncoroni A., Implementing Models in Quantitative Finance (2008) Geman H., Madan D., Pliska S.R. and Vorst T. (Editors), Mathematical Finance – Bachelier Congress 2000 (2001) Gundlach M. and Lehrbass F. (Editors), CreditRisk+ in the Banking Industry (2004) Jeanblanc M., Yor M., Chesney M., Mathematical Methods for Financial Markets (2009) Jondeau E., Financial Modeling Under Non-Gaussian Distributions (2007) Kabanov Y.A. and Safarian M., Markets with Transaction Costs (2009) Kellerhals B.P., Asset Pricing (2004) Külpmann M., Irrational Exuberance Reconsidered (2004) Kwok Y.-K., Mathematical Models of Financial Derivatives (1998, 2nd ed. 2008) Malliavin P. and Thalmaier A., Stochastic Calculus of Variations in Mathematical Finance (2005) Meucci A., Risk and Asset Allocation (2005, corr. 2nd printing 2007, Softcover 2009) Pelsser A., Efficient Methods for Valuing Interest Rate Derivatives (2000) Platen E. and Heath D., A Benchmark Approach to Quantitative Finance (2006, corr. printing 2010) Prigent J.-L., Weak Convergence of Financial Markets (2003) Schmid B., Credit Risk Pricing Models (2004) Shreve S.E., Stochastic Calculus for Finance I (2004) Shreve S.E., Stochastic Calculus for Finance II (2004) Yor M., Exponential Functionals of Brownian Motion and Related Processes (2001) Zagst R., Interest-Rate Management (2002) Zhu Y.-L., Wu X., Chern I.-L., Derivative Securities and Difference Methods (2004) Ziegler A., Incomplete Information and Heterogeneous Beliefs in Continuous-time Finance (2003) Ziegler A., A Game Theory Analysis of Options (2004)

Giovanni Cesari John Aquilina Niels Charpillon Zlatko Filipovi´c Gordon Lee Ion Manda

Modelling, Pricing, and Hedging Counterparty Credit Exposure A Technical Guide

Giovanni Cesari UBS AG 100 Liverpool Street London UK EC2M 2RH [email protected]

Zlatko Filipovi´c UBS AG 100 Liverpool Street London UK EC2M 2RH [email protected]

John Aquilina UBS AG 100 Liverpool Street London UK EC2M 2RH [email protected]

Gordon Lee UBS AG 100 Liverpool Street London UK EC2M 2RH [email protected]

Niels Charpillon UBS AG 100 Liverpool Street London UK EC2M 2RH [email protected]

Ion Manda UBS AG 100 Liverpool Street London UK EC2M 2RH [email protected]

ISBN 978-3-642-04453-3 e-ISBN 978-3-642-04454-0 DOI 10.1007/978-3-642-04454-0 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2009942279 Mathematics Subject Classification (2000): 60H10, 60H20, 60H35, 62P05, 65C05, 65C20, 91Bxx, 91-08 JEL Classification: C02, C61, C63, E43, E47, G12, G13, G32, G33 © Springer-Verlag Berlin Heidelberg 2009 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: WMXDesign GmbH Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Preface

It was the end of 2005 when our employer, a major European Investment Bank, gave our team the mandate to compute in an accurate way the counterparty credit exposure arising from exotic derivatives traded by the firm. As often happens, exposure of products such as, for example, exotic interest-rate, or credit derivatives were modelled under conservative assumptions and credit officers were struggling to assess the real risk. We started with a few models written on spreadsheets, tailored to very specific instruments, and soon it became clear that a more systematic approach was needed. So we wrote some tools that could be used for some classes of relatively simple products. A couple of years later we are now in the process of building a system that will be used to trade and hedge counterparty credit exposure in an accurate way, for all types of derivative products in all asset classes. We had to overcome problems ranging from modelling in a consistent manner different products booked in different systems and building the appropriate architecture that would allow the computation and pricing of credit exposure for all types of products, to finding the appropriate management structure across Business, Risk, and IT divisions of the firm. In this book we describe some of our experience in modelling counterparty credit exposure, computing credit valuation adjustments, determining appropriate hedges, and building a reliable system. What do we mean by all of this? Counterparty credit exposure is the amount a company could potentially lose in the event of one of its counterparties defaulting. At a general level, computing credit exposure entails simulating in different scenarios and at different times in the future, prices of transactions, and then using one of several statistical quantities to characterise the price distributions that has been generated. Typical statistics used in practice are (i) the mean, (ii) a high-level quantile such as the 97.5% or 99%, usually called Potential Future Exposure (PFE), and (iii) the mean of the positive part of the distribution, usually referred to as Expected Positive Exposure (EPE). With these measures and default probability information or counterparty CDS premia, it is then possible to price counterparty risk. In the financial industry, the economic value of this risk is generally called Credit Valuation Adjustment (CVA). v

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Preface

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As we will have occasion to see later in this book, it can be computed as the price of a Credit Default Swap paying the Expected Positive Exposure. Equivalently expressed, CVA is the price of a new type of hybrid product, the so-called Contingent Credit Default Swap (C-CDS), which pays the value of the exposure (floored at zero) upon default of the counterparty. The credit crisis which started in 2007 clearly shows why it is of crucial importance for any company entering a derivative business to (i) measure counterparty exposure, (ii) compute capital requirements, and (iii) hedge counterparty risk. Measuring counterparty exposure is important for setting limits on the amount of business a firm is prepared to do with a given counterparty; hedging it gives a possibility of mitigating it and transferring risk; and from a regulatory perspective there is significant pressure on financial institutions to have the capability of producing accurate risk measures to compute capital. In addition, computing counterparty exposure can also give insights into prices of complex products in potential future scenarios. It seems that what was until recently a Risk Control function attracting relatively limited attention, is now becoming a central activity of all major financial institutions, requiring significant resources from all parties. Our approach to counterparty credit exposure analysis is quantitative. The focus is on mathematical modelling, simulation techniques using various Monte Carlo approaches, and pricing. In contrast, we are only marginally interested in assessing the quality of counterparties or in analysing historical market data in order possibly to forecast future behaviours of the economy, or in risk and regulatory aspects of the problem. We consider derivative products and complex structures which are usually traded in Investment Banks, and focus on practical aspects of the problems at hand. All models used in our analyses are tested with practical data and real transactions. Given this quantitative focus, we sometimes refer to our work as Credit Quantification. The book is divided into four parts, (I) Methodology, (II) Architecture and Implementation, (III) Products, and (IV) Hedging and Managing Counterparty Risk. In Part I we present a generic simulation framework, which can be used to compute counterparty exposure for both vanilla and exotic products. We show how the classical Monte Carlo framework, where price distributions are computed by generating thousands of scenarios and by explicitly pricing the product at each point in time and at each scenario, is a special case of our more general framework. The classical Monte Carlo approach works well only for products that can be priced in analytical or quasi-analytical form. It is not practical for products that cannot be priced in closed form and require, for instance, a Monte Carlo or lattice pricing approach. Typical examples are products with callability features or exotic interestrate transactions. We show how in these cases American Monte Carlo techniques used generally for pricing can also be applied efficiently to compute exposure, as they provide intermediate valuations over time and scenarios. Part II shows how our simulation framework naturally leads to the implementation of a software architecture and the definition of a programming language that allows the computation of both vanilla and exotic products in a scenario-consistent way. In practice, in a large financial institution one of the main problems in building

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counterparty exposure systems, is to integrate different products, booked in different systems and priced using libraries written in different languages and with different technologies, in order to compute portfolio exposure across different businesses. We show that our approach leads to an architecture that can integrate other systems in a natural way. In Part III we consider how to compute exposure for different products. We show how the general techniques and models described in Part I and the architecture described in Part II can be used in practice. Finally in Part IV things are put together. We consider how to perform risk management and risk control of counterparty exposure on a portfolio basis. We describe different aggregation techniques and a standard set-up that uses collateral to mitigate exposure. We also analyse how to model wrong-way/right-way exposure, where transaction price fluctuations and quality of the counterparty are correlated and we address the problem of changing the reference probability measure after the simulation has been performed. The final chapter is dedicated to pricing counterparty credit exposure and to computing CVA and CVA sensitivities not only to credit spread, but also to market risk factors. The whole book can be seen as a roadmap to achieve this goal. One note to conclude: in our work we describe and use well-established simulation and pricing techniques. Our goal is not to suggest new or more sophisticated algorithms. It is rather to show how well-known algorithms can be put together and used to compute counterparty credit exposure and which limitations have to be taken into consideration if we want to move from vanilla products to complex exotic transactions. London, September 2009

Giovanni Cesari John Aquilina Niels Charpillon Zlatko Filipovi´c Gordon Lee Ion Manda

Acknowledgements

This book developed from the experience gained during a long-term project within the Investment Bank we work for. As such it would have never been written without the support, advice and encouragement of several people to whom we would like to express our gratitude. Duncan Rodgers and Myles Wright gave us full support in the development of the ideas described in this book. Darryll Hendricks and Tom Daula helped us to have a global approach to credit exposure computation and inspired us in the early stages of this project. From Thomas Hyer and Trevor Chilton we gained a better understanding of the challenges of the American Monte Carlo (AMC) algorithms, while Yuan Gao engineered an early prototype that used AMC not just for pricing but also to estimate credit exposure. Helmut Glemser kept us on our toes by insisting on explanations (or corrections!) whenever our calculations gave results that puzzled him. Sincere thanks go to our colleagues Rong Fan and Yi Yuan, who, from across the Atlantic, contributed to the writing and testing of a significant part of our code. We would like also to thank Mark Davis and Martijn Pistorius for their very useful comments, and Catriona Byrne and her team for their continuous support during the publishing phase of this book. Thanks are also due to the following people for useful discussions during the course of the project: Richard Adams, Annette Alford, Rowan Alston, Philip Anderson, Ashima Bhalla, Rajinder Basra, Marc Baumslag, Alan Baxter, Lucia Bonilla, Gareth Campbell, Denton Capp, Ben Cassie, Dean Charette, Paul Charles, Dipak Chotai, Jack Chung, Mark Dahl, Valdemar Dallagnol, Gerald Elflein, Jesus Fernandez, Alex Ginzburg, Alasdair Gray, Lionel Guerraz, Stephen Johnston, Jeffrey Lin, Catarina Lopes, Felix Matschke, David Matthews, Peta McRae, Sourav Mishra, Andrew Morgan, Bruno Mugica, Logan Nerio, James Ntuk-Idem, Sarah Peplow, Tom Prangley, David Shieff, Andrew Tseng, and Winnie Zheng.

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Disclaimer

The views and opinions expressed in this book are those of the authors and are not those of UBS AG, its subsidiaries or affiliate companies (“UBS”). Accordingly, UBS does not accept any liability over the content of this book or any claims, losses or damages arising from the use of, or reliance on, all or any part of this book. Nothing in this book is or should be taken as information regarding, or a description of, any operations or business practice of UBS. Similarly, nothing in this book should be taken as information regarding any failure or shortcomings within the business, credit or risk or other control, or assessment procedures within UBS.

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About the Authors

Giovanni Cesari is Managing Director at UBS. He has more than 10 years’ experience in modelling and pricing counterparty credit exposure. Before moving to finance, Giovanni worked for several years in particle physics and in theoretical computer science. Giovanni holds a Laurea in Electrical Engineering from the University of Trieste, a Perfezionamento in Physics from the University of Padova, and a Ph.D. from ETH, Zurich. John Aquilina holds an M.Phil. in Statistical Science from the University of Cambridge and a Ph.D. in Mathematical Finance from the University of Bath. He has worked on modelling counterparty credit exposure at UBS since 2005. Niels Charpillon holds a Diplôme d’Ingénieur from Ecole des Mines, an M.Sc. in Financial Mathematics from Warwick Business School, and a Licence in Economics from University of St. Etienne. He joined the counterparty exposure team at UBS in 2006. Zlatko Filipovi´c started working for UBS in 2005 as a Quantitative Analyst in the counterparty exposure team. Before joining UBS, Zlatko had been working for Mako Global Derivatives, London, as a Financial Engineer. Zlatko obtained a Ph.D. in Quantitative Finance from Imperial College, London, after graduating from the Faculty of Mathematics, University of Belgrade. Gordon Lee joined the counterparty exposure team at UBS in 2006. Prior to UBS, he was a Senior Associate in quantitative risk and performance analysis at Wilshire Associates. Gordon holds an M.A. in Mathematics from Churchill College, University of Cambridge. Ion Manda holds a Diploma de Inginer from the University of Bucharest and a M.Sc. in Financial Engineering from University of London. He has been working in the credit exposure team at UBS since 2006. Ion has about 10 years’ experience as a software engineer. xiii

Contents

Part I

Methodology

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Preliminary Examples . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Vanilla Interest-Rate Swap . . . . . . . . . . . . . . . 1.2.2 Cancellable Swap . . . . . . . . . . . . . . . . . . . . 1.2.3 Managing Credit Risk—Collateral, Credit Default Swap 1.3 Why Compute Counterparty Credit Exposure? . . . . . . . . . 1.4 Modelling Counterparty Credit Exposure . . . . . . . . . . . . 1.4.1 Definition . . . . . . . . . . . . . . . . . . . . . . . . 1.4.2 Risk Measures . . . . . . . . . . . . . . . . . . . . . . 1.4.3 Netting and Aggregation . . . . . . . . . . . . . . . . . 1.4.4 Close-Out Risk . . . . . . . . . . . . . . . . . . . . . . 1.4.5 Right-Way/Wrong-Way Exposure . . . . . . . . . . . . 1.4.6 Credit Valuation Adjustment: CVA . . . . . . . . . . . 1.4.7 A Simple Credit Quantification Example . . . . . . . . 1.4.8 Computing Credit Exposure by Simulation . . . . . . . 1.4.9 Implementation Challenges . . . . . . . . . . . . . . . 1.4.10 An Alternative Approach: The AMC Algorithm . . . . 1.5 Which Architecture? . . . . . . . . . . . . . . . . . . . . . . . 1.6 What Next? . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Modelling Framework . . . . . . . . . . . . . 2.1 Counterparty Credit Exposure Definition . 2.2 Process Dynamics . . . . . . . . . . . . . 2.3 Interest Rate: Single Currency . . . . . . . 2.3.1 Simple Specifications . . . . . . . 2.3.2 HJM Framework . . . . . . . . . . 2.3.3 Libor Market Models . . . . . . . 2.4 Multiple Currencies and Foreign Exchange 2.5 Inflation . . . . . . . . . . . . . . . . . .

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2.6 Equity . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Credit . . . . . . . . . . . . . . . . . . . . . . . . 2.7.1 Default Probabilities from par CDS Spreads 2.7.2 Stochastic Default Probabilities . . . . . . . 2.7.3 Loss Simulation . . . . . . . . . . . . . . . 3

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Simulation Models . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Interest-Rate Models . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Separable Volatility . . . . . . . . . . . . . . . . . . . . 3.1.2 Example: Hull-White (Extended Vasicek) . . . . . . . . . 3.2 Equity and FX Models . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Black Model . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Local Volatility . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Stochastic Volatility . . . . . . . . . . . . . . . . . . . . 3.2.4 Jump Models . . . . . . . . . . . . . . . . . . . . . . . . 3.2.5 Extension to Stochastic Interest Rates . . . . . . . . . . . 3.2.6 A Simpler Approach: Independent Interest Rates . . . . . 3.2.7 Different Models for Different Markets . . . . . . . . . . 3.3 Credit Models . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Simulation of Single-Name Default Probabilities and Default Times . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Inter-Name Default Dependence . . . . . . . . . . . . . 3.3.3 Technical Note: Recursion . . . . . . . . . . . . . . . . . 3.3.4 Properties of the Loss Distribution: Large Homogeneous Portfolio . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.5 Calibration of Correlation . . . . . . . . . . . . . . . . . 3.4 Choice of Model . . . . . . . . . . . . . . . . . . . . . . . . . .

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Valuation and Sensitivities . . . . . . . . . . . . . . . . . . . . . . 4.1 American Monte Carlo: Mathematical Notation and Description 4.1.1 Mathematical Formulation . . . . . . . . . . . . . . . . 4.1.2 Practical Examples . . . . . . . . . . . . . . . . . . . . 4.1.3 Backward Induction Algorithm . . . . . . . . . . . . . 4.2 AMC Estimation Algorithms . . . . . . . . . . . . . . . . . . 4.2.1 Tilley’s Algorithm . . . . . . . . . . . . . . . . . . . . 4.2.2 Longstaff-Schwartz Regression . . . . . . . . . . . . . 4.2.3 Biases of Estimates . . . . . . . . . . . . . . . . . . . 4.2.4 An AMC Algorithm to Compute Credit Exposure . . . 4.3 Post-Processing of the Price Distribution . . . . . . . . . . . . 4.4 Practical Examples Revisited . . . . . . . . . . . . . . . . . . 4.5 Computing Price Sensitivities . . . . . . . . . . . . . . . . . . 4.5.1 The Classical Approach . . . . . . . . . . . . . . . . . 4.5.2 Price Sensitivities through Regression . . . . . . . . . 4.5.3 Removing Correlation . . . . . . . . . . . . . . . . . . 4.6 Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Contents

Part II

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Architecture and Implementation

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Computational Framework . . . . . . . . . . . . . . . 5.1 AMC Implementation and Trade Representation . . 5.1.1 Examples . . . . . . . . . . . . . . . . . . . 5.1.2 Expression Trees . . . . . . . . . . . . . . . 5.2 A Portfolio Aggregation Language . . . . . . . . . 5.2.1 PAL Examples . . . . . . . . . . . . . . . . 5.3 The Concept of Scenarios . . . . . . . . . . . . . . 5.4 The Concept of Super-Product . . . . . . . . . . . . 5.4.1 An Example of Super-Products: The C-CDS

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Implementation . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Spot and Forward Statistics . . . . . . . . . . . . . . . . 6.1.1 Libor Rates and Bond Prices . . . . . . . . . . . 6.1.2 Annuity . . . . . . . . . . . . . . . . . . . . . . 6.1.3 Swap Rate . . . . . . . . . . . . . . . . . . . . . 6.2 Path Dependent Statistics . . . . . . . . . . . . . . . . . 6.2.1 Extremum . . . . . . . . . . . . . . . . . . . . . 6.2.2 Average . . . . . . . . . . . . . . . . . . . . . . 6.2.3 In Range Fraction . . . . . . . . . . . . . . . . . 6.2.4 Credit Loss . . . . . . . . . . . . . . . . . . . . . 6.3 Monte Carlo Stepping . . . . . . . . . . . . . . . . . . . 6.4 Technical Notes . . . . . . . . . . . . . . . . . . . . . . 6.4.1 SDE Integration Schemes . . . . . . . . . . . . . 6.4.2 Milstein 2 Scheme . . . . . . . . . . . . . . . . . 6.4.3 Martingale Interpolation . . . . . . . . . . . . . . 6.4.4 Distribution of Maxima and Minima . . . . . . . 6.5 Error Analysis . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Choice of Model: Scenario and Exposure Analysis 6.5.2 AMC Error . . . . . . . . . . . . . . . . . . . . . 6.5.3 Numerical Errors . . . . . . . . . . . . . . . . . 6.5.4 Approximations: Arbitrage Conditions . . . . . .

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Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.1 Functional, Non-Functional Requirements, and Design Principles . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Conceptual View: Methodology . . . . . . . . . . . . . . . . . 7.3 Logical View . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 Portfolio Manager Components . . . . . . . . . . . . . 7.3.2 State of the World Components . . . . . . . . . . . . . 7.3.3 Quantification Components . . . . . . . . . . . . . . . 7.4 Physical View . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Alternative Approaches . . . . . . . . . . . . . . . . . . . . .

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Part III Products 8

Interest-Rate Products . . . . . . . . . . . . 8.1 Interest-Rate Swaps . . . . . . . . . . . 8.1.1 Swaps in Advance and in Arrears 8.1.2 Capped and Floored Swaps . . . 8.1.3 Cancellable Swaps . . . . . . . . 8.1.4 Cross-Currency Swaps . . . . . . 8.2 Constant-Maturity Swaps and Steepeners 8.3 Range Accruals . . . . . . . . . . . . . 8.4 Interest-Rate Options . . . . . . . . . .

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9

Equity, Commodity, Inflation and FX Products 9.1 Forwards and Options . . . . . . . . . . . . 9.1.1 Forwards Contracts . . . . . . . . . 9.1.2 Vanilla and Digital Options . . . . . 9.1.3 Bermudan and American Options . . 9.1.4 Asian Options . . . . . . . . . . . . 9.1.5 Barrier Options . . . . . . . . . . . 9.2 Asset Swaps . . . . . . . . . . . . . . . . . 9.2.1 Absolute Return Swaps . . . . . . . 9.2.2 Relative Return Swaps . . . . . . . . 9.2.3 Cliquets . . . . . . . . . . . . . . . 9.2.4 Target Redemption Swaps . . . . . .

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159 159 160 162 162 164 164 166 166 167 168 169

10 Credit Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 10.1 Credit Default Swaps . . . . . . . . . . . . . . . . . . . . . . . . 171 10.2 Collateral Debt Obligations . . . . . . . . . . . . . . . . . . . . . 172 11 Structures . . . . . . . . . . . . . . 11.1 Sinking Funds . . . . . . . . . 11.2 Accelerated Share Re-Purchase 11.3 Callable Daily Accrual Notes . 11.4 Call Spread Overlays . . . . .

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12 Counterparty Risk Aggregation and Risk Mitigation . . . . . . . 12.1 Risk Measures . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Choice of Measure . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Portfolio Risk Aggregation . . . . . . . . . . . . . . . . . . . 12.3.1 Reference Currency . . . . . . . . . . . . . . . . . . . 12.3.2 Netting and No-Netting Agreements . . . . . . . . . . 12.3.3 Break Clauses . . . . . . . . . . . . . . . . . . . . . . 12.4 Collateral Agreements . . . . . . . . . . . . . . . . . . . . . . 12.4.1 Counterparty Exposure of Collateralised Counterparties

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183 184 186 187 188 188 189 190 191

Part IV Hedging and Managing Counterparty Risk

Contents

xix

12.4.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 12.5 Close-Out Risk . . . . . . . . . . . . . . . . . . . . . . . . . . 12.6 Risk Allocation . . . . . . . . . . . . . . . . . . . . . . . . . 12.6.1 Naive Allocation . . . . . . . . . . . . . . . . . . . . . 12.6.2 Euler Allocation . . . . . . . . . . . . . . . . . . . . . 12.6.3 Comparison with Naive Allocation . . . . . . . . . . . 12.6.4 Contribution Calculation of Collateralised Transactions

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192 194 195 196 196 197 199

13 Combining Market and Credit Risk . . . . . . . . . . . . . . . 13.1 Change of Measure: Practical Implementation . . . . . . . . 13.2 Exposure under Real-World Measure . . . . . . . . . . . . . 13.3 Stress Testing . . . . . . . . . . . . . . . . . . . . . . . . . 13.4 Right-Way/Wrong-Way Exposure . . . . . . . . . . . . . . . 13.4.1 Right-Way/Wrong-Way Exposure: Merton Approach 13.4.2 The Inverse Problem . . . . . . . . . . . . . . . . . . 13.4.3 Example 1: Call Option on Stock . . . . . . . . . . . 13.4.4 Example 2: Call Put Structure on Oil . . . . . . . . . 13.4.5 Example 3: Cross-Currency Swap on USD-GBP . . . 13.4.6 Comparison with the Change of Measure Approach .

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201 202 203 204 205 206 209 210 212 212 212

14 Pricing Counterparty Credit Risk . . . . . . . . . . 14.1 Credit Valuation Adjustment and Static Hedging 14.2 Contingent Credit Default Swap . . . . . . . . . 14.2.1 American Monte Carlo Valuation . . . . 14.2.2 Example . . . . . . . . . . . . . . . . . 14.3 Dynamic Hedging of Counterparty Risk . . . . 14.4 Optimal Static Hedging . . . . . . . . . . . . . 14.5 CVA Sensitivities . . . . . . . . . . . . . . . . 14.6 Collateral Agreements . . . . . . . . . . . . . . 14.7 Right-Way/Wrong-Way Risk . . . . . . . . . . 14.8 Examples . . . . . . . . . . . . . . . . . . . . . 14.8.1 C-CDS on a Vanilla Interest-Rate Swap . 14.8.2 Impact of Discretization Schedule . . . . 14.8.3 Collateralised Equity Swap . . . . . . . 14.9 Case Study . . . . . . . . . . . . . . . . . . . .

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Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 A

Approximations . . . . . . . . . . . . . . . . A.1 Maximum Likely Exposure . . . . . . . A.1.1 MLE of Equity and FX Products A.1.2 MLE of Swaps . . . . . . . . . . A.2 Expected Positive Exposure . . . . . . . A.2.1 EPE and CVA of Equity Options A.2.2 Relation between MLE, EPE . . A.3 CVA of Swaps . . . . . . . . . . . . . .

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233 233 233 234 235 235 235 236

xx

B

Contents

Results from Stochastic Calculus and Finance . . . . . . . . . . B.1 Brownian Motion and Martingales . . . . . . . . . . . . . . B.2 Replication of Contingent Claims: Martingale Representation B.3 Change of Numeraire . . . . . . . . . . . . . . . . . . . . .

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239 239 241 243

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249

Part I

Methodology

Chapter 1

Introduction

The aim of this first chapter is to introduce basic notions of counterparty credit exposure, and to motivate with a few simple examples the problems and concepts we will be considering in more detail later in this book.

1.1 Basic Concepts Consider two parties, A and B say, who enter into an OTC transactions portfolio.1 This portfolio could consist of products ranging from interest-rate and crosscurrency swaps in different currencies with various exotic features, to exotic options on equity, foreign exchange and commodity underlyings. It could also include various types of credit derivatives contracts, such as credit default swaps (CDS) on single names or collateral debt obligations (CDO) tranches in swap form on portfolios of reference entities, or credit indices.2 In general a given company, say a financial institution A, will have portfolios with many other counterparties, varying among sovereign entities, corporates, hedge funds, insurance companies (including for examples monolines3 ). It may also happen that the credit quality of the counterparty is not independent of the performance of the transaction entered into, such as what happens for example, when an electricity generating oil-powered plant bets on the price of oil. Counterparty credit exposure is the amount a company, say A, could potentially lose in the event of one of its counterparties defaulting. It can be computed by simulating in different scenarios and at different times in the future, the price of the 1 An

OTC (Over The Counter) transaction is a transaction that is not traded through an exchange.

2A

typical credit index is for example iTraxx; it is composed of the 125 most liquid CDS names referencing European investment grade credits. 3 Monoline

insurance are companies that guarantee to bond investors the payment of coupon and notional. They insure different type of securities, such as CDO, structured products and municipal bonds. Monolines have been affected in the recent credit crunch, raising counterparty risk issues. G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0_1, © Springer-Verlag Berlin Heidelberg 2009

3

4

1 Introduction

transactions with the given counterparty, and then by using some chosen statistic to characterise the price distributions that have been generated. Typical statistics used in the industry are (i) the mean, (ii) the 97.5% or 99% quantile, called Potential Future Exposure (PFE), and (iii) the mean of the positive part of the distribution, referred to as the Expected Positive Exposure (EPE). We will also have occasion to speak about less commonly used statistical measures that can be more appropriate for certain products. As important as measuring counterparty exposure, via PFE or EPE, is the computation of the cost of hedging it, and the capability of having a dynamic hedging strategy, i.e. the computation of exposure sensitivities. In the financial industry the price of hedging is generally called Credit Valuation Adjustment (CVA). We will see that there are strong links between EPE and CVA computation.

1.2 Preliminary Examples Some simple examples will help clarifying these points.

1.2.1 Vanilla Interest-Rate Swap Consider counterparties A and B who enter into an interest-rate swap where A receives every six months the 6-month Libor rate on a notional of $100 million, while paying to B a fixed amount equal to the par 10-year swap rate on the same notional observed at inception. This is a typical swap contract with value zero at inception. As time passes and market conditions change, the value of the swap changes accordingly. Thus, if the

Fig. 1.1 Exposure profile for a typical USD 10-years swap contract, paying fix and receiving floating on a notional of 100 mUSD. The full distribution is shown in Fig. 1.2

1.2 Preliminary Examples

5

swap rate decreases (resp. increases), the transaction will be out of the money (resp. in the money) as seen from A’s point of view. Therefore, if B were to default at a point in the life of the trade when swap rates had increased, then A would need to replace in the market—at higher cost than the fixed amount being paid to B—the floating cashflows promised and not delivered by B. To compute the credit exposure for the swap, we would need to estimate the values the swap could take in different market scenarios at points in the future. Figure 1.1 shows the 97.5%, the 2.5% quantiles and the EPE of the swap price distribution, over its entire life, as seen from party A’s point of view. A plot like this is usually referred to as the exposure profile. Note that the 2.5% quantile seen from A’s perspective, corresponds to the 97.5% quantile seen from B’s perspective. Figure 1.2 shows in the top panel the full price distribution over time. The bottom panel shows three slices of this distribution at three different points in time. For this example, the 97.5% PFE quantile is a function that starts at zero, peaks at around the 4-year point and then decreases to zero. First, by definition, the fixed payment in the trade is the fair value for the swap, and this must therefore have value (and hence exposure level) identically equal to zero at inception. Similarly, towards the end of the transaction, when all payments but one due under the swap have been paid, the exposure remaining is that from only a single coupon exchange. This explains what happens at the right end of the profile. At intermediate times, the shape of the profile is the result of opposing effects. On the one hand, as the interest rates underlying the swap diffuse, there is more variability in the realised Libor rates, potentially leading to higher exposure. On the other hand, as time evolves there are fewer payments remaining under the swap, and this mitigates the effect of diffusing rates. The profile therefore tells us that with 97.5% probability, the loss of A due to default of B will not exceed roughly $28 million. Of course, this estimate is based on market information at inception of the swap, and would change if it were to be repeated at a different time.

1.2.2 Cancellable Swap We can make our example slightly more interesting. It is common for swaps to trade with an additional callability feature, whereby one counterparty would have the option, at certain times in the life of the swap, to cancel (“call”) the transaction for a fixed fee (which may be zero).4 Suppose that party A, from whose perspective we look at exposure, also holds the option to cancel the trade; one says that A is long callability. Assuming A behaves 4 We define “cancellable swap” a swap which has an embedded option to terminate it at zero cost (or at a given predetermined fee). Sometimes these swaps are also called “callable”. We use the term callable swap in a more generic way, considering the possibility that the swap is “called” into a new product. In this sense a cancellable swap is a simple example of a callable swap.

6

1 Introduction

Fig. 1.2 Future value distribution for a typical USD 10-years swap contract, paying fix and receiving floating. The PFE and EPE are shown in Fig. 1.1

1.2 Preliminary Examples

7

Fig. 1.3 Exposure of a typical cancellable 10-years swap, paying yearly the fair swap rate fixed at inception and receiving semi-annually the 6-month Libor rate on a notional of 100 mUSD. On the left (resp. right), the exposure represents long (resp. short) optionality to cancel the swap every year. The value of the swap at time-zero corresponds to the value of the option, which is assumed to be paid up-front by the counterparty

rationally, it would never decide to walk away from the swap in those scenarios where the swap has a high value (because the swap rate has increased and future receivables are worth more than at inception). This means that having the option to cancel, at zero cost, should not affect materially A’s exposure. On the other hand, suppose A is short callability, meaning that it is B who has the option to walk away from the swap. Rational behaviour on B’s part implies that B would cancel the swap when they are making a loss on the transaction, which is exactly when A would be in the money. Thus we would expect that with A being short callability, A’s PFE (and EPE) to B is reduced to zero at each date where B has the option to cancel the transaction. Figure 1.3 shows all this happening. In the left panel, we see that with A having the option to cancel the trade, the PFE profile is similar to that of a vanilla swap, with the exception of the time-zero level, which equates to the value of the cancellation option. On the right we see that A’s exposure is reduced to zero at dates where B can cancel; on remaining dates, the PFE is reduced to that arising from coupons due until the next allowed cancellation date. Note that the time-zero point is not zero but negative from A’s point of view, since it is B who holds the option in this case. Note that in practice the value of the option is often embedded in the fixed coupon of the swap, which has then value zero at inception. From the computational point of view, there is a fundamental difference between the vanilla swap example of the previous section and the cancellable swap we have just described. A vanilla swap can in fact be priced analytically and in a model independent way, and therefore, as we will see, exposure could be computed in a classical Monte Carlo framework, where scenarios are generated and then products are priced at each scenario and each time step. On the other hand, a cancellable swap is priced using a lattice or Monte Carlo simulation, making therefore impractical the

8

1 Introduction

computation of credit exposure itself by Monte Carlo simulation.5 This would entail in fact a Monte Carlo of Monte Carlo approach (with nested simulations), where one set of simulation is used for scenarios and one set, at each time step and scenario, for pricing. We will analyse this aspect in more details in the next chapters.

1.2.3 Managing Credit Risk—Collateral, Credit Default Swap When structuring a new transaction (or portfolio of transactions), one of the criteria is the amount of acceptable credit exposure. This will depend on several factors including risk appetite and quality of the counterparty. The most common way to reduce counterparty exposure is to set up a collateral agreement, whereby the client is required to deposit collateral into a separate account at regular time intervals. A collateral agreement between counterparties can take one of several forms. For instance, it can be in the form of cash or securities, can be called daily or at other regular intervals, and there can be thresholds and minimum transfer amounts. In addition, since the point of holding collateral is to be able to liquidate it in case of the counterparty defaulting, market liquidity plays an important role in determining the amount of collateral needed. When collateral agreements are in place, therefore, credit exposure computation has to take into account features of that agreement together with the dynamics of the trade itself, in order to compute so-called close-out risk. Close-out risk measures the amount by which the value of a transaction could change during the period from when the counterparty is deemed to have defaulted, until the collateral has been liquidated and used to fund, at current market conditions, the replacement of the defaulted counterparty in the transaction. In general this computation should also include change in value of the collateral, possibly taking into account the correlation between collateral and transaction value. A further possibility for A to manage counterparty credit exposure to B is to buy Credit Default Swap (CDS) protection on B from another counterparty C. The transaction between A and C is typically fully collateralised. This will transfer the risk of B to C. In case of default of B, the CDS would ensure that C will step in and make good any payments that were originally promised by B, or simply pay the value of the transaction. This should cover the value of the products (e.g. the interestrate swap we described before) as calculated at the time of B’s default. The value of the protection is called Credit Valuation Adjustment (CVA) and in principle should be charged to the client (in our case counterparty B) in order to reflect its credit risk. For instance, suppose the credit spread of B is 100 bps,6 the amount to protect $100 million, the trade maturity 10 years. Under these market conditions the price 5 Note that a cancellable swap is the combination of a vanilla swap and a Bermudan option. If the option is European (i.e. the swap can be called only on one date), the cancellable swap can be priced in closed form. 6 bps:

basis points, a hundredth of a percent.

1.2 Preliminary Examples

9

of buying protection on B will be in the order of $10 million.7 Such protection is sufficient only at the time of calculation, and one would need to compute exposure sensitivities to the underlying factors in order to dynamically hedge the required amount of protection on B, as the future exposure to B will evolve with market conditions. As we mentioned, the usage of CDS transfers the credit exposure from B to C. So, even if one assumes the exposure to B to be perfectly hedged via the CDS, there will be counterparty exposure to C, which offers protection.8 Consider again the example above where A buys CDS protection on B on a notional amount of $100 million. Figure 1.4 shows a typical profile for such a transaction, assuming it is un-collateralised. For such a default protection product, the exposure one observes Fig. 1.4 Exposure of a typical credit default swap on a notional of 100 mUSD and spread about 100 bps. PFE (on different quantiles), EPE and ES are shown

results from the effects of (i) movements in the simulated credit spread of B and (ii) defaults. Clearly, the payment triggered by B’s default, equal to about 1 − R = 60% of notional, would imply that in a default scenario, A would have an exposure to C of $60 million. R is the recovery rate, i.e. the amount which can be recovered upon default of the counterparty. Now in Fig. 1.4, the PFE profile (which we recall is the 97.5% quantile of the distribution) does not show such high levels of exposure. This must mean that in the simulated scenarios, fewer than 2.5% of the scenarios involve B defaulting. Or in other words that the event of B defaulting is a rare event. To take into account this event one could display higher quantiles of the distribution, say the 99.9% quantile. Alternatively, one can calculate the Expected Shortfall (ES) of the distribution, which is simply the expected value of the tail of the distribution (see Chap. 12 for more details); this measure will uncover any large outliers in the distribution (such as the rare event of default of B, and hence large payment by C, in this case). Figure 1.4 displays this quantity, and clearly shows that defaults are indeed occurring even if they are not frequent enough to affect the 97.5% PFE. 7 This 8 This

is roughly equal to the 10-years duration multiplied by the 10-years spread.

is one of the reasons why, after the 2007–08 credit events, it is under discussion to use clearing houses when dealing with credit default swaps.

10

1 Introduction

This example shows that with credit products, where events of small probability can lead to large payments, the PFE might not be the appropriate exposure measure to consider. We will have more to say on this in due course.

1.3 Why Compute Counterparty Credit Exposure? Counterparty risk is at the root of traditional banking. Historically, the first form of financial instruments were bonds, and their value was mainly driven by the market’s view of how creditworthy the issuers of these bonds were. However, today’s financial world is much more complex, and the process of estimating counterparty risk much more challenging. While for loans and other traditional products the focus is mainly on estimating the capability of the borrower to repay its obligation, for derivative transactions, estimating accurately the future value of the transaction is as important and challenging as having a view on the ability of the counterparty to honour its obligations. Accuracy is important because credit exposure models are used for several purposes in financial institutions, such as (i) Setting limits on the amount of business allowed with a particular counterparty. (ii) Dynamic hedging of counterparty risk, by buying credit protection on the counterparty. This in effect allows one to trade away counterparty credit risk. (iii) Computation of risk weighted assets and capital requirements. (iv) Obtaining insight about prices of complex transactions in potential future scenarios. For example, while counterparty risk is concerned with measuring how high the value of a transaction can go (and therefore how much a counterparty would owe), there are similarities between this and computing Value at Risk, or stress testing, where one would be interested in how much the value of a transaction could drop.

1.4 Modelling Counterparty Credit Exposure In the previous sections we have introduced the concept of counterparty exposure and have provided some simple examples. We focus now on a more formal approach which will give a flavour of the mathematical tools we will need in the next chapters.

1.4.1 Definition Given a portfolio of positions traded with a counterparty, the main quantity we need to model to compute the counterparty credit exposure at time t, is the distribution Vt of the portfolio prices, computed at time t > 0 and seen from today. We will see in the next chapter how Vt can be described in its full generality. For the moment

1.4 Modelling Counterparty Credit Exposure

11

we consider the case of products without callability features and where cashflow payments are performed at discrete time points (Ti ), i = 1, . . . , n, with Tn being the maturity of the trade. Define Xt to be the (generally stochastic) payment made by the portfolio at any time t (Xt = 0 if t is not a member of (Ti ), i = 1, . . . , n). Then at any time t ≥ 0, Vt can be expressed as: Tn XTi Vt = Nt E (1.1) Ft , NTi Ti >t

where Nt indicates the numeraire, E is the expectation in the numeraire measure and Ft the usual filtration. More details of the concept of numeraire, pricing measure, and filtration can be found in the literature (see for example Baxter & Rennie [10] for an intuitive description, or Rogers & Williams [93, 94] and Shreve [98] for a more formal approach) and will also be given later in this book (see Appendix B). For our purposes here it is enough to think of the numeraire as being the cash account, used to discount cashflows, and the filtration as the information available at time t . At time t = 0 the distribution degenerates into the current price of the portfolio. We are interested in the distribution of Vt under either the real or the pricing (called also risk-neutral) measure. In general the price distribution Vt will change with time due to changes in market conditions, portfolio composition (for example due to payment of cashflows), and time value. If the portfolio is collateralised, it can be extended to take into account additional positions representing the collateral value. The computation of the price distribution Vt depends also on specific contractual features with the counterparty, e.g. netting agreements between short and long positions in the portfolio, or break clauses held by the counterparties. The industry practice to compute exposure is to use a simple Monte Carlo framework implemented in three steps: (i) scenario generation, (ii) pricing, and (iii) aggregation. The first step involves generating scenarios of the underlying risk factors at future points in time. Simple products can then be priced on each scenario and each time step, therefore generating empirical price distributions. From the price distribution at each time it is then possible to extract convenient statistical quantities. Exposure of portfolios can be computed by consistently pricing different products on the same underlying scenarios and aggregating the results taking into account possible netting and collateral agreement with the counterparty. If taken literally, this approach works only for relatively simple products which can be priced analytically, or which can be approximated in analytical form, and which do not need complex calibrations depending on market scenarios. More exotic products requiring relatively complex pricing, cannot be treated in this way. As already mentioned, even a cancellable swap, which is a relatively simple product, cannot be computed easily in this framework. In the next chapters we will show how (1.1) can be generalised and which algorithms can be implemented to compute exposure for more exotic products. We will also challenge the simple Monte Carlo approach we have just described, and see how more sophisticated modelling frameworks can provide answers to some of the common problems faced when building a counterparty exposure system.

12

1 Introduction

1.4.2 Risk Measures For practical reasons it can be useful to characterise the distribution Vt with some statistical quantities which can then be used for various risk controlling or risk management purposes. The Potential Future Exposure (PFE), computed at time t is defined as PFEα,t = qα,t = inf{x : P(Vt ≤ x) ≥ α},

(1.2)

where α is the given confidence level, and P indicates the probability distribution of Vt . Note that this is a function of time t and is the price of the obligation in the future given a set of scenarios. This pricing is called sometimes Mark-to-Future. The graph of PFEα,t as a function of t is known as the exposure profile of the trade. Similarly the Expected Positive Exposure (EPE) will be computed as9 EPEt = E Vt+ ,

(1.3)

where the expectation can be taken under the real or pricing measure depending on the usage of EPE. An alternative measure to the quantile is the Expected Shortfall, called also Expected Tail Loss, defined as ESα,t = E Vt | Vt > PFEα,t . (1.4) Expected shortfall is used especially when it is convenient to have a measure which takes into account events of significant magnitude, which, however, can occur with only very small probability. As we have shown above, typical examples are credit derivatives, where the default of the reference entity protected by the derivative is a low probability event, which, however has significant impact.

1.4.3 Netting and Aggregation In general, the credit exposure to a particular counterparty arises not from a single transaction but several ones. For any particular market scenario, some of these transactions will have positive, and others negative value. Consider, for example, a long and a short position on an option on highly correlated stocks, a portfolio of payers and receivers swaps10 in different currencies, or, as a more sophisticated example, 9 We will see later in Chap. 12 and Chap. 14 that other definitions of EPE are more appropriate to compute CVA. 10 A

payer (resp. receiver) swap, is a swap that pays (resp. receives) a fixed rate and receives (resp. pays) a floating Libor rate.

1.4 Modelling Counterparty Credit Exposure

13

a long position on ABX11 and a short position on a tranche of pool of MBS. One would expect that, at a given time as one position increases in value, the value of the other position decreases. Since both transactions are facing the same counterparty, it is natural to think about the possibility of netting these positive and negative values together, in order to reduce the overall exposure. The possibility of treating risk in this way will depend on the legal agreement in place. Netting agreements can have different flavours. For example for a given counterparty it could be possible to net together interest-rate swaps, but not swaps with e.g. equity transactions. From the quantitative and computational perspective netting and no-netting agreements will determine how aggregation is performed within a pool of transactions. The main challenge is the requirement of being scenario consistent across trades. This means that the price distributions of all transactions have to be computed together in order to choose the correct risk measure of the whole portfolio together with the correct netting agreements. This can pose significant constraints on the software architecture as well as on the computational capacity. Once counterparty exposure is computed at portfolio level, one can be interested in assigning a portion of the exposure to each single transaction. It is interesting to note that this is not equivalent to computing exposure for each single transaction separately. This process of redistributing exposure is often called exposure allocation or disaggregation and can be performed in different ways leading to different results. We will analyse quantitative aspects of both aggregation and allocation in Part IV where we discuss hedging and managing counterparty risk.

1.4.4 Close-Out Risk Close-out risk refers to the possibility of loss during the time period between when a counterparty is deemed to be in default and when the transaction with that counterparty has been wound down or replaced in the market. The length of this time period, referred to as the close-out period, is typically assumed to be ten business days. In practice it may be shorter for liquid transactions or longer for specialised and bespoke transactions. To mitigate close-out risk, a collateral agreement is often included in the transaction. Under such an agreement, the counterparty would have a commitment to post assets (be they in form of cash or other highly-rated assets) whenever the exposure from the transaction is observed to increase. There are several components that may be specified in a collateral agreement, such as (i) an initial upfront collateral amount called the initial margin, (ii) the threshold exposure above which extra collateral 11 The

ABX Index is a series of credit-default swaps based on 20 MBSs that relate to subprime mortgages.

14

1 Introduction

would need to be posted, (iii) the minimum amount of collateral that may be posted on each collateral exchange date, and (iv) the frequency of the margin calls. The collateral agreement is a legal agreement also referred to as the CSA (Credit Support Annex). Typically the terms of this agreement will depend on the jurisdiction where it applies. In Part IV we will analyse some quantitative and modelling aspects of close-out risk, without addressing all the intricacies of the legal aspects.

1.4.5 Right-Way/Wrong-Way Exposure In all the examples we have analysed previously, we did not consider the quality of the counterparty, assuming in effect that counterparty exposure is equivalent to the future replacement value of the trade at time of counterparty default. In general, however, the level of exposure caused by the trade and the quality of the counterparty are not independent of each other. Information about one would force us to re-evaluate information we have about the other. We refer to such dependence as right-way or wrong-way exposure. The question is how to factor this effect into a credit quantification computation. Typical examples where such considerations are called for are when call or put options are written on the counterparty’s own stock. These are limiting cases with practically no need of accurate modelling. The problem becomes more interesting when the product is complex and the correlation between counterparty quality and level of exposure cannot be clearly determined. Consider for example an energy producer which swaps energy futures for a stream of coupons. In general, increases in energy prices could be beneficial to the company, therefore reducing its probability of default. One way of taking into account the Right Way risk is to measure correlation between energy prices and company credit spread. Another interesting example12 of wrong way risk are negative basis swaps performed with monoline insurance companies. Typically in this case the insurance company receives a premium and pays default protection on missing payments from a pool of mortgages. These swaps are called negative basis as, in normal market conditions, the price for the protection is lower than the value implied by the spread paid by the mortgages.

1.4.6 Credit Valuation Adjustment: CVA Once counterparty exposure has been computed it is necessary to find ways of mitigating it. The simplest way is to compare the portfolio PFE with pre-defined limits and constrain the amount of transacted notional or, as we have seen previously, enter 12 . . . especially

in light of the 2007/2008 credit events. . . .

1.4 Modelling Counterparty Credit Exposure

15

into a collateral agreement. A possible alternative consists in buying credit protection on the counterparty. Its price corresponds to the value of the protection leg of a CDS that pays the exposure amount in case of default of the counterparty. This value is called in the industry credit valuation adjustment, CVA. Intuitively we can see this as follows. Within a pricing framework the value of credit exposure can be seen as the expected value of the positive part of the price distribution weighted by the default probability. Assuming that prices are independent from defaults, we can separate expectations, obtaining that CVA is the value of a CDS with the notional being the EPE profile of the underlying transaction. Suppose for simplicity that the EPE profile is a piece-wise constant function over a time interval (Ti − Ti−1 ). CVA = EPEi (Ti − Ti−1 )D0,Ti si , (1.5) i

where si is the spread corresponding to the time interval Ti − Ti−1 and D0,Ti the discount bond maturing at time Ti . We can see that the CVA corresponds to a portfolio of forward starting CDSs (or equivalently long and short CDS positions) with piecewise constant notional. The availability of CDSs of different maturities will dictate how the EPE profile is discretized. The CVA depends on the level of exposure as well on the credit spread of the counterparty. As counterparty exposure and spread change with time, the amount of credit protection needs to be adjusted accordingly. The process of balancing of exposure with CDSs and other instruments sensitive to market parameters corresponds to dynamically hedging counterparty credit exposure. More details on how to compute and hedge CVA are given in Chap. 14.

1.4.7 A Simple Credit Quantification Example We will discuss in detail in Chap. 9 the computation of credit exposure for equity products. We consider here a very simple example where the form of the exposure profile and the maximum values of the PFE can already be deduced from an approximation. Suppose company A has bought from counterparty B a call option of strike K on a stock S. Our goal is to compute the credit exposure and close-out risk company A is facing. As mentioned in the previous section we need to calculate the price distribution Vt . In the case of the call option, in a simplified context where rates are deterministic, (1.1) becomes (ST − K)+ −r(T −t) (1.6) E (ST − K)+ | Ft , Vt = N t E Ft = e NT where S is the stock price, K the strike, and r the interest rate assumed to be constant. The notional (number of options) has been assumed equal to one. As mentioned previously, Ft is the usual filtration, Nt the numeraire, and the expectation is taken in the measure N.

16

1 Introduction

To solve this equation we need for the stock price S a model, with which simulate the stock value till maturity T . A simple model, which is often used in credit, is the geometric Brownian motion with constant volatility σ , interest rate r and dividend yield d. dS = (r − d)dt + σ dWt , S

(1.7)

where Wt is a standard Brownian motion. As it is well known this stochastic differential equation can be solved analytically. Thus, to compute exposure we need to simulate the stock with (1.7), and then, using the Black and Scholes formula [15] we can price the option at each time step and in each scenario. As the exposure of an equity option is generally monotonic in the underlying and is growing with time, and a vanilla stock option depends only on the current stock value (the product is not path dependent), the max PFE will be in general at maturity T .13 We can compute it at let’s say 97.5% confidence level as (see also Appendix A) √ σ2 (1.8) PFET = S0 e(r−d− 2 )T +1.965σ T − K, assuming it is a positive quantity. The expected exposure can be computed in this simple case as EPEt = V0 ert ,

(1.9)

where V0 is the option premium. We can see this as following. Given that the value of a call option is always positive, we can write (in our simplified set-up), EPEt = E[Vt+ ] = E[Vt ] = E[E[e−r(T −t) (ST − K)+ |Ft ]] = V0 ert .

(1.10)

As for approximating the close-out exposure for a short close-out period, one can use a first-order Taylor approximation. CloseOutt ≈ V0 + Δ(St − S0 ),

(1.11)

where Δ is the first order derivative of the call option price with respect to the stock and St is the value of the stock during the close-out period. This is the close-out risk for the initial period, i.e. for the time between time-zero and t . We will see later in this book that the computation of close-out risk presents subtleties which go far beyond this simple computation.

13 The

exact shape of the PFE curve will depend on the interest rate, dividend curve, and option characteristics.

1.4 Modelling Counterparty Credit Exposure

17

1.4.8 Computing Credit Exposure by Simulation Within a Monte Carlo framework, to compute exposure we could simulate the stock price from today to maturity using (1.7), and then price the option on each path and each time step using Black and Scholes, again with constant rate, dividend, and volatility. As we will see in the next chapters, it is more convenient to simulate martingale processes, for which only the volatility structure is relevant, while the drift (and thus the dividends) does not need to be specified (for a definition of martingale see Appendix B and for more details see for example the books by Baxter & Rennie [10], Rogers & Williams [93, 94] and Shreve [98]). In practice a convenient quantity we can simulate are forward prices. By considering our example in these terms, we can write (1.6) as (FT ,T − K)+ Vt = Nt E (1.12) Ft , NT where Ft,T is the t-value of the T -forward. The link with the notation in the previous section is, Ft,T = St e(r−d)(T −t) .

(1.13)

As before, assume for simplicity that the numeraire N is independent from the stock price, and impose also the simple specification dFt,T = F0,T σ dWt ,

(1.14)

with the volatility being a constant σ > 0 and with W being a standard N-Brownian Motion. This is the quantity we simulate in t. The paths generated by integrating this SDE are our scenarios which we show in Fig. 1.5 in a stylised representation. Note that (1.14) can be integrated analytically at each time step, thus avoiding discretisation errors, σ2 Ft,T = F0,T exp − t + σ Wt . (1.15) 2 We can then price at each scenario and each time step the stock option using again Black and Scholes, expressed in terms of the forward price Ft,T . PFE can be computed analytically at maturity, where the option price is given by the stock price minus the strike.

+ √ 1 PFET = F0,T exp σ T q˜α,T − σ 2 T − K , (1.16) 2 where q˜α = Φ −1 (α) is the α-quantile of the standard normal distribution. This is the equivalent of (1.8) generalised for any quantile. We have also floored at zero the exposure, as in some cases one is interested only at the amount the counterparty should pay. Negative exposure represents the amount we owe to the counterparty.

18

1 Introduction

Fig. 1.5 Computing exposure by Monte Carlo simulation. The paths on the left panel represent stock prices. At each scenario and each time step, the price of the option is computed using the analytical Black and Scholes formula. Resulting prices are represented on the right panel. From the price distributions generated in this way at each time step, various statistical quantities (e.g. PFE and EPE) can be extracted. The bigger circles indicate a mean

1.4.9 Implementation Challenges The Monte Carlo framework we have shown in the previous section, seems to give a good implementation recipe. For a given portfolio of transactions we could (i) identify the underlying risk factors and simulate forward (or spot) prices, taking into account correlations if required, (ii) use functions already implemented to price each product, and then (iii) derive statistical quantities. As we have mentioned already, this could be the approach followed by a financial institution to assess the counterparty credit risk of its OTC derivatives portfolios. In the implementation phase, however, there can be issues which need to be addressed. (i) The generation of correlated scenarios is not trivial, as there can be thousands of different risk factors driving the dynamics of products in the portfolio. Consider for example an equity portfolio, where each underlying stock needs, at least in principle, a specific simulation. (ii) The scenarios have to be consistent across systems to build a counterparty view. This is a requirement which is much more stringent than what is generally specified in the design of a Front Office system used for pricing or a Risk system used to monitor the Profit and Loss (P&L) of a bank. Basically what we need here is the same underlying models, or the same family of models, for all types of products. In fact, even if the correlation between asset classes can be in some cases ignored (e.g. equity could be considered not correlated

1.4 Modelling Counterparty Credit Exposure

19

with interest rate), still all these models need to be expressed using the same numeraire (the discount factors in equity have to be consistent with the discount factors used in FX or rates). This consistency can be difficult to achieve, as often large financial companies have different systems to book and value, for example, interest-rate, equity, or FX products. (iii) Pricing functions developed in various libraries are not necessarily designed to be integrated in a counterparty exposure framework. This has implications from both a software and architecture, as well as from a methodological point of view. Consider for example path dependent products. Counterparty exposure depends on the whole scenario history, which could be in different formats across different pricing systems. (iv) Not all products can be computed in analytical form. Most exotics are priced on grids using PDEs or using Monte Carlo approaches. In these cases the exposure computation would require a Monte Carlo simulation for scenarios and a Monte Carlo simulation, or a PDE computation, for each scenario and time step to price the instrument. This becomes quickly unfeasible from a computational point of view. In addition, depending on the model used for pricing, calibration could also become problematic, as it has to be performed at each scenario. In practice credit systems based on the classical Monte Carlo scheme approximate products using a simplified representation. While these approximations could have their justification in a risk environment, they are difficult to use when counterparty risk has to be priced and hedged.

1.4.10 An Alternative Approach: The AMC Algorithm The points highlighted in the previous section clearly show that the classical Monte Carlo scheme has intrinsic limitations and that we need an alternative approach. As we will see at length in the rest of the book, there are possibilities to circumvent in a systematic way some of the problems related to valuation and architecture. The basic idea is to approach the counterparty exposure problem as a pricing problem, and thus to use pricing algorithms, which generate not just the value of a trade at inception, but rather a price distribution at predetermined time steps. One possibility is to use the so called American Monte Carlo algorithm, which we will refer to as, simply, the AMC algorithm. The main feature of this algorithm is that, instead of building a price moving forward in time, it starts from maturity, where the value of the transaction is known, and goes backwards, till the inception. In general AMC is used for pricing products with callability, i.e. products whose values depend on a strategy which can be determined by only knowing future states of the world. From a counterparty exposure perspective, the benefit of this approach is that, not only a price at time-zero is provided, but also the price distribution at each time step. In addition, the algorithm is generic, in the sense that using simply a payoff description, we can obtain the information needed to compute counterparty

20

1 Introduction

Fig. 1.6 A simplified graphical representation of the AMC algorithm. In the left hand panel we show the scenarios generated according to some underlying model. At maturity the payoff of the trade is known. To estimate the value at intermediate scenarios we need to proceed with a backward induction step

exposure. This suggests the possibility of having a generic trade representation and thus the possibility of having a modular software architecture that incorporates trade descriptions without explicit knowledge of each type of product. The challenge is that we need to develop an underlying model capable of pricing a hybrid product, consisting potentially of a large portfolio of transactions. This hybrid model will need to take into account all stochastic drivers of the portfolio in a consistent, arbitrage free way. It is natural to ask what is the performance of the AMC algorithm for vanilla products. We will see that by a careful implementation the prices computed via AMC are very close to those computed using for example closed form formulae.

1.5 Which Architecture? Building a system that computes credit valuation adjustments and counterparty exposure for the book of a large financial firm is a very challenging task, not only from the modelling and algorithmic perspective, but also from the technical and IT point of view. One of the problems is that often in large institutions such as Investment Banks, products are not booked on one system. They are in general recorded on several systems, which do not necessarily communicate between each other. To overcome this situation we suggest developing not only a common modelling platform, but also a programming language, which allows the representation of different types of products. As we will see later in this book, we have called our language

1.6 What Next?

21

Fig. 1.7 High level architecture description

PAL, Portfolio Aggregation Language, to highlight the fact that we need to aggregate trades at counterparty (or at netting pool) level. Once we have this common booking language, we can translate bookings made in other systems into PAL, bridging the difference between these systems. In the figure below we show how the system architecture could be implemented.

1.6 What Next? We have introduced all basic concepts needed to understand counterparty credit exposure. We have now to analyse in detail the steps necessary to build a system designed to compute and hedge counterparty risk for large portfolios of exotic transactions. This is what we do in the rest of this book. We start from a generic modelling

22

1 Introduction

and simulation framework based on American Monte Carlo techniques, and then we present a software architecture, which, with its modular design, allows the computation of credit exposure in a portfolio-aggregated and scenario-consistent way.

Chapter 2

Modelling Framework

Our goal is to define a general framework which can be used to compute counterparty credit exposure for all types of transactions. As highlighted in the Introduction, computing counterparty exposure consists of computing distributions of prices at future times. For simple products this can be achieved by scenario simulation, followed by pricing on each scenario, at each time step. However, in the case where no analytical form is known for the price of the product, this approach is not practical and a different approach is required. The framework we define is intended to cater for the need to estimate price distributions in a consistent way, not just for a single product, but for complex portfolios of products admitting no closed-form pricing. Price distributions will be obtained through simulating underlying processes specified in a generic way, and this chapter is dedicated to defining the problem at hand and describing the various processes to be simulated. There are several ways to model the stochastic processes driving the price of derivatives. We start from a generic representation and show how to adapt it to cope with practical constraints. By making certain independence assumptions, we then analyse how to modify the model so that new asset classes can be introduced in a modular way that maintains scenario consistency across products but does not require the re-evaluation of models already in place. Such assumptions have consequences for pricing, and these are assessed in Chap. 6. After having prescribed the theoretical framework, the next chapters will be focused on implementation. In Chap. 3 we will specify simulation models. For vanilla trades, which can be priced analytically, this will be the main step to compute credit exposure, as there is a direct mapping between scenarios of the underlying risk factors and price distribution. In Chap. 4 we will analyse valuation techniques that can be applied when this mapping is not obvious, as, for example, with products having callability features and for which no closed-form valuation is available.

2.1 Counterparty Credit Exposure Definition Consider a financial product, which we denote generically by P , and denote by X ≡ (Xt )0≤t≤TX the cashflows which the holder of the product is entitled to. In G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0_2, © Springer-Verlag Berlin Heidelberg 2009

23

24

2 Modelling Framework

general, P might entitle the holder to not just the cashflow X, but also the option to replace, at predetermined points in time, the cashflow X with an alternative product, Q say. Associated with the product Q are cashflows Y . We also generally assume in the following that the option allows exercise only once. Now let τE∗ denote the optimal (stochastic) time at which the option should be exercised to replace the cashflows X with the product Q. The basic problem at hand is to compute at all times t the value of the product P whose payoff may depend on decisions made after time t . The fair value for product P should be that attainable by employing the optimal among all possible exercise strategies τE . Solving the pricing problem entails solving for the best possible exercise strategy. We now formalise what we have said above with a definition, and introduce the required notation that will serve us in Chap. 4 when we describe valuation techniques in detail. First, denote by T the set T := {τ1 , τ2 , . . . , τnE } ∪ {∞}

(2.1)

of points where the holder of the option is allowed to exercise from P into Q. Even if the optionality is of American type and exercise is allowed at any t within a time interval, any attempt at numerical solution would need to approximate the problem with one where exercise is allowed at a discrete set of points. The point at infinity is included in T , as this corresponds to the possibility of the holder never deeming it optimal to exercise into Q. Q Write V Q ≡ (Vt )t≥0 for the value process1 of the product Q, and let N denote our chosen numeraire process. At any time t before exercise, the holder of the product P chooses the optimal exercise time τE∗ ∈ T so as to attain, VtP = Nt sup

τE ∈Tt

E t

τE ∧TX

Q Xu VτE du Ft + E Nu NτE

Ft

(t < τE∗ ),

(2.2)

where Ft is the usual filtration at time t , Tt = {τ ∈ T | τ ≥ t} is a subset of T , and where the expectation is taken in the pricing measure relating to the numeraire N . For vanilla products without callability features, we have T = {∞}, so that the question of optimising the time of exercise does not arise. Here on the left hand side of (2.2), VtP is the time-t value of P conditional on exercise not having happened prior to t . This is the pre-exercise value of product P . On the right-hand side, the two terms, τE ∧TX Xu no Πt := Nt E du Ft Nu t (2.3) Q VτE Q Vt = Nt E Ft , NτE we work with processes defined on a filtered probability space (Ω, F , (Ft )t≥0 ). More details are given in Appendix B.

1 Throughout

2.1 Counterparty Credit Exposure Definition

25

represent, respectively, the time-t value of the non-exercise portfolio containing the X cashflows,2 and the time-t value of entering into the exercise portfolio containing the Y cashflows at time τE . Q Upon exercise at τE∗ , the product will have value Vτ ∗ . The credit exposure at E times t > τE∗ will depend on the way in which the option inherent in P may be exercised. We will discuss in Chap. 4 several types of exercise that are possible. Here it suffices to point out that exercising into Q might give rise to credit exposure for the lifetime of the product Q, which may exceed the maturity TX of the preexercise flows X. Putting everything together, we may write the price of product P as

Vt =

VtP , Q

Vt ,

t < τE∗ t ≥ τE∗ .

(2.4)

In a complete market, there will be unique prices for any derivative that can be replicated. In particular, in a simulation framework, the price calculated will not depend on the numeraire (and therefore the numeraire measure) chosen for the simulation. However, the distribution of prices that drives the credit exposures will depend on the simulated processes, which in turn depend on the measure chosen for the simulation. Thus, for instance, there could be good reasons for generating scenarios in the physical measure even if pricing is accomplished by taking expectations in the numeraire measure corresponding to numeraire N . We will analyse this point in more detail in Part IV, where we discuss Hedging and Managing Counterparty Risk. From (2.2), it is apparent that the elements required to simulate values of the distribution Vt are (i) Simulations of the numeraire process N . (ii) The ability to simulate the cashflows X provided by P , and similarly the cashflows offered by Q. Since all these cashflows are usually stochastic, we need to specify a simulation model tailored to the particular products P and Q. This is what we deal with in Chap. 3. (iii) Estimation of the price of the no-exercise portfolio Π no and that of the product Q for each point in time at which we are interested in having a simulated value available. Similarly, we need to account for the sup operator numerically within the simulation. We will consider this aspect in Chap. 4. The rest of this chapter is dedicated to describing our general approach to modelling the dynamics of the numeraire and the cashflows of the generic products P and Q mentioned in (i) and (ii) above. that X in general can be decomposed as Xt = Xtc + δtj xj , where X c is a continuous process, xj is a cashflow at time tj and δt is the Dirac delta function. 2 Note

26

2 Modelling Framework

2.2 Process Dynamics Most approaches to simulating the financial quantities required for credit exposure estimation choose to model stochastic variables that are directly observable, e.g. Libor rates, swap rates, stock prices, foreign exchange rates. While specific payoffs can usually suggest which observable quantity it is more sensible to simulate, there is the inherent drawback that a new payoff may require simulation of new quantities, which the framework in place had not been designed to handle. We take an alternative approach, for which (i) Extraction of all stochastic quantities of interest (for all possible payoffs) is done from the same basic set of martingale3 processes. This set does not need to be augmented in order to tackle a new payoff type. (ii) Time-zero term structures are reproduced exactly by construction. The specification of a new model is therefore reduced to specifying the volatility structure. Such specification must allow for fast and accurate calibration to vanilla option market prices. The basic idea is as follows. Suppose we have a contingent claim XT paying off at time T . The risk-neutral price at t < T of XT is given by4

p0 = EN (NT /N0 )−1 XT , (2.5) the expectation being in a measure that makes martingales out of (non-dividendbearing) tradable assets when expressed in units of the numeraire N . We can equivalently use the T -bond as numeraire, so that T p0 = EN (DT ,T /D0,T )−1 XT = D0,T EN [XT ] T

= D0,T EN [MT ,T XT ],

(2.6)

where the expectation is now in a measure where prices of (non-dividend bearing) assets expressed in units of the T -bond are martingales. The usefulness of this second representation is that by modelling the change-of-measure process5 dNT Dt,T /D0,T Mt,T ≡ = , (2.7) dN Ft Nt /N0 3 A martingale is a stochastic process whose expected future value, conditional on present information, is its current value. In mathematical terms this can be expressed as E[Mt | Fs ] = Ms , for all s ≤ t (see Appendix B for definitions and, for example, Williams [106], or Shreve [98] for a detailed treatment of martingale theory). 4 For clarity we explicitly indicate the measure in which expectation is taken; in the rest of the book we will ignore the superscript N when expectation is taken in the numeraire measure N. 5 See

Appendix B for more details.

2.3 Interest Rate: Single Currency

27

the time-zero price of XT is a multiple of time-zero observed bond prices. That is to say, the yield curve at time zero is replicated by construction. Comparing (2.5) and (2.6), we see moreover specifying the martingale M·,T forces the numeraire to take the form NT−1 = D0,T MT ,T .

(2.8)

At a general level, our approach is similar to that followed in Constantinides [28], Flesaker [45], and Rogers [90]. These papers model directly (in some fixed reference measure P∗ ) the state-price density process ζ , which has the property that for any non-dividend-bearing asset with price process S, ζt St is a P∗ -martingale.

(2.9)

If the numeraire process is increasing, then ζ is a positive supermartingale. It is this property that Flesaker in [45] and Rogers in [90] exploit to come up with a surprising variety of interest-rate models, essentially writing ζt = e−αt f (Xt )/f (X0 )

(2.10)

for a given Markov process X and inspired choices of the function f . We choose to work in the reference measure P∗ = N and to model the numeraire directly. The recipe for modelling assets other than the chosen reference currency (foreign currencies, equity and inflation) is similar and will be elaborated on further below. Conceptually, there will be two new sources of randomness introduced with each new asset class. The first of these is the output derived from owning one unit of the asset (bond yield volatility in the case of a currency, and stock dividend volatility in the case of equity), and is modelled by introducing for each asset i a martingale (i) process M·,T , analogous to (2.7) above. The second source of randomness is the fluctuation of the asset price itself (that is to say, the exchange rate process in the case of a currency foreign to the reference one, and the stock price process in the case of equity) when expressed in some existing unit of value; this will be modelled through a second process, which we typically denote Y (i) for the i’th asset. Credit derivatives present different challenges; nevertheless, the idea there will still be to find the relevant martingale and to directly specify dynamics for it in our chosen reference measure.

2.3 Interest Rate: Single Currency We start by considering the modelling of the reference currency itself. All the ingredients for this have been given by way of motivation in Sect. 2.2, namely the change-of-measure martingales (2.7) Mt,T =

Dt,T /D0,T Nt /N0

(t ≤ T )

(2.11)

28

2 Modelling Framework

which in fact can also be interpreted as the price process of the T -bond, expressed in units of the numeraire N (for t > T we assume Mt,T = MT ,T ). We will take Mt,T as the fundamental quantity to be simulated and from which the other interest rates-related stochastic quantities we need can be extracted. Table 2.1 Single currency interest-rate product notation Dt,T

T-bond in the local (domestic) currency

Nt

Numeraire in the local currency

Mt,T

Martingale used to simulate the T -bond in the local measure

N

Measure in which prices of non-dividend-bearing assets expressed in units of N are martingales

NT

Measure in which prices of non-dividend-bearing assets expressed in units of the T -bond are martingales

While, for each t, Nt depends only on the “diagonal” values Mt,t of the martingale M·,t , it is clear that simulated values of Mt,T will be required for all t ∈ [0, T ], and for each T ≥ 0. Indeed, from (2.11) it follows that D0,t Mt,T = Dt,T , Mt,t D0,T

(2.12)

which means that the time-t price of a T -bond is Dt,T =

D0,T Mt,T . D0,t Mt,t

(2.13)

Other quantities of interest, such as lending rates, can similarly be written in terms of the martingales Mt,T . All that is left now is to choose an SDE for the strictly positive martingale M. Consider, for example, for each T ≥ 0 the SDE dMt,T = Mt,T σt,T dWt,T ,

(2.14)

where W·,T is a Brownian Motion process and σt,T is a volatility term. For deterministic σ , (2.14) can be written in integral form as t 1 t 2 σu,T dWu,T − σu,T du , (2.15) Mt,T = M0,T exp 2 0 0 where

2 tΣt,T

:= 0

t

2 σu,T du

(2.16)

is the total variance of ln Mt,T . The framework described by (2.15) gives the simulation recipe: first choose a volatility structure σ , then simulate the martingale Mt,T and finally derive the

2.3 Interest Rate: Single Currency

29

stochastic quantities which are needed to estimate price distributions of the relevant product. For example, the numeraire can be written in integral form (for deterministic volatilities) as Nt −1 = D0,t Mt,t

t 1 2 = D0,t M0,t exp σu,t dWu,t − Σt,t t . 2 0

(2.17)

Similarly, the Libor rate of interest Lt,[T1 ,T2 ] observed at t for the period [T1 , T2 ], is (with α ≡ T2 − T1 ) −1 Dt,T1 Lt,[T1 ,T2 ] = α −1 Dt,T2 −1 D0,T1 Mt,T1 −1 =α D0,T2 Mt,T2 M t,T1 − α −1 , (2.18) = L0,[T1 ,T2 ] + α −1 Mt,T2 or, in other words, Lt,[T1 ,T2 ] + α −1 Mt,T2 = L0,[T1 ,T2 ] + α −1 Mt,T1

(2.19)

showing that simulation of the martingale process and the knowledge of the initial yield curve allows simulation of Libor rates.

2.3.1 Simple Specifications We have chosen our simulation approach to best serve our goal of computing exposure for all types of products across all asset classes in a consistent way. More familiar models from the literature usually take a different approach, and it is instructive to see how these models can be represented within our framework. This is what we do in this section. The first practical problem we need to tackle is of course that of dimensionality. In general, the cashflows that need to be simulated for a particular product will depend on values of the martingale Mt,T for arbitrary values of T . For example, we see from (2.18) that the Libor rate for two different periods will depend on two different martingales. This means that all martingales for all values of T are needed if one is to ensure that cashflows of any product can be extracted. In practice, of course, only finitely many stochastic drivers can be simulated, so one needs to find ways of projecting the richness provided by the infinite family of martingales M·,T onto a finite set of Brownian Motions which can be simulated. The richness retained by such a dimensional reduction will depend on how many different Brownian Motion processes one chooses. In particular, one can force the

30

2 Modelling Framework

martingales M·,T to all depend on the same Brownian Motion, as would be the case in any one-factor short rate model. In this case (2.14) becomes, dMt,T = Mt,T σt,T dWt ,

0 ≤ t ≤ T.

(2.20)

Note that the Mt,T depends on t but also, thanks to σt,T , on T . In fact, if σ (and therefore M) were to not depend on T , then bond prices in the model would be deterministic, because the bond price Dt,T =

D0,T Mt,T D0,T = , D0,t Mt,t D0,t

(2.21)

would be determined at time zero. Similarly, for the forward rate one would get f (t, T ) = −

D˙ 0,T ∂ log Dt,T =− , ∂T D0,T

(2.22)

where D˙ represents derivative with respect to maturity of the bond. From the point of view of implementation, a separable specification of the volatility term σt,T in (2.20) turns out to be very useful. In detail, write σt,T = ft gT

(2.23)

for positive deterministic functions f and g. The integral form of Mt,T then becomes, t 1 t 2 Mt,T = M0,T exp σu,T dWu,T − σu,T du 2 0 0 1 2 = M0,T exp gT Xt − Σt,T t , (2.24) 2 where

Xt :=

t

fu dWu 0

2 Σt,T

:= t

−1

0

t

(2.25) fu2 gT2 du ≡ Ft2 gT2 .

This specification is simple to extend to a multi-factor setting as follows. Starting from an Rn -Brownian Motion W, we write6 dMt,T = Mt,T gT · (ft RdWt ), 6 Throughout

(2.26)

this work we use boldface to indicate vectors and matrices. The dot (scalar) product between two vectors a and b is indicated by a · b, and the row by column product between two matrices (or a matrix and a vector) X and Y, by XY. The transpose of matrix X is XT . We indicate with X(i) (or Xi ) the i-th column (vector) of matrix X, and with aj the j -th element (scalar) of vector a. Xi,j is the i, j element of matrix X.

2.3 Interest Rate: Single Currency

31

where now (1)

(n)

ft = Diag(ft , . . . , ft )

(2.27)

is a diagonal matrix with the ft(i) on the diagonal, (1)

(n)

gT = (gT , . . . , gT )

(2.28)

(i)

is the vector of the gT , and R is such that RT R is a positive semi-definite matrix making the i’th component of RW a standard Brownian Motion, which then gets time-changed by ft(i) . We will see in Chap. 3, for instance, that the Hull-White model is a special case of a parametrization of this type. The impact of separability is that simulation of a small number of Brownian Motions t (j ) (i) fu(i) Ri,j dWu (2.29) X = j

0

(i) together with parametrization of the volatility function Σt,T = Ft(i) gT(i) , allows simulation of Mt,T for all T . Without a separable specification for σ , the integral t 0 σu,T dWu would need to be computed on the fly, or stored, for each maturity T . Because of its simplicity, we will employ the separable volatility specification in most of what follows in this book.

2.3.2 HJM Framework The well-known HJM framework [60] can also be accommodated within our framework, as we will see in the following. In the standard way of specifying an HJM model with d > 0 factors, one takes the forward rates ft,T defined via (2.22) and writes down the SDE dft,T = αt,T dt + st,T · dWt ,

(2.30)

where W is an Rd -Brownian Motion and st,T ∈ Rd is a vector specifying the instantaneous volatility at time t of the T -forward rate. Given that, in our framework, we model directly the bond price Dt,T , it is natural to compare the bond price dynamics in our framework to those implied by the HJM dynamics (2.30). Indeed, using Itô calculus and (2.30), we can see that in the HJM parametrization the discounted log-bond price satisfies T αt,u du dt, (2.31) d[log Dt,T Nt−1 ] = St,T · dWt − t

with

St,T = −

T

st,u du. t

(2.32)

32

2 Modelling Framework

The drift term α in (2.30) cannot be chosen without restriction, as doing so might introduce arbitrage between bonds of different maturities. Arbitrage will be excluded, however, if there exists a process γ ≡ (γ t )t≥0 in Rd such that (2.33) αt,T = st,T · γ t − St,T , as this will guarantee that bond prices discounted by the numeraire are martingales. Notice the key fact that the process γ should make (2.33) true for all T simultaneously. In our framework, the drift condition (2.33) is in fact satisfied automatically. With hindsight this should be expected, since our model laid down at the outset that the discounted bond prices are martingales. To see this, recall that Dt,T =

D0,T Mt,T . D0,t Mt,t

(2.34)

Hence, d[ln Dt,T Nt−1 ] = d[ln Mt,T ], and 1 d[log Dt,T Nt−1 ] = σ t,T · dWt − σ t,T · σ t,T dt, 2

(2.35)

showing that the volatilities of forward rates within our framework satisfy st,T = −

∂ σ t,T . ∂T

(2.36)

2.3.3 Libor Market Models Libor Market Models, also referred to as BGM models, choose to model Libor rates directly, Lt,[T1 ,T2 ] (for the original description of the model see [17]; for a comprehensive Libor market model analysis see for example Rebonato [89]). In essence, BGM models require a discretization of the set {Ti } of Libor fixing dates. Recall that, at time t < Ti−1 , the Libor rate for time period [Ti−1 , Ti ] is defined by −1 Lt,[Ti−1 ,Ti ] = αi−1 Dt,Ti−1 − Dt,Ti Dt,T , (2.37) i where αi = Ti −Ti−1 . Since the term in brackets is the difference of two bond prices, Lt,[Ti−1 ,Ti ] Dt,Ti

(2.38)

is a tradable process. Consequently, the process Lt,[Ti−1 ,Ti ] will be a martingale under NTi , which is the measure specified by taking the Ti -bond as numeraire. This is the cornerstone of the Libor Market Models, which allows the application of the Black formula for pricing caplets by modelling Libor rates as log-normal stochastic processes.

2.4 Multiple Currencies and Foreign Exchange

33

In our framework, the SDE satisfied by the Libor rate is cumbersome because our starting point is martingale modelling of the bond prices. Nevertheless, the SDE can be written down, as we do now. Let us consider, instead the shifted Libor rate L¯ t,[Ti−1 ,Ti ] := Lt,[Ti−1 ,Ti ] + αi−1 .

(2.39)

From (2.19), we can express Mt,Ti in terms of the Libor rates as Mt,Ti =

i ¯j i ¯j L0 L0 M = . t,T0 j ¯ ¯j j =1 Lt j =1 Lt

(2.40)

Therefore, if we are to model directly Libor rates as log-normally distributed satisfying the SDE dLit = Lit μit dt + σti dWti , (2.41) then we need to derive the drift μit of each rate under N. To this end, we apply Ito’s formula to (2.40) to obtain the finite-variation part of Mt,Ti as i i i ∂ 2M i j ∂M j j j j t t dMti L Lk σ σ k dWt dWtk + μ L dt j t t ¯ jt ∂ L¯ kt t t t t ∂ L ∂ L¯ t j =1 k=1 i i j k L L j j t t Mti σt σtk ρi,j − μt dt, j L¯ kt L¯ t j =1

(2.42)

k=1

where signifies that the two sides differ by a martingale term. Since M i is an Nmartingale, the drift μit of the Libor rate Lt,[Ti−1 ,Ti ] under N must therefore satisfy μit = σti

i j Lt

¯j j =1 Lt

j

ρi,j σt .

(2.43)

While the BGM approach is appealing because it allows easy calibration to caplet prices, its application to the purpose of exposure computation is hindered by the fact that not all Libor rates are directly simulated, and therefore interpolation across tenors is required. More importantly, the simulated Libor rates are not directly obtainable from a common stochastic process, making any simulation computationally intensive.

2.4 Multiple Currencies and Foreign Exchange We now turn to modelling of interest rates in a currency other than the reference one, leading naturally to modelling the exchange rate between the two currencies.

34

2 Modelling Framework

Table 2.2 Cross currency interest-rate products notation Dt,T D˜ t,T

T-bond in the local (domestic) currency

Nt N˜ t

Numeraire in the local currency

Mt,T t,T M

Martingale used to simulate T-bonds in the Nt (local) measure t (foreign) measure Martingale used to simulate T-bonds in the N

χt

Spot FX rate from foreign to reference currency

Yt

˜ Change of measure from the reference measure N to the foreign measure N

T-bond in the foreign currency Numeraire in the foreign currency

The basic tradable asset in the reference currency remains the bond, and we have modelled this in the previous section through the N-martingale M: Mt,T =

Dt,T /D0,T . Nt /N0

(2.44)

Whatever holds for the reference bond in the reference currency holds for the foreign one in the foreign currency, so in analogous manner we define another process (in terms of the foreign bond and foreign numeraire) D˜ t,T /D˜ 0,T , M˜ t,T = N˜ t /N˜ 0

(2.45)

˜ ˜ being the measure that makes martingales out of N which is now an N-martingale, all foreign tradable assets expressed in units of foreign numeraire. Now, some relation is going to have to hold between the change-of-measure induced by M and M˜ and the exchange rate linking the foreign and reference currencies. Indeed, if χ ≡ (χt )t≥0 is the FX process representing the value in reference currency of one foreign currency unit, then it is a standard no-arbitrage argument that the process Y ≡ (Yt )t≥0 defined by ˜ χt N˜ t dN = Yt = χ0 Nt dN Ft

(2.46)

is the Radon-Nikodym derivative7 for changing from the measure N to the mea˜ By this, Yt is an N-martingale. sure N. Inserting Nt = D0,t Mt,t and N˜ t = D˜ 0,t M˜ t,t in (2.46), we get Yt =

7 See

Appendix B.

χt D0,t Mt,t . χ0 D˜ 0,t M˜ t,t

(2.47)

2.4 Multiple Currencies and Foreign Exchange

35

It turns out to be more convenient to express Y in terms of the basic FX forwards that one would observe in the market. To this end, define F¯t,T to be χt D˜ t,T /Dt,T , F¯t,T := Ft,T /F0,T = χ0 D˜ 0,T /D0,T

(2.48)

the time-t FX forward normalised by its time-zero value. In terms of this, Y has the simple representation ˜ t,T /χ0 D˜ 0,T D χ t Yt M˜ t,T = F¯t,T Mt,T = (2.49) Nt /N0 which is in fact an identity involving measure changes, namely T ˜ dN ˜T ˜T ˜T dN = d N dN ≡ d N , ˜ Ft dNT Ft dN Ft dN Ft d N dN Ft

(2.50)

corresponding to changing from the reference numeraire measure to the foreign T forward measure. It will prove useful to read off from (2.49) the facts that Yt M˜ t,T ,

F¯t,T Mt,T ,

Yt ,

and Mt,T

(2.51)

are all N-martingales. Intuitively, in (2.49) the volatility structure for F¯ will relate to that of foreign exchange options,8 while the volatility of M and M˜ arises from stochasticity of interest rates. The only component in (2.49) that is free to model is Y , and one would then obtain the FX process from this as χt = F0,t F¯t,t = F0,t Yt M˜ t,t /Mt,t .

(2.52)

The dynamics of Y have therefore to be chosen in such a way that market-observed FX option prices can be reproduced, given the dynamics of M and M˜ which would have already been calibrated to their respective interest-rate markets. In particular, the volatility structure for Y will depend on the volatilities and covariances of M, M˜ and F¯ . In practice, doing this becomes very tedious especially when a large number of currencies are involved, so it is useful to look at a less exact but simpler approach. 8 The volatility for the FX forwards F¯ would be implied from market-observed option prices so that the price of an FX call struck at K, of maturity T , is

C(K, T ) = EN NT−1 (χT − K)+

˜ T [χ −1 ≤ K −1 ] − D0,T KNT [χT ≥ K], = D0,T F0,T N T where we recall F0,T D0,T = χ0 D˜ 0,T and where we have used the identities (2.50) to switch from one measure to another. Note in the last equality that F¯ (resp. F¯ −1 ) is a martingale in the forward ˜ T ). measure NT (resp. N

36

2 Modelling Framework

We go about this by modelling F¯ (and therefore χ ) as if it were independent of the ˜ In effect, we mimic (2.49) by interest-rate martingales M and M. χt D˜ t,T /χ0 D˜ 0,T Yt M˜ t,T = ≈ Fˆt,T Mt,T Nt /N0

(2.53)

with Fˆ independent of M and M˜ but having the same marginal distributions as F¯ . The reason this helps is that the market prices for FX options tell us directly what the volatility structure for F¯ must be. The immediate consequence of this independence assumption9 is that Fˆt,T is an N-martingale. The cost of this assumption is of course that while M F¯ /M˜ and M F¯ are Nmartingales, M Fˆ /M˜ and M Fˆ are not; to mitigate this we impose a drift correction on M˜ t,T so as to ensure at least that their expected values stay constant (and equal to one), that is E Mt,T Fˆt,T /M˜ t,T = 1 = E Mt,T F¯t,T /M˜ t,T = E(Yt ) ˜ χt Dt,T /χ0 D˜ 0,T . E Mt,T Fˆt,T = 1 = E Nt /N0

(2.54)

˜ it can be seen that Using the independence between Fˆ and the pair (M, M), −1 ˜ E M˜ t,T = 1 − Cov Mt,t M˜ t,t , Mt,T .

(2.55)

We re-iterate here that the above expectations are taken on time-zero information, that is, in the filtration F0 . Thus, the drift correction that we derive for M˜ is the expected drift seen at time zero. Attempting similar calculations for arbitrary Ft fail because the processes Fˆ M and Fˆ M/M˜ are not bona-fide martingales. This arises as a direct consequence of the independence assumption for Fˆ ; this independence is of course inconsistent with the fact that the conditional expectations of the FX forwards do depend on observed bond prices. In practice, the input to the model is the correlation between the drivers of M and M˜ in the N-measure, and in concrete examples, the covariances above need to be expressed in terms of this correlation. In particular, when M and M˜ are modelled with deterministic volatilities (see (2.15)), the condition (2.55) is expressed by saying that the Brownian Motion X˜ t driving the martingale M˜ t,T (see (2.24)) has, in the N-measure, the drift αt,T := σ˜ t,t − ρσt,t ≡ α(t),

(2.56)

9 . . . it also implies that while rates in different currencies can have co-dependence, all other asset classes are independent of interest rates. . . .

2.5 Inflation

37

where ρ is the instantaneous correlation between the Brownians X˜ and X that drive ˜ respectively. Note that (2.56) is a function only of t, so the martingale M and M, that the Brownian Motion X˜ t looks like t α(u)du, (2.57) Bt + 0

in terms of an N-Brownian Motion B having correlation ρ with X.

2.5 Inflation The basic inflation product, an inflation-linked bond, is designed to preserve the purchasing power of money in some given currency. Modelling of inflation products is generally similar to modelling of foreign exchange since for each currency, one can consider the nominal and real rates to be like a conventional (local and foreign, respectively) currency pair. Inflation yield curves (arising from inflation-linked bonds) and volatility term structures (deduced from prices of inflation-linked options) serve to calibrate the inflation model, in the same way that foreign currency yield curves and FX options allow calibration of foreign exchange models. In concrete terms, we let the exchange rate χ represent the inflation index denominated in the reference currency, with M˜ being calibrated to the volatility of real rates. For example, if the reference currency is GBP, the UK RPI inflation rate (denominated in GBP) would be modelled as some process χ , as if the RPI index were a foreign currency.

2.6 Equity Equity can be tackled within our framework by treating it as an asset foreign to the reference currency, similar to what we do for foreign currencies. In this way, one (i) lets the ‘exchange rate’ χ represent the value in its denomination currency of one unit of stock and (ii) uses M˜ to control the volatility of the stock dividend yields. In more detail, recall the identity (2.49) χt D˜ t,T /χ0 D˜ 0,T Yt M˜ t,T = F¯t,T Mt,T = . Nt /N0

(2.58)

The third term here represents the tradable asset for the foreign currency, namely the price of the foreign bond expressed in the reference currency. Now consider a stock denominated in a foreign currency. The stock forward price, St,T , discounted by the foreign bond and expressed in units of the foreign numeraire, is the relevant ˜ traded asset for the stock and has the representation as the N-martingale St,T D˜ t,T /S0,T D˜ 0,T (S) (S) Y˜t Mt,T := S¯t,T M˜ t,T = , N˜ t /N˜ 0

(2.59)

38

2 Modelling Framework

¯ being the stock forwards (respectively, forwards normalised with S (respectively, S) by their initial values), denominated in their own currency, and with Y˜ (S) and M˜ ˜ being N-martingales. The martingale M (S) will control the volatility of the stock dividends, so the element in (2.59) that is free to model is now Y˜ (S) , exactly as what happens for Y ˜ when modelling the FX rate. What remains is to ask what the N-martingale Y˜ (S) looks like in N. (S) is an N-martingale, ˜ ˜ But because Yt = d N/dN| we have that Y Y˜ (S) Ft , and Y˜ is an N-martingale. Thus, for each s < t, (S) Y˜s(S) Ys = E Y˜t Yt Fs (S) (S) (2.60) = Ys E Y˜t Fs + Cov Yt , Y˜t Fs , where we have used the N-martingale property of Y . Dividing out Ys we get that (S) (S) (2.61) Y˜s(S) = E Y˜t Fs + Ys−1 Cov Yt , Y˜t Fs . The second term on the right here is commonly referred to as a quanto adjustment. In the case where Y and Y (S) are both log-normally distributed with deterministic volatility, the equation above will result in a drift term for Y (S) of the form (S)

μt

(S)

= −ρt σt σt ,

(2.62)

(S)

where ρt , σt and σt are respectively the instantaneous correlation linking Y and Y˜ (S) , the instantaneous volatility of Y and the instantaneous volatility of Y˜ (S) . Table 2.3 Notation for equity products denominated in a foreign currency

St,T S¯t,T Y˜ (S)

Forward stock price in the stock’s currency Normalised stock forward price, St,T /S0,T ˜ Change of measure dN(S) /d N

2.7 Credit For credit products, exposure is driven by (i) the likelihood and occurrence of defaults, (ii) the dynamics of credit spreads, and (iii) the inter-dependence between defaults of different entities. Often the fundamental quantities chosen to be modelled are the credit spreads, as they are directly observable in the market (see for example the books by Bielecki & Rutkowski [14], Duffie & Singleton [39], Lando [72] or Schönbucher [95] for a discussion about credit models). We opt for a model whereby stochastic default probabilities are simulated directly. As we will see this allows us

2.7 Credit

39

Fig. 2.1 Change of measures between the local measure N, the T-forward local measure NT , the ˜ and the T-forward foreign measure N ˜T foreign measure N

to work in a framework similar to what we have introduced in the previous sections. Section 2.7.1 below describes how the par CDS spreads observed in the market can be used to obtain initial default probabilities. After that, we propose a method for evolving default probabilities of single entities, simulating default times, and modelling inter-dependence of credit spreads and default times of different entities. The model as we present it imposes a Gaussian dependence structure, which is used to achieve dependence between default probabilities and simulated default times of any pair of reference entities. Such Gaussian models are widely used in practice, even if it is well known that the Gaussian copula does not build dependence in the tail of the distribution. Other, more realistic, ways of introducing dependence can be used (for a discussion of other types of copula see, for example, [78]), but the mathematics is bound to become much more tedious.

2.7.1 Default Probabilities from par CDS Spreads At any time t, the observed CDS curve consists of a set of spreads, st,Ti for a set of maturities Ti , i = 1, 2, . . . , n. The spreads indicate, for a given entity which is not yet in default at time t, the market view on the propensity of that entity to default in the future. By writing down the value of the CDS contract,10 we can express default probabilities for the horizons Ti in terms of the set of spreads st,Ti observed at time t. 10 A

CDS (credit default swap) is a product entitling its holder to receive, at time of default and in return for a bond issued by an entity that has just defaulted, the face value of that bond. A common convention when pricing CDSs is to assume that the value of a defaulted bond is a fraction R of its face value. Under such a convention, the net value of the CDS at the time of default is then R¯ := (1 − R) per unit of face value. We will describe CDSs in detail in Chap. 10.

40

2 Modelling Framework

Table 2.4 Notation for credit products τ, τ (i)

Generic default time, default time for reference name i

qt,T ≡ 1 − pt,T

Probability of surviving beyond T , conditional on t-information and survival until t

M¯ t,T

The N martingale E[1τ >T | Ft ]

To see this, consider a CDS contract of maturity Tn , for which the observed spread is st,Tn , and write T1 , . . . , Tn for the fee payment dates of the CDS. The distance between payment dates is αi = Ti − Ti−1 , with t = T0 < T1 . Set pt,T := N (τ ∈ (t, T ) | Ft , τ > t)

(2.63)

to be the probability in the time-t filtration that the default time τ of the entity underlying the CDS lies in (t, T ] (given survival until t). Then the fee leg of the CDS has value n At := st,Tn Dt,Ti αi (1 − pt,Ti ). (2.64) i=1

Similarly, if R¯ ≡ 1 − R represents the payment per unit notional made by the CDS upon default, the value of protection offered by the CDS is given by Bt := R¯

n

Dt,Ti (pt,Ti − pt,Ti−1 ),

(2.65)

i=1

where we have assumed that default payments are made at time-points in a discretized grid. The observed (fair) CDS spread st,Tn is that which makes At and Bt equal. That is to say, the unknown probabilities pt,T1 , . . . , pt,Tn satisfy n

¯ i−1 − αi st,Tn + R¯ pi = 0, Dt,Ti αi st,Tn + Rp

(2.66)

i=1

where we have abbreviated pj ≡ pt,Tj . For n = 1, (2.66) is an expression in p1 and p0 ≡ 0, and therefore gives us the value of p1 as α1 st,T1 + R¯ . (2.67) p1 = α1 st,T1 Similarly, given values of p1 , . . . , pj −1 , we obtain the value of pj in terms of pi , i = 1, . . . , j − 1 and st,Tj . Explicitly, pj =

−1 Dt,T j

j −1 i=1

¯ i−1 − (αi st,Tn + R)p ¯ i} Dt,Ti {αi st,Tn + Rp

αj st,Tj + R¯ +

¯ j −1 αj st,Tj + Rp , αj st,Tj + R¯

j = 2, 3, . . . , n.

(2.68)

2.7 Credit

41

The inductive recipe (2.68) gives us the per-period default probabilities pt,Ti , which at time t are consistent with CDS market prices observed as the spreads st,Ti . In particular, at t = 0, we can obtain probabilities p0,Ti which are consistent with timezero observed CDS spreads.

2.7.2 Stochastic Default Probabilities The time-zero default probabilities (2.68) serve as the starting point for the simulation of stochastic default probabilities for a given entity. Our modelling hinges on specifying dynamics for the N-martingale (2.69) M¯ t,T = E 1τ >T | Ft . Here, expectation is taken in the martingale measure corresponding to numeraire N , where we will assume that credit quantities and interest rates are independent.11 The initial values M¯ 0,T in (2.69) are chosen to replicate exactly the initial term structure of survival probabilities, namely M¯ 0,T = (1 − p0,T ) ≡ q0,T .

(2.70)

The filtration F = (Ft )t≥0 contains information about the underlying drivers of the economy, but not about actual defaults. In this filtration, the martingale M is related to survival probabilities via the intensity process, λ = (λt )t≥0 , for τ , that is, M¯ t,T = E 1τ >T | Ft t T = exp − λu du E exp − λu du Ft 0

t

=: M¯ t,t qt,T ,

(2.71)

where the last equality serves to define qt,T as the probability that conditional on having survived until time t, and conditional on the information in Ft , the reference name does not default before T . From (2.71), default probabilities qt,T can be obtained from values of martingales M¯ ·,T . This approach is in a sense akin to those followed by Sidenius [99] and Bennani [12]. The form of M¯ t,t also points to how one can simulate values of the default time for any given reference name: (i) draw a random uniform U ; (ii) set τ to be the least member of the set {t ≥ 0 : M¯ t,t ≤ U }. 11 The

implication of this is that M¯ t,T is a martingale under any rate-based numeraire measure.

42

2 Modelling Framework

Looking at the recipe above, correlation between different reference names can be introduced by (i) imposing dependence between U (i) and U (j ) for any i, j , and/or (j ) (i) (ii) setting up diffusion dynamics for the martingales M¯ t,T and M¯ t,T and then allowing the driving Brownian Motions to be correlated. The question of how to introduce dependence between default times has been approached in several ways in the literature, as any standard book on credit derivatives will reveal (see for example Lando [72] or Schoenbucher [95]). We approach the problem using a copula method. One of several copulas can be used, each type of copula building dependence in a different way (see, for example Madan et al. [77]). In our exposition below, we use the example of the well-known Gaussian copula, for which dependence is equivalent to a linear correlation parameter. Thus, we specify for each reference name i, U (i) = 1 − Φ ρ (i) · M + ρ¯ (i) M (i) (2.72) dW (i) = η(i) · dZ + η¯ (i) dZ (i) . Here, M and M (i) are standard normally distributed random variables (the former of dimension possibly larger than one) driving dependence between default times. Similarly, Z and Z (i) are standard Brownian Motions (with the former again of dimension possibly larger than one) driving thestochasticity of survival probabilities. The reals ρ¯ (i) = 1 − ρ (i) · ρ (i) and η¯ (i) = 1 − η(i) · η(i) ensure that U (i) and dW (i) have uniform and normal distributions, respectively. For each pair (i, j ), Z, Z (i) , Z (j ) , dW (i) and dW (j ) are all independent; expressions (2.72) build up dependence between U (i) , U (j ) , and between dW (i) , dW (j ) , because defaults of all entities depend on Z and on M. Elements in M and Z are thought of as market factors impacting defaults and spread evolutions for different entities, while M (i) and Z (i) affect only the spread evolution and default time of reference name (i) . We will see in Chap. 3 the specific form the inter-name dependence takes for a ¯ particular choice of dynamics for the martingale M.

2.7.3 Loss Simulation For credit products that depend on defaults of several entities, the simulation of defaults for individual names implies, assuming an appropriate dependence structure has been imposed, a corresponding simulation of losses suffered by a chosen portfolio of names. Consider an investor who holds several corporate bonds B¯ (i) , i = 1, 2, . . . , n. The loss li incurred by bond i upon default is li = (1 − Ri )Ai ≡ R¯ i Ai ,

(2.73)

2.7 Credit

43

where Ai is the nominal amount on bond i and Ri is the fraction of face value that is retained by the bond upon default. Given this, the fractional total loss suffered by the portfolio of bonds in the time interval [0, t] is defined to be Lt =

n

n

R¯ i Ai 1{τ (i) ≤t}

i=1

Ai .

(2.74)

i=1

The law of Lt depends of course on the dependence between default times of different entities. The typical observable quantity that provides information on such dependence is the market price of a CDO tranche, which is a product whose payoff at time t is of the form n + n + Lt − ka − Lt − kd , (2.75) Πt = i=1

i=1

where ka (respectively, kd ), satisfying 0 ≤ ka ≤ kd ≤ 1, are referred to as the attachment (respectively, detachment) point.12 We will see in Chap. 3 how market quotes for different tranches can be used to calibrate the dependence coefficients ρ and η appearing in (2.72).

12 CDO

(Collateral Debt Obligation) products will be described in details in Chap. 10.

Chapter 3

Simulation Models

In Chap. 2 we defined a general framework to enable estimation of counterparty exposure for different product classes. Throughout, we highlighted the importance of being able to simulate price processes of different asset classes simultaneously and in consistent fashion. This was accomplished by simulating a martingale process for each asset class. By doing so, the models fit time-zero forward curves by construction, so that calibration involves only choosing the volatility structure for the martingale pertaining to each asset class. In this chapter we focus on specific choices of models for different asset classes, discussing how they can be implemented and calibrated within our framework.

3.1 Interest-Rate Models For relatively simple interest-rate products, arbitrage-free models with deterministic volatility have dynamics rich enough to reproduce simulated price distributions with the correct properties. In this section we describe in detail, within the framework defined in Chap. 2, the model with separable volatility structure introduced in (2.23), and we show in particular how the familiar Hull-White model is a special case of such a separable specification. For ease of exposition, we will mostly refer to a separable model driven by a single Brownian Motion. Products such as steepeners, however, which depend on different points of the yield curve, may require models with more than one stochastic driver or a richer volatility structure.1

1 There is a vast literature on interest-rate models. The reader can refer to the following books, Brigo

& Mercurio [18], Cairns [21], Filipovic [44], Hunt & Kennedy [64], Pelsser [85], or Rebonato [89] for more details. G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0_3, © Springer-Verlag Berlin Heidelberg 2009

45

46

3 Simulation Models

3.1.1 Separable Volatility In Sect. 2.3 we expressed all bond prices and the numeraire in terms of a family of N-martingales M.,T . In turn, for deterministic volatilities, (2.15) gives the integral form of Mt,T as t 1 2 σu,T dWu,T − Σt,T t , (3.1) Mt,T = M0,T exp 2 0 in terms of N-Brownian Motions WT ≡ (Wt,T )0≤t≤T and volatility functions σ ≡ (σt,T ). What we study here is the special case, already alluded to in (2.23), where the dependence of the volatility function on t and T can be separated into two terms. That is, we look at cases where the SDE for M can be written as (see also Sect. 2.3.1) dMt,T = Mt,T gT · (ft RdWt ),

(3.2)

with W a Brownian Motion in Rn , gT a deterministic vector in Rn , and with RT R being a positive semi-definite matrix such that the i’th component of RW is a standard Brownian Motion, which then gets time-changed by the (i, i)’th entry in the diagonal (n × n) matrix ft . Thus, by defining a new, time-changed process (also in Rn ) t Xt = fu RdWu , (3.3) 0

the expression for Mt,T can be written as 1 2 Mt,T = M0,t exp gT · Xt − Σt,T t . 2

(3.4)

2 t is the variance of g · X , that is, The term Σt,T T t 2 := t −1 Var (gT · Xt ) = gT · Σt,T

=: gT · F2t gT ,

t

(fu R)T (fu R)du gT

0

(3.5)

serving to define the matrix F. While we have written the model in general for n factors, to simplify the notation we will mostly restrict ourselves to the one-factor case in what follows; the vector gT and the matrix ft are then real-valued functions of T and t , respectively. In general Σt,T will be characterised by a number of parameters, and in a simulation framework, it is necessary to first calibrate these to instruments whose values are observable in the market. Thus, for instance, by writing the time-t price of a T -bond in terms of the martingale M, we get that Dt,T =

D0,T Mt,T D0,t Mt,t

3.1 Interest-Rate Models

47

=

1 2 D0,T exp (gT Xt − 2 Σt,T t) , 2 t) D0,t exp (gt Xt − 12 Σt,t

(3.6)

where in this one-factor case the variance is simply 2 tΣt,T = tFt2 gT2 .

(3.7)

The functional dependence of F and g on T and t may now be chosen to have the desired behaviour for the volatility of the T -bond. For instance, taking ft ≡ 1 and (3.8) gT = a 1 − e−κT results in bond volatilities (gT − gt )Ft = ae−κt 1 − e−κτ

(τ ≡ T − t),

(3.9)

which decrease with with time t but increase with tenor τ . In other words, bond options (or, equivalently, caplets; see below) priced with this model would exhibit implied volatilities that decrease with expiry but increase with tenor. One will need to choose the number of model parameters so as to strike the right balance between having a model that is parsimonious enough and having a fit to market prices that is good enough. It is also important to note that calibration can influence materially the counterparty risk profile. In the next sections we will show how to calibrate the model, using caps, floors, and swaptions.

3.1.1.1 Calibration to Caps A cap is a market instrument that pays, at pre-specified points in time, T1 < · · · < Tn , the amount by which the observed Libor rate exceeds a given level, K, referred to as the cap strike. Thus, the time-zero value, denoted C0,Tk (K), of the k’th payment made by the cap is C0,Tk (K) = D0,Tk EN

Tk

+

αk LTk−1 ,[Tk−1 ,Tk ] − K ;

(3.10)

the payoff above is referred to as a caplet. Tk indicates the maturity of the caplet, and NTk the Tk -forward measure. On the right, the payoff is a call option on the Libor rate, fixed at Tk−1 , for the period [Tk−1 , Tk ] of length αk = Tk − Tk−1 . Changing measure from NTk to N in (3.10) using (2.7), we get that + C0,Tk (K) = αk D0,Tk E MTk−1 ,Tk LTk−1 ,[Tk−1 ,Tk ] − K = αk D0,Tk E

¯ Tk−1 ,Tk L¯ k MTk−1 ,Tk−1 − KM

+

(from (2.19))

48

3 Simulation Models

MTk−1 ,Tk L¯ k ≤ MTk−1 ,Tk−1 K¯

K¯ Tk MTk−1 ,Tk−1 ¯ − αk D0,Tk KN ≥ , MTk−1 ,Tk L¯ k

= αk D0,Tk L¯ k NTk−1

(3.11)

where L¯ k = L0,[Tk−1 ,Tk ] + αk−1 and K¯ = K + αk−1 . Define M¯ tk =

Mt,Tk−1 . Mt,Tk

(3.12)

Since M¯ k and (M¯ k )−1 are respectively martingales in NTk and NTk−1 , evaluating the probabilities in (3.11) involves knowing the volatility structure of M¯ k . In particular, in the case of a separable volatility structure, we can write these probabilities as NTk−1

M¯ Tkk−1

N

Tk

−1

M¯ Tkk−1

where dk :=

L¯ k = Φ (dk ) , and K¯ K¯ = Φ dk − sTk−1 ,Tk Tk−1 , ≥ L¯ k ≤

√ ln L¯ k /K¯ sTk−1 ,Tk Tk−1 . + √ 2 sTk−1 ,Tk Tk−1

(3.13)

(3.14)

The variance of M¯ Tkk−1 is sT2k−1 ,Tk = gTk−1 − gTk · F2Tk−1 gTk−1 − gTk ,

(3.15)

where FTk−1 is the diagonal matrix defined in (3.5).

3.1.1.2 Calibration to Swaptions An interest-rate swaption is an instrument that gives the holder the right to enter into a fixed-for-floating interest-rate swap. Such instruments can be valued in closedform in the case of separable volatility one-factor models, and can therefore be used for calibration to observed market prices. Consider for concreteness a swaption which can be exercised at time T to enter into a swap where the holder pays a fixed annualised coupon of K and receives the Libor rate of interest. Write, again, T1 < · · · < Tn for the times when the swap payments are exchanged, and define (Tk − Tk−1 ) = αk for all k = 1, 2, . . . , n. Assume that T = T0 = T1 − α1 , so that the swaption exercise time coincides with the time of the first fixing on the floating rate component of the underlying swap. Then the

3.1 Interest-Rate Models

49

time-zero value, S0,T ,Tn (K), of the swaption is given by + n NT αk DT ,Tk S0,T ,Tn (K) = D0,T E 1 − DT ,Tn − K k=1

=E

NT

D0,T −

n k=1

where

ak =

Kαk , 1 + Kαk ,

MT ,Tk ak D0,Tk MT ,T

+ ,

k = 1, 2, . . . , n − 1, k = n.

(3.16)

(3.17)

In the special case of a one-factor model, (3.16) can be expressed as a sum of put M is a deterministic, monotonic options of different strikes. Indeed, if for any T , Mt,T t,t function of a single stochastic process Xt , then we can define hi (Xt ) := Mt,Ti /Mt,T h(Xt ) := D0,T −

n i=1

Mt,Ti ai D0,Ti = D0,T − ai D0,Ti hi (Xt ) Mt,T n

(3.18)

i=1

with h and the hi ’s being bijective. Therefore, there exists a family of strikes Ki∗ , i = 1, 2, . . . , n, such that D0,T −

n i=1

MT ,Ti ai D0,Ti MT ,T

+ = MT−1 ,T

n

ai D0,Ti (Ki∗ MT ,T − MT ,Ti )+ . (3.19)

i=1

In fact, the above is satisfied by having Ki∗ = hi h−1 (0) .

(3.20)

Once the strikes Ki∗ have been found using a root-searching algorithm, pricing the swaption is reduced to pricing put options of the form + M T T ,T i Ki∗ − EN MT ,T √ (3.21) = Ki∗ Φ (d) − Φ d − sT ,Ti T , where

√ ln Ki∗ sT ,Ti T d := , √ + 2 sT ,Ti T 2 sT2 ,Ti = FT2 gT − gTi .

(3.22) (3.23)

50

3 Simulation Models

3.1.2 Example: Hull-White (Extended Vasicek) We analysed in the previous section how to calibrate with caps and swaptions a generic model specified in terms of separable volatility structure. We consider here a specific example of separable volatility model, the Hull-White model. The Hull-White model introduced in [63] is a short rate model with deterministic volatility, where the short rate r satisfies the SDE drt = (θt − art )dt + σ dWt ,

(3.24)

with speed of mean reversion a and volatility σ (see also Baxter & Rennie [10], Brigo & Mercurio [18], or Hull [62] for further details). Knowing the instantaneous forward rate f (0, t) at time zero for different values of t (that is, knowing the current yield curve) allows determination of the unknown θt in (3.24). Note also that (3.24) prescribes that all bonds in the economy will depend on the same source of randomness W . We can write for each t ≥ 0, rt = f (0, t) +

2 σ2 1 − e−at + σ 2 2a

t

e−a(t−u) dWu ,

(3.25)

0

in terms of f (0, t). From this it is immediate that for each t , the short rate rt is a normal random variable with variance σ2

1 − e−2at . 2a

(3.26)

Moreover, note that from (3.25), the Hull-White numeraire process satisfies Nt−1

t = exp − f (0, u)du −

2 σ2 1 − e−au du 2 0 0 2a t u −a(u−s) − σ e dWs du 0

t

0

= D0,t exp −

t

0

= D0,t exp −

0

t

2 σ2 1 − e−au du − 2a 2 2 σ2 1 − e−au du − 2 2a

t

u

σ 0

e 0

t

σ 0

−a(u−s)

t

e

−a(u−s)

dWs du

dudWs , (3.27)

s

where the boundedness of the integrand allows us to use the Fubini theorem2 in the last equality. We now attempt to write our martingale M so that bond-prices in our framework have the same volatility structure as in the above Hull-White model. First, in the 2 For

the generalisation of this result to stochastic integrals, see, for example, Protter [86].

3.1 Interest-Rate Models

51

framework of Sect. 2.3 we have that Dt,T = and

D0,T Mt,T , D0,t Mt,t

t 1 2 Nt−1 = D0,t M0,t exp σu,t dWu,t − Σt,t t , 2 0

where 2 Σt,t :=

1 t

0

t

2 σu,t du.

(3.28)

(3.29)

(3.30)

Comparing (3.27) and (3.29) we can then set 2 σt,t =

2 σ2 1 − e−at . 2 a

(3.31)

To identify the form of Σt,T for T = t in terms of the Hull-White parameters, we write Dt,T in terms of the martingale Mt,T : D0,T M0,T + log D0,t M0,t t t 1 2 1 2 + σu,T dWt − Σt,T t − σu,t dWt − Σt,t t , 2 2 0 0

log Dt,T = log

which means that

(3.32)

. d log Dt,T = σt,T − σt,t dWt ,

(3.33) . where = signifies that the two sides differ by a term in dt only. On the other hand, in the Hull-White model, we can write

. d log Dt,T = −σ Bt,T dWt , (3.34) where Bt,T :=

1 1 − e−a(T −t) . a

We therefore have σt,T − σt,t = −σ Bt,T = −

σ 1 − e−a(T −t) a

(3.35)

(3.36)

with σt,t as in (3.31). Putting everything together, our framework can therefore be specialized to HullWhite volatility dynamics by choosing σ σ σt,T = eat e−aT + − e−at . (3.37) a a

52

3 Simulation Models

Referring to the separable specification of volatility discussed in Sect. 3.1.1, the Hull-White model is achieved by having n = 2 and ft = Diag eat , e−at ; σ σ T ; e−aT , − gT = (3.38) a a 1 1 1 . R= √ 2 1 1 In effect, what happens here is that our general volatility function σt,T is a linear combination of two separable forms. The degeneracy of the matrix R means that the components of the √Brownian Motion RW are both equal to the single Brownian Motion (W1 + W2 )/ 2, consistent with there being only one source of noise as in the original Hull-White model.

3.2 Equity and FX Models The vast literature on option-pricing models, stemming from the seminal BlackScholes paper [15], consists of various attempts at dealing with the fact that stock prices and foreign exchange rates do not follow the simple dynamics of the BlackScholes model.3 In particular, financial asset price returns are not normally distributed (exhibiting skewness and fat tails), and volatilities vary with time and with the price level itself. In fact, it is market practice to quote option prices in terms of implied volatilities, that is, the value at which to evaluate the Black option-pricing formula that recovers the market option price. At a general level, the devices for modelling these departures from asset price log-normality are, among others, (i) Building a market model whereby stock and FX forward prices are modelled directly, employing Black-Scholes volatilities implied from observed option prices; (ii) Making the volatility function depend on the level of the underlying, a class of models commonly referred to as local volatility models; (iii) Allowing the volatility of the underlying process to be itself stochastic, thus introducing an additional source of noise. This class of models is referred to as stochastic volatility models; (iv) Having a stock price process with a discontinuous component such as a Lévy process. Our goal in this section is to specify equity and FX models within our modelling framework, and to show how they can be calibrated to market instruments. Recall 3 For

a survey of stylised facts about asset price returns, see, for example, Cont [29].

3.2 Equity and FX Models

53

from Sect. 2.4 the expression Ft,T = F0,T F¯t,T = F0,T Yt

M˜ t,T Mt,T

(3.39)

for the evolution of the FX forward price in terms of the martingales M and M˜ driving the reference and foreign rates of interest. Typically, models of the types enumerated above, designed to reproduce some desired feature of stock prices or foreign exchange rates, do away with complexity arising from discounting by assuming interest rates to be deterministic or constant. In terms of our framework this amounts to having M˜ ≡ M ≡ 1 (whence Y ≡ F¯ ), and therefore in what follows we will sketch the details of how the Black, local volatility and stochastic volatility models can be applied to the process Y . To see how this relates to the more familiar situation where the spot price is modelled directly, note that Y = F¯ means Yt = Ft,T /F0,T . In particular, for T = t we then have St ≡ Ft,t = F0,t Yt , so that F˙0,t dYt dSt = dt + ; St F0,t Yt

(3.40)

that is, imposing a volatility on Y amounts to doing the same on the spot price S. Because we need to deal with all asset types (including interest-rate products) simultaneously, we also outline in Sect. 3.2.5 the form these models take when rates are stochastic, in which case the martingale terms in (3.39) contribute additional volatility to F¯ . The key idea will be to reduce the general case to the case of nonstochastic rates by taking an appropriate conditional expectation. We also look at the simpler approach, mentioned in (2.53), where we model forward rates (for equity and foreign exchange assets) that are independent of interest rates.

3.2.1 Black Model The Black model extends the original Black-Scholes model, by writing localmartingale dynamics for asset forward prices, of the form dYt = Yt g(t)dWt ,

(3.41)

with W an N-Brownian Motion and g(t) a deterministic function in t . In order that this specification of forward prices match observed option prices on the underlying asset, one simply compares the variance of Y in (3.41) to the observed Black-Scholes implied volatility for options of different maturities. That is, if maturities T1 , . . . , Tn are available, then g has to be consistent with

1 Ti

0

Ti

12

gu2 du

= Σ BS (F0,Ti , Ti ),

i = 1, 2, . . . n,

(3.42)

54

3 Simulation Models

where Σ BS (K, T ) is the Black-Scholes implied volatility for an option of expiry T and strike K. An alternative approach would be to calibrate to variance swap strikes, if a market for these is available. The constraint on g in this case is 1 Ti

Ti

0

gu2 du = E[VTi ],

(3.43)

where for each t > 0, the time-t realised variance Vt is defined to be 1 t [ln Fu,t ]u du, Vt := t 0

(3.44)

in terms of the quadratic variation process,4 [ln F ], of ln F . Now, the realised variance can be expressed as an integral over call and put prices. Indeed5 we find +∞ F0,T P (K, T ) C0 (K, T ) 0 −1 2 E[VT ] = D0,T dK + dK , (3.45) T K2 K2 0 F0,T in terms of the time-zero prices of calls (resp. puts) C0 (K, T ) (resp. P0,T (K, T )). In practice, there are subtleties in how to compute the fair variance swap strike, which we will not discuss here.

3.2.2 Local Volatility The Black model of the previous section is calibrated to a different volatility for each at-the-money option maturity, but still makes the implicit assumption that options of different strikes exhibit the same implied volatility. In practice, implied volatilities observed in the market vary not just with option expiry but also with the strike of the option, an effect usually called a smile or skew. That is to say, market prices 4 If

M is a continuous local martingale, then there exists a unique increasing continuous process, [M], called the quadratic variation process of M, such that M 2 − [M] is a continuous local martingale. See Rogers & Williams [94] for the full story. 5 Taylor’s

expansion series with integral remainder gives for any C 2 (R) function that b f (b) = f (a) + f (a)(b − a) + f (x)(b − x)dx a

= f (a) + f (a)(b − a) +

a

b

f (x)(b − x)+ dx −

b

f (x)(x − b)+ dx.

a

We apply this to f (x) = ln x with a = F0,T and b = FT ,T . Taking N-expectations and noting that E[FT ,T ] = F0,T yields the result.

3.2 Equity and FX Models

55

for options are inconsistent with assuming that the volatility of the underlying price process does not change with the price level. One way of building this effect into the model is to modify the Black SDE (3.41) to dYt = g(t, Yt )Yt dWt ,

(3.46)

where the volatility function now depends also on Y . Models of the form (3.46) are referred to as local-volatility models. The departure of (3.46) from the Black model can be gauged by analysing the special case where the volatility term takes the form g(t, x) = g(t)f (x). Expanding the function f around the value 1 and comparing to the implied volatility, we have (to second-order) m m 1 f (x)dx ≈ α + β(x − 1) + γ (x − 1)2 dx 2 1 1 ≈ (m − 1)

Σ BS (mF0,T , T ) , Σ BS (F0,T , T )

(3.47)

where α ≡ f (1), β ≡ f (1) and γ ≡ f (1), and where for each T , g relates to the at-the-money implied volatilities,

T

g 2 (u)du = (Σ BS (F0,T , T ))2 T ,

0

as in the Black model. Doing the integration, we see that we need to have 1 1 Σ BS (mF0,T , T ) 2 ≈ 1 + β(m − 1) + γ (m − 1) , Σ BS (F0,T , T ) 2 6

(3.48)

which can be used in the Black formula to calculate approximate prices for options in this model, an observation that is important for calibration. From (3.48), we interpret the coefficients β and γ in terms of the at-the-money skew and convexity, that is 1 ∂Σ BS (K, T ) = βΣ BS (F0,T , T ) (3.49) ATM Skew = K ∂K 2 K=F0,T 2 BS (K, T ) 1 2∂ Σ = γ Σ BS (F0,T , T ) (3.50) ATM Convexity = K 2 3 ∂K K=F0,T showing that if it is deemed suitable to have f quadratic, then (3.48) characterises f in terms of observed implied volatilities. We also read off Derman’s approximate rule of thumb (see Derman [36]) that near m = 1, the local volatility f changes with m twice as fast (at a rate of β) as the implied volatility (which changes at a rate of 1 2 β). The most well known example of a local volatility model is the Constant Elasticity of Variance (CEV) model, first studied by Cox & Ross [32] (see also

56

3 Simulation Models

Schroder [97]). In our notation, the CEV model is obtained by choosing g(t, x) = σ x β ,

(3.51)

where σ > 0 is a positive real and β ≡ f (1) is a constant skewness parameter6 as seen in (3.48). For β = 0, the CEV model is obviously the Black Model in (3.41). For β < 0 (resp. β > 0), the volatility decreases (resp. increases) with x. This results in the distribution of Yt being skewed to the left (resp. to the right), as shown in Fig. 3.1. Fig. 3.1 Distribution of FT ,T = ST with F0,T = 100, σ = 20% and T = 1

Taken at face value, local volatility models impose on asset prices a volatility term that is a deterministic function of the asset price level, an assumption that might be questionable in practice. However, Dupire [41] showed the existence of a diffusion process consistent with the observed local volatility surface, and the diffusion coefficient for this process is a local volatility function. We review the derivation of this result, following closely the analysis of Dupire’s idea in Gatheral [48]. Consider the time-zero price, C(F0,T , mF0,T , T ) say, of an option of expiry T and strike mF0,T , where F0,T is the time-zero price of the T -forward and m > 0 is the strike moneyness. If we write pt (·, ·) for the risk-neutral transition density of Y , the call price has the representation C(F0,T , mF0,T , T ) =

∞

D0,T (yF0,T − mF0,T )pT (1, y)dy.

(3.52)

m

The Kolmogorov equations for the transition density pT (·, ·) read p˙ t (x, y) = Gx pt (x, y) = Gy∗ pt (x, y), 6 The

(3.53)

parameter β is also related to the elasticity of the variance in this model, which is x(f 2 ) (x)/f 2 (x) = 2β.

3.2 Equity and FX Models

57

where G , G ∗ are, respectively, the infinitesimal generator7 of Y and its adjoint, and where p˙ t represents differentiation in t . Differentiating (3.52) in T and using the first part of (3.53), we obtain the Black-Scholes differential equation. Because we are interested in a differential equation involving derivatives in the strike and not in the initial forward price F0,T , we use the second part of (3.53) to get ˙ C(y, t) =

˙ F˙0,t F˙0,t D0,t + yC (y, t) C(y, t) − D0,t F0,t F0,t 1 + g 2 (t, y)y 2 C (y, t), 2

(3.54)

where C(y, t) is an abbreviation for C(F0,T , yF0,T , t), C and C are derivatives of C in its argument y, and C˙ is the derivative in t . Inverting (3.54) gives us an expression for the local volatility function g in terms of option moneyness and maturity, that is g 2 (t, y) =

˙ C(y, t) −

D˙ 0,t D0,t

+

F˙0,t F˙0,t F0,t C(y, t) + F0,t yC (y, t)

1 2 2 y C (y, t)

.

(3.55)

Although C and the derivatives C and C are related to market call prices, the final goal will be reached only when we express g in terms of what is directly observed in the market, namely the implied volatility surface. Gatheral [48] shows the required expression to be ϕ ∂w 1 1 1 ϕ2 ∂w 2 1 ∂ 2 w g (t, y) = w˙ 1− + + − − + 2 , w ∂ϕ 4 4 w w ∂ϕ 2 ∂ϕ 2 2

(3.56)

where w(F0,T , y, T ) = T Σ BS (F0,T , yF0,T , T ),

(3.57)

is the Black-Scholes implied total variance, w˙ ≡ ∂w/∂T ,

(3.58)

ϕ ≡ ln y

(3.59)

and where

is the log-moneyness of the option strike. In practice, to get around difficulties arising from the volatility surface not being smooth or granular enough, one would parametrize the total variance surface w in t X is a diffusion, then for smooth f the process Yt = f (Xt ) − 0 (G f )(Xs )ds is a martingale. ∗ is then defined by having g(x)(G f )(x)dx = (G g)(x)f (x)dx for smooth f and g. From this characterisation, we see that Y is a martingale, supermartingale or submartingale according as f is harmonic (G f = 0), superharmonic (G f ≤ 0), or subharmonic (G f ≥ 0) for G .

7 If

G∗

58

3 Simulation Models

terms of powers of moneyness K/F0,T , and use derivatives of this parametrized form in computing (3.56). Figure 3.2 shows the implied volatility as a function of strike for chosen values of skew and convexity. Similarly, Fig. 3.3 shows the effect of skew on the resulting distribution for the log-price of the stock.

3.2.3 Stochastic Volatility The class of stochastic volatility models allows volatility of an asset price and the asset price itself to be altogether different processes, by writing dynamics of the form √ dYt = Yt vt dWt

(3.60)

dvt = α(Yt , vt , t)dt + β(Yt , vt , t)dZt

(3.61)

with W and Z being N-Brownian Motions having instantaneous correlation ρ. Now from Dupire’s result we know that prices of options (equivalently, the implied volatility surface) of some given expiry T are consistent with a distribution of asset prices at T arising from some local-volatility model. In fact, it is a consequence of a more general result of Gyöngi [54] that if the SDE for Y in (3.60) admits a unique solution, then the SDE dXt = Xt b(t, Xt )dWt ,

X0 = Y0 ,

(3.62)

has a weak solution8 having the same law as Y . Moreover, the diffusion term b has the representation b2 (t, y) = E[vt | Yt = y],

(3.63)

thus exposing the link between the stochastic volatility process v and the Dupire local volatility function b. For more details on diffusion and stochastic volatility models we refer the reader to Andersen & Piterbarg [3], Cox [31], Dupire [41], Derman & Kani [37], and Hagan et al. [55]. A well-known example of a stochastic volatility model is the Heston model (see Heston [61]), in which the asset price variance is a diffusion of CIR type, that is √ dvt = κ(θ − vt )dt + σ vt dZt .

(3.64)

This diffusion is mean-reverting to a level θ > 0, with κ > 0 controlling the speed of reversion. The real parameter σ > 0 is commonly known as the volatility of volatility. Correlation between the Brownian Motions Z (driving volatility) and W (driving the asset price Y ) controls the implied volatility skew, with negative skew resulting 8 For the concept of a weak solution to an SDE, see any standard text on the subject, such as Rogers & Williams [94].

3.2 Equity and FX Models

59

from negative correlation (much like negative β in the CEV model). Euler and Milstein schemes are not the best choice for discretizing and simulating the pair of SDEs (3.60)–(3.61); Andersen [1] presents an efficient method that makes use of moment-matching techniques. For the Heston model (part of a bigger class of generally tractable models—see Duffie et al. [40]), the characteristic function of the asset price distributions is known in closed form. By this, and as shown by Carr & Madan [24], the characteristic function of the option price can itself be written down in closed form, allowing option prices to be recovered by numerical inversion. In general, if st = ln (Yt ) and φT (u) = E[exp (iusT )]

(i 2 ≡ −1),

(3.65)

is the characteristic function of sT , then the modified call prices −1 ζ m c(m, T ; ζ ) ≡ F0,T e C(F0,T , em F0,T , T )

(3.66)

with strike K, log strike moneyness m = ln (K/F0,T ), maturity T and real ζ > 0, have Fourier transform ∞ D0,T φT (v − (ζ + 1)i) , (3.67) eivx c(x, T ; ζ )dx = 2 ψT (v) ≡ ζ + ζ − v 2 + i(2ζ + 1)v −∞ which shows why positivity of ζ is required to avoid having ψT singular at the origin. Fourier techniques such as FFT can be used to calibrate the model using observed call prices, by numerically inverting the option price transform (3.67). In the particular case of the Heston model9 the transform (3.65) is log-affine in the starting point V0 of the variance process V , namely φT (u) = exp {C(T , u) + D(T , u)V0 },

(3.68)

where

κθ c(u)ed(u)T − 1 C(T , u) = 2 (κ − ρσ ui + d(u)) T − 2 ln , c(u) − 1 σ κ − ρσ ui + d(u) ed(u)T − 1 , D(T , u) = σ2 c(u)ed(u)T − 1

with c(u) =

κ − ρσ ui + d(u) , κ − ρσ ui − d(u)

d(u) =

(ρσ ui − κ)2 + (iu + u2 )σ 2 .

(3.69)

(3.70)

Note that in (3.68), we have φT (−i) = E[YT ] = 1, because C(T , −i) = D(T , −i) = 0; this is of course as expected from the martingale property of Y . 9 See,

for example, Kahl & Jaeckel [67].

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3 Simulation Models

3.2.4 Jump Models Processes with jumps are an attractive tool for modelling asset prices, with Merton [81] being the first to explore option pricing in models where the stock price has a discontinuous component. Geman et al. [49] and Carr et al. [26] present a case for using pure jump processes (that is, with no diffusion component) to model asset price returns. Linked to this idea is the use of non-decreasing jump processes10 to time-change continuous diffusions, thus providing a powerful device for matching the implied volatility skew (see also Carr & Wu [25], Eberlein et al. [42], Kou [71], and Mendoza et al. [79]). The basic class of jump processes with which anything tractable can be done is Lévy processes, because the infinite divisibility property gives the characteristic function a form that enables calculation of various Laplace transforms and therefore opens the way to inversion of option prices as for the Heston model. For more information on Lévy processes, we refer the reader to Barndorf-Nielsen et al. [9], Bertoin [13], Boyarchenko & Levendorskii [16], and Rogers & Williams [94]. For work on models combining stochastic volatility with Lévy processes see, for example, Carr et al. [27]. There is a vast literature on the use of jump processes in financial modelling, of which Cont & Tankov [30] is a good survey. Schoutens & Symens [96] studies the pricing of options by simulation in jump models.

3.2.5 Extension to Stochastic Interest Rates The previous sections have reviewed standard specifications for the Black, local volatility and stochastic volatility models. Because the primary goal of these models is to explain or fit observed volatilities implied from option prices, the complexities brought about by stochastic interest rates are avoided by assuming rates and bond prices to be deterministic. The assumption of deterministic rates is a severe constraint if models are to be used for credit exposure computation, but what we have done above has not been in vain. In fact this section will describe how, by introducing an extra conditioning step, the models for deterministic interest rates can be re-used to take into account stochastic interest rates. In our framework, where we have (see (3.39)) Ft,T = F0,T F¯t,T = F0,T Yt

M˜ t,T , Mt,T

(3.71)

deterministic rates are equivalent to the martingales M and M˜ being identically equal to one, and the asset pricing models as we have described them then apply to the process Yt = Ft,T /F0,T , where YT then has the interpretation as the moneyness of the time-T spot price relative to the time-zero forward price. 10 Real-valued

increasing Lévy processes are termed subordinators.

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61

We now review how such models can be extended to the case when the rates martingales are not deterministic, similar to what is done by Andreasen in [5], and assuming throughout that M and M˜ follow log-normal dynamics. The key idea is to write the process Y as the product Y Y ⊥ of two independent martingales. Conditioning on the value of Y , an option on Y ⊥ can be priced in the stochastic or local volatility models, with the final option price then being an integral over the law of Y of the conditional option prices. To see this, in (3.71) we start by writing ˜ t,T ˜ t,T M M =: Yt Yt⊥ , (3.72) Yt⊥ ≡ F¯t,T F¯t,T = Yt Mt,T Mt,T ˜ 11 ). Intuwith Yt independent of Yt⊥ (but not, of course, independent of (M, M) ⊥ itively, as the notation is intended to help convey, Y relates to the stochastic component in asset forward prices that is ‘orthogonal’ to rates. Of course, for deterministic rates, Y ⊥ coincides with Y and Y is identically equal to one. The advantage of this decomposition is that it allows to calibrate the model in two distinct steps, separating the asset price and interest rate components, as we see now. Consider a call option of maturity T and strike K = mF0,T , which has price + C(F0,T , mF0,T , T ) = E NT−1 FT ,T − mF0,T . (3.73) Conveniently switching to the T -forward measure, we have + T F¯T ,T F0,T − mF0,T C(F0,T , mF0,T , T ) = D0,T EN + T F¯T ,T − m = D0,T F0,T EN = D0,T F0,T EN

T

+ F¯T,T YT⊥ − m/F¯T,T .

(3.74)

The crucial next step is now to invoke the independence of Y and Y ⊥ to develop the call price as C(F0,T , mF0,T , T ) + T T YT⊥ − m/y y = F¯T,T = F0,T EN F¯T,T D0,T EN = F0,T EN

T

F¯T,T C ⊥ 1, m/F¯T,T , T ,

(3.75)

where C ⊥ (y ⊥ , k, T ) is the price of an option on Y ⊥ (whose initial value is y ⊥ ) struck at k, of expiry T . 11 We emphasise again that, for tractability reasons, we will assume Y follows a log-normal prot cess.

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3 Simulation Models

The usefulness of decomposing Y is now clear: to compute the price of an option on F , we first compute prices of an option on Y ⊥ , using some preferred asset-pricing model and unencumbered by rates stochasticity, and then incorporate the effect of rates by integrating over the law of F¯ . ˜ then F¯ will be If we take log-normal Black models for each of Y , M and M, T also log-normal (and a N -martingale). We then have the equality in law √ 1 2 ¯ FT ,T ∼ exp Z T ΣT − (ΣT ) T ≡ E (Z, T , ΣT ) (Z ∼ N (0, 1)), (3.76) 2 where the total variance T (ΣT )2 is a function of the volatilities and covariance ˜ Thus, if gt = (gt , g˜ t , gt ) is the vector of Black volatilites matrix of Y , M and M. ˜ M), then we have of (Yt , M, T (ΣT )2

=

T

gu · Rgu du,

(3.77)

0

where, with obvious notation, the matrix ⎛ 1 −ρY,M˜ ⎝ 1 R = −ρY,M˜ ρY,M ρM,M˜

⎞ ρY,M ρM,M˜ ⎠ 1

(3.78)

has as entries the pair-wise correlations between Brownian Motions driving Y , M ˜ and M. With this setup, we can now write the call price fairly explicitly as a Gaussian integral, for we have from (3.75) that C(F0,T , mF0,T , T ) ∞ E (z, T , ΣT )C ⊥ 1, m/E (z, T , ΣT ), T φ(z)dz = D0,T F0,T −∞ ∞

= D0,T F0,T

−∞

√ C ⊥ 1, m/E (z + T Σ , T , ΣT ), T φ(z)dz,

(3.79)

where the last equality follows immediately once we interpret the ‘Doléans’ exponential12 E defined in (3.76) as a change of measure. The integral in (3.79) readily lends itself to numerical methods (such as gaussian quadrature) as long as the call price is available. In the case of stochastic volatility models such as the Heston model, call prices are known only up to a Fourier transform, so FFT methods will be needed to compute the call prices by inversion, as discussed in Sect. 3.2.3. a martingale M with M0 = 0, the Doléans exponential of M is the exponential martingale process Et (M) = exp (Mt − 12 [M]t ), where [M] is the quadratic variation process of M. Our use of the name is because the law of (3.76) is the law of ET (ΣT B) for some Brownian motion B. 12 For

3.2 Equity and FX Models

63

There is nothing to stop us from decomposing the process Y ⊥ itself as the product of independent processes, as long as the characteristic functions of the time-t laws of those processes (and hence the characteristic functions of call option prices) are known in closed form. This allows the pricing of options in models where not only are rates stochastic, but also where different features of the volatility surface can be accommodated by employing independent local volatility, stochastic volatility and even jump components, in the spirit of Andreasen’s [4, 5].

3.2.6 A Simpler Approach: Independent Interest Rates It is worth contrasting the decomposition of Y in (3.72), whereby Yt⊥

˜

Mt,T Yt Mt,T

= F¯t,T

−1 ,

Y , Y ⊥ independent,

(3.80)

to the approximative one of (2.53), where we had simply Yt ≈ Fˆt,T

M˜ t,T Mt,T

−1 ,

˜ M) independent, Fˆ , (M,

(3.81)

with Fˆ and F¯ having the same marginal laws. Expression (2.54) showed how the inaccuracy brought about by (3.81) can be mitigated somewhat, and in Chap. 6 we will analyse by means of concrete examples the impact this inaccuracy has on pricing. Benhamou [11] analyses the bias introduced by neglecting the stochasticity of interest rates when deriving the Dupire formula.

3.2.7 Different Models for Different Markets Certain models may be better suited to equity markets than to FX markets. For example, equity markets usually have negative skews, whereas FX markets tend to have smiles. In other words, while the market-implied volatilities in equity markets are impacted by skew, those in FX markets are impacted by kurtosis. One possible economic reason for this is that traditional investors in equities can only be long, whereas one can always go short in foreign exchange markets by switching one’s holdings from one currency to another. Figure 3.2 shows typical implied volatilities for equities and foreign exchange markets. Figure 3.3 shows how the shape of the implied volatility surface impacts the distribution of log-FX rates. As described in Chap. 2, inflation markets can be seen as an extension of foreign exchange markets, with the exception that inflation markets are less complete. Indeed, while options on FX rates are commonplace, options on inflation indices

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3 Simulation Models

Fig. 3.2 Typical implied volatilities for Equity Markets (at-the-money skew = −10%, at-the-money convexity = 0%) and Foreign Exchange Markets (at-the-money skew = 0%, at-the-money convexity = 30%, see (3.49))

Fig. 3.3 Implied distributions for Foreign Exchange Markets (above: at-the-money skew = 0%, at-the-money convexity = 30%) and Equity Markets (below: at-the-money skew = −10%, at-the-money convexity = 0%) compared to the standard normal distribution, see (3.49)

are not. Therefore using complex dynamics for inflation indices and real rates may be unnecessary, or even counterproductive since there are not enough liquid instruments available to calibrate the model.

3.3 Credit Models

65

3.3 Credit Models Credit Derivatives are products whose payoff is related to credit quantities such as credit spreads, credit default losses, or rating migrations. For credit derivatives relating to a portfolio of more than one entity, an essential modelling element is the inter-dependence between the spread and default times of the individual entities. Indeed, computing prices and simulating price distributions for credit products is challenging as (i) the choice is not clear as to what the best model is for simulating credit spreads and default times, and (ii) there are several ways of introducing default dependence between different credit entities (see for example Duffie & Singleton [39], Lando [72], or Schönbucher [95]). This section describes a possible model for computing the credit exposure posed by credit-related products. The starting point, as described in Sect. 2.7, is to use CDS term structure to compute values at time zero of the default probabilities for each single reference name. Once this is done, stochastic default probabilities for each reference name can be simulated following some chosen dynamics for the martingales M¯ appearing in Sect. 2.7.2; this is what we do in Sect. 3.3.1 below. We also describe how the volatility parameters in the model can be calibrated to market quotes for options on CDSs. For the dependence between different reference entities, we have chosen to work with a Gaussian dependence structure, as hinted at in (2.72). We detail in this section how we propose to calibrate this dependence structure to quotes on CDO tranches. At this point it is worth highlighting the difficulties one encounters in simulating price distributions of credit portfolios. When faced with the task of pricing correlation-dependent products such as, say, CDO tranches, it is standard practice to match market prices for different tranches by tuning model parameters (usually the correlation input) individually for each tranche.13 Such an approach is useless, however, in a simulation model attempting to produce correct default loss distributions for different tranches simultaneously. Consequently, the correlation structure in our model has no meaning beyond that given to it in the modelling expressions (2.72). In what follows, we consider a portfolio of n defaultable entities, and let Ai , Ri and τ (i) respectively define the nominal amount, the recovery fraction upon default, and the time of default of name i. Recall from (2.74) the portfolio loss process L ≡ (Lt )t≥0 , n n (1 − Ri )Ai 1τ (i) ≤t Ai . (3.82) Lt = i=1

i=1

The quantity (i) qs,t ≡ (1 − p (i) (s, t)) = N τ (i) ≥ t | Fs , τ (i) > s , 13 Often

this parameter is called base correlation by practitioners.

(3.83)

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3 Simulation Models

will denote the probability, based on time-s information, that name i survives beyond time t , having survived until time s < t.

3.3.1 Simulation of Single-Name Default Probabilities and Default Times Here we specify concrete dynamics for the martingale process M¯ in (2.69). We choose a volatility term in the SDE for M¯ that has a separable dependence on time and maturity, and which leads to normally-distributed survival probabilities.14 More specifically, we set M¯ 0,T = q0,T ,

d M¯ t,T = ft gT dWt ,

(3.84)

or, in integral form, M¯ t,T = q0,T + gT Xt ∼ N (q0,T , gT2 Ft2 t), with

Xt =

(3.85)

t

fu dWu ,

(3.86)

0

and with Ft2

=t

−1

t 0

fu2 du

(3.87)

being the normalised variance of the time-changed process X. Note that the separability of the volatility coefficient of M¯ t,T is essential to allow fast access to any martingale value and therefore to any needed quantity (for instance, a par CDS spread). We now look at how to parametrize the functions f and g and calibrate them to prices of options on CDSs. Consider an option, expiring at time T ≡ T0 , to enter into a CDS contract that pays protection on a chosen reference name in return for a fixed strike coupon K at coupon payment dates T1 , T2 , . . . , Tn . Assuming a fixed ¯ the no-arbitrage time-zero price of such an option CDS recovery rate of R ≡ 1 − R, is + n ! " D T ,T i CDSS(0, K, T , Tn ) = E R¯ qT ,Ti−1 − qT ,Ti − αi KqT ,Ti , M¯ T ,T NT i=1

(3.88)

14 We

will see later in this chapter how to put bounds on the proportion of simulated probabilities not in [0, 1].

3.3 Credit Models

67

with αi ≡ (Ti − Ti−1 ). Expressing now the bond DT ,Ti in terms of the martingale MT ,Ti and the survival probability qT ,Ti−1 in terms of M¯ T ,Ti−1 , we get CDSS(0, K, T , Tn ) = E

n DT ,T

i

i=1

NT

+ {R¯ M¯ T ,Ti−1 − (R¯ + αi K)M¯ T ,Ti }

. (3.89)

This formulation allows one to price semi-analytically any CDS option, as long as the expectation in (3.89) can be computed in closed form. This is the case for the normal model in (3.84). Indeed, in (3.89), because M¯ is normally distributed, so is the difference (M¯ T ,Ti−1 − M¯ T ,Ti ). Thus (neglecting rates stochasticity), CDSS(0, K, T , Tn ) = E

n

D0,Ti

√ γi (qi−1 , qi ) + γi (gi−1 , gi )FT T Z

+ ,

i=1

(3.90) where we have abbreviated gi ≡ gTi , qi ≡ q0,Ti , Z ∼ N (0, 1), and where ¯ − (R¯ + αi K)y. γi (x, y) ≡ Rx

(3.91)

The sum appearing in (3.90) is just a normal random variable.15 We can write (3.90) as CDSS(0, K, T , Tn ) = μΦ(μ/η) + ηφ(μ/η),

(3.92)

where μ and η are the mean and standard deviation, respectively, of the sum in (3.90), i.e. μ≡

n

D0,Ti γi (qi−1 , qi )

i=1

η≡

n

D0,Ti |γi (gi−1 , gi )|FT

√

(3.93) T.

i=1

In what we have above, M¯ t,T is a normal variate centered around q0,T . It is desirable to have the martingale M¯ stay within the interval [0, 1] with high probability. This condition puts constraints on the forms one can choose for the volatility functions f and g. In more detail, if we want 0 ≤ M¯ t,T ≤ 1 15 Note

that given a random variable Z ∼ N(0, 1), a ∈ R, and b > 0, we can write E[a ± bZ]+ = aΦ(a/b) + bφ(a/b),

where φ is standard normal density and Φ is its primitive.

(3.94)

68

3 Simulation Models

Fig. 3.4 Quantiles of 5Y CDS spreads resulting from simulating survival probabilities according to the separable model in (3.84). Model parameters for the volatility functions f and g were (κ, κg , σ ) = (0.4, 0, 1)

to be true with probability p, then f and g will need to satisfy √ q0,T + qgT Ft t ≤ 1 √ 0 ≤ q0,T − qgT Ft t,

(3.95)

where q = Φ −1 ( 12 (p + 1)). Both these conditions can be summarised as √ min(q0,T , q¯0,T ) , gT Ft t ≤ q

(3.96)

q¯0,T = 1 − q0,T . Since the right hand side above involves only T , it is preferable to work on the assumption that F is known and then to ensure that gT ≤

min(q0,T , q¯0,T ) √ ; q maxt>0 (Ft t)

(3.97)

one satisfactory choice is gT = σ exp(−κg T )

min(q0,T , q¯0,T ) √ q maxt>0 (Ft t)

(3.98)

with κg ≥ 0 and σ ≤ 1. From (3.84), choosing, then, say, ft = exp(−κt),

κ ≥ 0,

(3.99)

whence tFt2 = (1 − e−2κt )/(2κ)

(≤ (2κ)−1 ),

(3.100)

3.3 Credit Models

we have finally the specific form of g as √ gT = q −1 σ 2κ exp (−κg T )(min(q0,T , q¯0,T )).

69

(3.101)

We can now use this parametrization to compute options on CDS (as described above) and we can derive Black implied volatilities. In this model Black-implied volatilities of CDS option prices can be seen to (i) decrease with option expiry; (ii) increase as a function of maturity of the underlying CDS, then decrease again; (iii) increase with the level of par CDS spread. Figure (3.5) displays implied volatilities for options of different expiries, exercising into CDSs of varying maturities.

Fig. 3.5 Black-implied volatilities for options of various expiries giving the holder the right to enter into CDSs of different maturities. Volatility generally decreases with option expiry (x-axis). It increases to a peak (for the CDS of 5Y maturity) and then decreases again with maturity of the underlying CDS. Model parameters used were (κ, κg , σ ) = (0.4, 0, 1)

3.3.2 Inter-Name Default Dependence In the spirit of the Gaussian dependence model prescribed in (2.72), and in the con¯ we now write down expressions for the joint law text of the normal model for M, of default times of different entities; these are of course a function of the correlation parameters ρ and η in (2.72). In turn, these joint laws will be used to compute semi-analytically the prices of the CDO tranches that are chosen as calibration instruments, allowing calibration of the correlation structure to market information.

70

3 Simulation Models

Consider the martingale process M¯ (i) driving the stochastic default probabilities for reference name i, namely (i) (i) (i) (i) d M¯ t,T = ft gT dWt ,

(i) (i) M¯ 0,T = q0,T .

(3.102)

We will force the M (i) to depend on each other by decomposing the Brownian Motion W (i) as dWt(i) = η(i) · dZt + η¯ (i) dZt(i) ,

(3.103) (i)

as in (2.72). Zt is a d-dimensionalBrownian Motion and Zt is a univariate independent Brownian Motion; η¯ (i) = 1 − η(i) · η(i) . This results in t (i) Xt = fu dWu(i)

0

t

= 0

(i) fu η(i) · dZt + η¯ (i) dZt t η(i) · Z + η¯ (i) Z (i) ,

(i) √

∼ Ft

(3.104)

where tFt2 is the variance of X and where Z, Z (i) now denote random variables √ (i) √ with same laws as Zt / t , Zt / t , respectively. Similarly, we force the uniform variables16 U (i) to depend on each other by setting (3.105) U (i) = 1 − Φ ρ (i) · M + ρ¯ (i) M (i) , again as in (2.72). With this setup, we can now write down the joint law of default times of different entities. Indeed, the cumulative distribution function of the default time of entity i is (i) N τ (i) < t = N M¯ t,t < U (i) (i) (i) (i) = N q0,t + gt Xt ≤ U (i) (i) (i) (i) = N 1 − U (i) ≤ (1 − q0,t ) − gt Xt (i) (i) (i) (i) (i) (p¯ 0,t ≡ 1 − q0,t ) = N Φ ρ (i) · M + ρ¯ (i) M (i) ≤ p¯ 0,t − gt Xt = N Φ ρ (i) · M + ρ¯ (i) M (i) (i) (i) √ (i) tFt η(i) · Z + η¯ (i) Z (i) ≤ p¯ 0,t − gt 16 . . . from

which we insist that default of i happen as soon as M¯ t,t < U (i) is true. . . . (i)

3.3 Credit Models

71

= N ρ (i) · M + ρ¯ (i) M (i) (i) (i) √ (i) η(i) · Z + η¯ (i) Z (i) , ≤ Ψ p¯ 0,t − gt tFt

(3.106)

where Ψ ≡ Φ −1 and, we recall, ρ¯ (i) ≡ 1 − ρ (i) · ρ (i) , η¯ (i) ≡ 1 − η(i) · η(i) . Note that the special case η ≡ 0 (or, equivalently, gt ≡ 0) corresponds to survival probabilities that are not stochastic, so that for each T , (i) (i) (i) M¯ t,T = q0,T = qt,T ,

t ≤ T.

(3.107)

The model we have presented collapses in this case to the familiar static one-factor Gaussian copula model. The reason for decomposing the uniform U (i) and the time-changed process Xt in terms of sums of independent terms is that by conditioning on the values of M and Z, which are common to all reference names i, we can obtain the conditional joint law of default times of several reference names. Indeed, from (3.106), expanding (i) Ψ ≡ Φ −1 to first order around p¯ 0,t , we end up approximating the conditional default probability for name i as N τ (i) < t M, Z (i) (i) ≈ N ρ¯ (i) M (i) + Ψ σ η¯ (i) Z (i) ≤ Ψ (i) − Ψ σ η(i) · Z − ρ (i) · M Ψ (i) − Ψ (i) σ η(i) · Z − ρ (i) · M , (3.108) =Φ 1 [(ρ¯ (i) )2 + σ 2 (η¯ (i) )2 (Ψ (i) )2 ] 2 (i)

where Ψ (i) ≡ Ψ (p¯ 0,t ) indicates the inverse cumulative normal distribution function,

Ψ (i) ≡ Ψ (p¯ 0,t ) is its derivative, easily written in terms of the Gaussian density, and √ where we have abbreviated σ ≡ gt Ft t. Because, by construction, the default times τ (i) are conditionally independent given M and Z, the conditional loss distribution at any time t of a portfolio of names is the distribution of a sum of independent single-name loss distributions. Thus, if L(i) is the cumulative loss process for name i, then n (i) (3.109) N(Lt ∈ (x, x + dx) M, Z) = N Lt ∈ (x, x + dx) M, Z (i)

i=1

is the law of a sum of independent random variates. This law can be computed numerically either using Fourier inversion or, as we describe in Sect. 3.3.3 below, by recursive methods. Having obtained the conditional loss distribution, the full unconditional loss distribution is only an integration step away, since the integral N (Lt ∈ (x, x + dx)) = N (Lt ∈ (x, x + dx) | M = m, Z = z) φ(dm, dz) (3.110)

72

3 Simulation Models

with respect to the gaussian densities of M and Z can be accomplished efficiently using a quadrature method. Knowing the distribution of the portfolio loss, given in (3.110), allows us to compute prices of derivatives on the portfolio loss, in particular CDO tranches, by computing the required expectations numerically.

3.3.3 Technical Note: Recursion The law of a sum of independent discrete random variables can be computed by a simple recursive procedure. Suppose we are given n independent discrete random variables Y1 , Y2 , . . . , Yn , assume that the support of Yi is the set {0, 1, 2, . . . , yi }, i = 1, 2, . . . , n, and let pi (k) = N(Yi = k). Consider the random variables Sj =

j

Yi ,

j = 1, . . . , n.

(3.111)

i=1

#j This has support in {0, 1, . . . , sj := i=1 yi }, and distribution {p(j, k) = N(Sj = k)}. The probabilities p(j, k) can be found by a recursive procedure, as follows: (i) Start with p(0, 0) = 1, p(0, k) = 0, k = 1, 2, . . . , sn . (ii) For each j = 1, . . . , n, compute p(j, k) for each k from 0 to sj : p(j, k) =

k

p(j − 1, i)pj (k − i).

(3.112)

i=0

Note that in the case where the Yi are Bernoulli variables with two possible values, as in the case of reference names that either default or not, all terms but two vanish in the sum (3.112). Now, as we have seen above, conditional on the market variables M and Z, the portfolio loss distribution Lt is a sum of independent distributions, so recursion can be applied once the loss distribution is suitably discretized. To do this, consider the process for the i’th reference name, Lt := A−1 R¯ i Ai 1τ (i) ≤t , (i)

A :=

n

Ai ,

(3.113)

i=1

with Ri = 1 − R¯ i being the fractional recovery for i and Ai the corresponding notional at risk for name i. In terms of this the fractional portfolio loss (3.82) is Lt =

n i=1

(i)

Lt .

(3.114)

3.3 Credit Models

73

We discretize the support set of Lt , namely −1 ¯ Ai Ri for some I ⊆ {1, 2, . . . , n} , A =0∪ z:z=A

(3.115)

i∈I

by choosing a real number and integers ψi and writing A−1 Ai R¯ = ψi + ri ,

i = 1, 2, . . . , n,

(3.116)

with each remainder term ri satisfying ri < . In this way, the random variable Lt , with support A , may be approximated by a discrete random variable of support {0, , 2, . . . , K} for some sufficiently large integer K. In other words, the loss suffered by each reference name upon default, expressed as a fraction of the total portfolio notional A, is an integer multiple of a basic loss quantum, . In particular, the loss suffered by reference name i takes values in {0, ψi } and the number of loss quanta has distribution ⎧ (i) ⎪ ⎨1 − q0,t , k = 0 (i) (3.117) pi (k) = q0,t , k = ψi ⎪ ⎩ 0, otherwise, with the quantities on the right being obtainable from the observed credit spreads for reference name i. Having discretized the support set A , the conditional portfolio loss process n (i) Lt (M, Z) = Lt (M, Z)

(3.118)

i=1

is a sum of discrete independent random variates, whose law can now be obtained using recursion. Clearly, the smaller the value one chooses for , the better will Lt be approximated by the corresponding discretized distribution, but this will necessitate a larger value of the integer K and result in longer computational time.

3.3.4 Properties of the Loss Distribution: Large Homogeneous Portfolio The portfolio loss distribution, and the prices of CDO tranches, can be written in closed form in the special case of a one-factor Gaussian copula model, under the assumptions that the number of names in the portfolio is arbitrarily large. To derive the closed form limiting distribution, we assume that the linear dependence parameter ρ i in (2.72), the recovery fraction Ri , and the default probabilities

74

3 Simulation Models (i)

1 − q0,t are identical for all i. That is, the portfolio is homogeneous, and contains an arbitrarily large number of identical reference names. Denoting by Lˆ the process whose value is the fraction of names that default in the portfolios, Vasicek [105] showed that the law of the time-t loss can be written as ρΦ ¯ −1 (h) − Φ −1 (1 − q0,t ) ˆ , h ∈ [0, 1], 0 ≤ ρ ≤ 1, (3.119) N Lt ≤ h = Φ ρ where we have dropped the now-irrelevant superscripts on the survival probability (i) q0,t and the correlation ρ (i) . Note that for ρ = 0, when defaults happen independently, the above says what we expect from the law of large numbers, namely that the proportion of losses will coincide with the probability (= 1 − q0,t ) that a single name defaults. In the limiting case ρ → 1, Lˆ is a Bernoulli distribution. More generally, for intermediate values of ρ, increasing ρ serves to skew the loss distribution to the right, with the consequence that larger mass is assigned to a larger number of defaults. Figure 3.6 shows this effect. What we infer from this is that the protection value of CDO tranches of the form [0, kd ], the so called equity- or basetranches, will decrease as ρ increases. Conversely, protecting senior tranches of the form [ka , 1] will cost more as ρ increases. For mezzanine tranches with ka and kd strictly different from 0 or 1, the behaviour of the protection price as a function of correlation will depend on the values of ka , kd and ρ. Fig. 3.6 Inverse CDF of Lˆ for a probability of default of 5%, and for various values of correlation ρ ∈ [0, 1]

It is tedious but not to difficult to also write down the price of protection for a CDO tranche in this limiting model. Recall that a CDO tranche is the difference of two options on the portfolio loss, so we need to be able to compute expectations of the form + , (3.120) E Lˆ t − Kˆ where Lˆ t , a random variable with support [0, 1], is the proportion of losses suffered by the portfolio by time t , and where Kˆ ∈ [0, 1] is the call strike. The expecta-

3.4 Choice of Model

tion (3.120) can be shown to equal + ˆ γ ; −ρ¯ , E Lˆ t − Kˆ = Φ2 −Φ −1 (K),

75

(3.121)

with Φ2 (·, ·; η) being the bivariate normal cumulative distribution with correlation ˆ owing to the η. Noticing that the real portfolio loss at time t will be Lt = (1 − R)L, recovery fraction R, we have the price of a call option on the portfolio loss, struck ˆ as at K = (1 − R)K,

+ E (Lt − K)+ = (1 − R)E Lˆ t − Kˆ , (3.122) whence

ˆ γ ; −ρ¯ . E (Lt − K)+ = (1 − R)Φ2 −Φ −1 (K),

(3.123)

3.3.5 Calibration of Correlation We have seen that within the context of a Gaussian dependence model, the expression (3.110) for the portfolio loss distribution can be computed semi-analytically after conditioning on M and Z. Clearly, the loss distribution obtained will depend on the dimensionality of the factors M and Z and on the dependence parameters, ρ (i) and η(i) chosen for each reference entity i. This points to a way for calibrating the chosen dependence model, namely by choosing dependence parameters in such a way that model prices for chosen tranches are sufficiently close to market-observed prices. The elements of M and Z can be thought of as market factors explaining the co-dependence between reference names; for example, geographical region and credit spread level (rating).

3.4 Choice of Model This chapter has described various models for asset and derivatives pricing, placing them in the context of the framework of Chap. 2. What we have built is a hybrid model tailored to price hybrid products whose underlying elements are the transactions in the counterparty portfolio. The particular choice of model for each asset class is driven mainly by a balance between accuracy and simplicity, and to a certain extent by the type of products present in the portfolio. In any case, however, we need to take into account that (i) The goal is the pricing and hedging of counterparty exposure, and accuracy is therefore key. (ii) Scenario consistency, and therefore the simulation of all processes simultaneously, is essential.

76

3 Simulation Models

3.4 Choice of Model

77

(iii) When portfolios are large, a compromise between accuracy and speed of computation needs to be found. (iv) The interest-rate model has to be common across all asset classes. Given the constraint of simultaneous simulation of scenarios for a large number of processes, and the valuation of portfolios of thousands of (exotic and plain vanilla) transactions, we found that one factor models with separable volatility structures (as described in Chap. 2) worked well for our purposes. In our experience, the sophistication of models used under such constraints is less important than having a setup that allows a consistent framework that can be extended in a modular way as new products require.

Chapter 4

Valuation and Sensitivities

Conceptually there are two steps in computing credit exposure: simulation followed by pricing. First, one needs to simulate scenarios from the distribution of the underlying processes that drive the price of the product concerned. Secondly, the price of this product needs to be evaluated at each time in the simulation schedule for each of the simulated scenarios. In the previous chapters we have considered a general simulation framework and we have specified simple models used in practice. The aim of this chapter is about the second step, pricing. In simple cases pricing can be performed in closed form or semi-analytically. If a closed-form valuation, which maps scenarios to price, is not available, then the pricing step needs also to be carried out by simulation. This implies that for products with no closed-form valuation, the problem of computing price distributions entails performing simulations (for pricing) within simulations (of scenarios for the underlying processes), an approach that quickly becomes unfeasible for any reasonable simulation size. American Monte Carlo (AMC) is a simulation technique that has been applied to the problem of pricing financial products with features of early exercise. As we describe later, when used in this way, the AMC method yields not just (an estimate of) the price of the product but also the price distribution at each point in a grid of discretized time-points. This price distribution is exactly what can be exploited for estimating the quantiles that define the level of credit exposure, so that the AMC technique can be applied to problems not just of pricing but also of credit exposure computation. In this chapter we focus on computing exposure using AMC. We briefly describe the theoretical basis of AMC, show different algorithms used in practice, and indicate our choice of algorithm for the computation of credit exposure. Finally, we describe different techniques to compute price sensitivities, which will be key later in Chap. 14, when discussing pricing and hedging counterparty risk. G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0_4, © Springer-Verlag Berlin Heidelberg 2009

79

80

4 Valuation and Sensitivities

4.1 American Monte Carlo: Mathematical Notation and Description When it comes to pricing financial products with very intricate payoff structures, or which depend on several underlying factors with complex dynamics, (Monte Carlo) simulation has become the tool of choice. As long as one has a means of sampling from the random distribution of the underlying drivers, there is virtually no limit to the payoff structures that can be priced. However, applying a simulation method to pricing of products with early-exercise features is not a straightforward task, the reason being that at any point in the life of such a product, the value depends on exercise decisions made at times in the future. In turn, the decision to exercise or not would depend on the perceived value of not exercising as compared to the intrinsic value upon exercise. In principle, this causes the pricing problem for an early-exercise product to mushroom into similar pricing problems on each simulated path, at each time-point considered. The number of simulations required quickly grows enough to thwart any attempt at pricing by straightforward simulation. The technique that has now become known as American Monte Carlo attempts to get around this problem by performing one set of simulations and then estimating (rather than pricing through new simulations) at each point in time the value of not exercising (once this value is known, the task of comparing it with the intrinsic value of exercising is relatively easy). One by-product of pricing by American Monte Carlo1 is that apart from the desired price, the method produces also samples from the price distribution at times between the pricing time and the expiry of the product. This feature makes it well-suited (with modifications) to estimating the counterparty or market risk posed by a particular product, such exposure being based merely on the quantiles of the product price distribution at different times.

4.1.1 Mathematical Formulation We presented in Chap. 2 a definition of credit exposure for a generic product with early-exercise features which we have denoted by P . At the outset, the holder of P is entitled to a cashflow X ≡ (Xt ). We denote by TX the maturity of X, so that Xt = 0 if t > TX . Apart from the cashflows X, P also gives the holder the option to replace, at specific points in time, their entitlement to X with an alternative product, which we call the post-exercise portfolio, denoted by Q, and which has maturity TY so that Q has value zero at times after TY . We write T = {τ1 , τ2 , . . . , τnE } ∪ {∞}

(4.1)

for the set of nE times at which the option may be exercised to give up X in exchange for Q. If exercise happens at τE ∈ T , then the value provided by P until the exercise 1 . . . for

products both with early-exercise and without it. . . .

4.1 American Monte Carlo: Mathematical Notation and Description

81

time is embodied in (2.3), Πtno

= Nt E

τE ∧TX

t

Xu du Ft , Nu

(4.2)

where the superscript on the left indicates that this is the value of the no-exercise flows X. The optimality criterion by which the holder chooses the optimal time, τE∗ , at which to exercise the option, will be defined shortly below. There are several possibilities for the form that the alternative holdings represented by Q may take. (i) Physical Settlement. In this case, the cashflows (Xt ) provided by the noexercise portfolio Π no are replaced by cashflows (Yt )0≤t≤TY changing also the maturity of the transaction from T = TX to T = TY . The price distribution of the trade will take values for all t in [0, TX ∨ TY ]. An example of this type of product is a physically settled swaption. (ii) Cash Settlement is different from physical settlement in that the net present value at time of exercise of all the flows (Yt ) is exchanged at exercise time τE , and the transaction then terminates. The price distribution will take values for all t in [0, τnE ]. An example is a cash settled swaption. (iii) Intrinsic Exercise. Here, the option holder receives the time-τE flow, YτE , and no further cashflows. The price distribution will take values for at most all t in [0, τnE ]. An example is a Bermudan option, or a cancellable swap. (iv) No exercise at all. For this case, we simply set T = {∞} =: T ∞ ,

(4.3)

expressing the fact that exercise will never happen, and therefore that the holder of the no-exercise portfolio Π no will receive flows (Xt ) until expiry time TX . As highlighted in Chap. 2 we can then write the price distribution of product P as,

Vt =

VtP , Q Vt ,

t < τE∗ t ≥ τE∗ .

The first element VtP is given by, Q τE ∧TX X VτE u VtP = Nt sup E du Ft + E Nu NτE t τE ∈Tt

(4.4)

Ft

(t < τE∗ ),

(4.5)

where Tt = {τ ∈ T | τ ≥ t}. Q Vt ,

(4.6)

The second element, can have different formulations depending on the type of callability. In practice, the flows X and Y ensuing from P and Q are not continuous but occur at discrete time points. For simplicity, we will nevertheless consider X

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4 Valuation and Sensitivities

and Y to be defined for any t ≥ 0 and set Xt = 0 (resp. Yt = 0) if X (resp. Y ) provides no cashflow at t . Note that physical and cash settlement provide the same value to the option holder. Exercising intrinsically into the cashflow (Yt ), however, provides less value, since the option holder is then not entitled to flows Yt for t > τE . Q Symbolically, if Vt is the time-t value to the option holder who has exercised at τE < t, we will have, in terms of notation introduced in Chap. 2,

T Πtex = Nt E[ t Y NYuu du | Ft ], for non-intrinsic exercise Q (4.7) Vt = for intrinsic exercise. πtex = Yt 1t=τE , The value of non-intrinsic exercise is unaffected by whether settlement is physical or in cash form, save for the fact that the holder receives the flows (Yu ) in the former case, and the one-off payment Πtex at τE in the latter. At each time t , the holder of the product P attains the value Vt by choosing his exercise time τE∗ ∈ T so as to maximise the net present value of his cashflows. The problem we want to consider is how to evaluate Vt . At each time τi ∈ T where the option holder may potentially exercise his option, the decision whether to exercise or to continue will be based on the information observed in the economy. Formally, then, we suppose that at any time t, the information set for the model consists of a σ -algebra Ft , part of a filtration (Ft )0≤t≤T generated by J underlying stochastic processes, say Ξ ≡ (ξ1 , . . . , ξJ ),

(4.8)

that drive the economy. For our model, these stochastic drivers will be, for instance, the collection of Brownian Motions, which the martingales (Mt,T ) depend on. We also suppose, not unreasonably, that the holder of the option knows at t whether exercise has taken place yet, that is, Ft ⊃ {τE ≤ t}. The price processes VtP and Q Vt are assumed to be adapted to (Ft ). Further, we suppose that there is a vector process Θ ≡ (θ1 , . . . , θnobs ),

(4.9)

which generates a filtration σ (Θ) with σ (Θt ) ⊆ Ft , for each t ∈ [0, T ]. Thus, for instance, θj could be the value process of a market instrument whose value depends on the same underlying factors, Ξ , in the economy as does the price process V P . The instruments whose price processes are the θj are referred to as observables. Their importance will become clearer when we will discuss the optimal decision algorithm within the American Monte Carlo framework. The key is that at any particular time t , we will assume that the decision whether or not to exercise will be driven by time-t values of the observables.2 2 Taken

at face value, this assumption would seem to exclude path-dependent products; in fact it does not. It is perfectly legitimate to take as observable the price process of a path-dependent instrument, and then allow its time-t price to drive time-t exercise decisions.

4.1 American Monte Carlo: Mathematical Notation and Description

83

Pricing of P , and estimation of the optimal exercise rule τE∗ , is via Monte Carlo simulation; to this end we assume we have simulated realisations {ξˆj,k }, (ν)

1 ≤ j ≤ J, 0 ≤ k ≤ K, 1 ≤ ν ≤ n

(4.10)

of the driving factors Ξ in the economy; here, j indexes the j ’th driving factor, k indexes the k’th time-point tk in a partition, say P := {0 = t0 , t1 , . . . , tK = T }

(4.11)

of the time-interval [0, T ], and ν indexes the ν’th simulation out of a total of n. The ˆ serves to indicate a sampled value. Thus, for each tk , we have samples of size n drawn from the distributions of each of the driving factors ξj . Similarly, we assume we also have simulated realisations {θˆm,k }, (ν)

1 ≤ m ≤ nobs , 0 ≤ k ≤ K, 1 ≤ ν ≤ n,

(4.12)

from the laws of the nobs observables θ1 , . . . , θnobs .

4.1.2 Practical Examples Let’s consider some examples that illustrate how these equations should be interpreted in practice.

4.1.2.1 Non Exercisable Trades For a transaction that does not allow early exercise, it is necessary to set, T ≡ T ∞ = {∞}.

(4.13) Q

In principle, we could allow T to be unrestricted and set Vt = −∞ for each t ≥ 0.3 Algorithmically, the first approach is neater, since then one does not need to even consider exercising at any t.

4.1.2.2 Simple Examples with Exercise We now turn to look at how simple trades, a Bermudan put option, a cancellable swap and a European swaption, are represented in this framework. Bermudan Put Option: Consider a 5 year contract on a stock S, which, at given dates (e.g. every year), gives the right to the holder of the option, to sell the stock at a predefined strike K. 3 This

Q

is the case of long callability. In the case of short callability we write Vt = +∞.

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4 Valuation and Sensitivities

The set of exercise dates T is defined as, T = {1, 2, 3, 4, 5} ∪ {∞}.

(4.14)

The no-exercise portfolio is defined as Xt = 0

∀t,

(4.15)

and the exercise portfolio as, K − S, Yt = 0,

if t ∈ {1, 2, . . . , 5} otherwise.

(4.16)

Note that we have written K − S and not (K − S)+ as the optimal exercise strategy is performed by the decision algorithm. Cancellable Swap: Consider a contract in which we start off with a 10-year swap with unit notional where we pay yearly coupons of 5% per annum and receive the 12-month libor fixed a year in advance, and in which we have the option to cancel the swap after 5 years, each year, for a fixed fee of 1%. In this case, the non-exercise payoffs (Xt ) would be defined as: Xt =

Lt−1 [t − 1, t] − 0.05, if t ∈ {1, 2, . . . , 10} 0, otherwise.

(4.17)

The set of exercise dates T is defined as: T = {5, 6, 7, 8, 9, 10} ∪ {∞}. As for the exercise portfolio, the cash flows (Yt ) are defined as −0.01, if t ∈ {5, 6, . . . , 10} Yt = 0, otherwise,

(4.18)

(4.19)

to reflect the penalty due at time of exercise. Given that there is a fixed one-off penalty upon exercise, the transaction has an intrinsic optionality feature, with Q

Vt = πtex = Yt 1t=τE ,

t ∈T .

(4.20)

Physically Settled European Swaption: Consider a contract where we have the right but not the obligation to enter in 5 years’ time, into a 5-year swap of unit notional, in which we would pay yearly coupons of 5% per annum and receive the 12-month libor rate. If, in 5 years’ time, we decide to exercise, then we are subject to market and counterparty risk for the remaining 5 years of existence of the swap. Otherwise, the trade terminates with no further exchange of cashflows.

4.1 American Monte Carlo: Mathematical Notation and Description

85

In this example, the set of exercise dates is: T = {5} ∪ {∞}.

(4.21)

Xt = 0,

(4.22)

The non-exercise payoffs X are t ≥ 0.

For the exercise payoffs, we have Lt−1 [t − 1, t] − 0.05, if t ∈ {6, 7, . . . , 10} Yt = 0, otherwise.

(4.23)

Choosing to exercise entails entering into the swap, so that we have non-intrinsic optionality with 10 Yt Q i Ft , t ≥ 0. Vt = Nt E (4.24) N t ≥t ti i

In particular, we have Q

V5 = A5 (S5 − 0.05),

(4.25)

where At is the annuity at t of a 5-year swap with yearly coupons, and St is the par rate of such a swap. Hence the optimisation program (4.5) reduces to V5 = max (0, A5 (S5 − 0.05)) , and

A5 (S5 − 0.05)+ Vt = Nt E N5

Ft ,

t ∈ [0, 5].

(4.26)

(4.27)

Many more types of transactions will be considered in detail at a later stage of this book.

4.1.3 Backward Induction Algorithm There are several approaches that may be employed to compute the optimal exercise decision rule. In general a recursive procedure is used. This involves estimating at each time step tk the expected value of not exercising, conditional (on not having exercised prior to time tk and) on the time-tk value of the observables θj . The base case for the induction is the point in time where the prices of both products P and Q are trivial. This happens at time T ≡ TX ∨ TY , after which X and

86

4 Valuation and Sensitivities

Y are both identically zero by definition. At this time, we have (ν) Q (Base case) VˆT = max (VˆTP )(ν) , (VT )(ν) 1T ∈T + (VˆTP )(ν) 1T ∈T / (ν) (ν) 1T ∈T ≡ max Xˆ T Ξˆ T , YˆT Ξˆ T (ν) + Xˆ T Ξˆ T 1T ∈T (4.28) / , 1 ≤ ν ≤ n, where the ˆ indicates sampled/estimated values and where we have made explicit that the payoffs XT and YT depend on the samples of the underlying driving factor Ξˆ T . For valuation times tk < T , we proceed inductively. Suppose that exercise has not happened prior to tk , and write Ftk for the expected value that would be gained by an agent who does not exercise at time tk , but who follows the optimal strategy at times after tk . By our assumptions, the expected value Ftk , (the continuation value), is a function of the time-tk observables Θtk : Ftk ≡ Ftk (Θtk ).

(4.29)

In practice, what we have is a finite sample of size n from the distribution of the time-tk observables Θtk , and from this we can hope to get a sample of size n from the law of the conditional-expected non-exercise value, Ftk (Θtk ): Fˆtk Fˆt ≡ k (Θˆ tk ) N tk N tk tk+1 Xu du Θˆ tk := E Nu tk τE ∧T N Ntk+1 Q 1 tk+1 + sup Xu du + VτE 1τE ≤T Θˆ tk E . Ntk+1 tk+1 Nu NτE τE ∈Tt k+1

(4.30) Again, the ˆ indicates sampled or estimated values.4 The expectation in the first line in the above expression is merely the value accumulated from hesitating (at time tk ) to exercise for one more time step (until tk+1 ). The remaining terms constitute the value to be gained from following the optimal exercise rule from time tk+1 onwards. To see this, note that the term in {} becomes

τE ∧T N Ntk+1 Q 1 tk+1 ˆ ˆ sup E Xu du + V 1τ ≤T Θt E Θt Ntk+1 Nu NτE τE E k+1 k tk+1 τE ∈Tt k+1

4 For

clarity, we have suppressed the explicit ν superscript indexing simulations, but (4.30) consists of n equations. In particular, the value Fˆtk is of course not the exact solution to the decision problem, as the supremum and expectation are obtained numerically from sampled values.

4.1 American Monte Carlo: Mathematical Notation and Description

=E

1 Ntk+1

E sup

τE ∈Ttk+1

τE ∧T tk+1

Nt Ntk+1 Xu du + k+1 VτQE 1τE ≤T Nu NE

87

ˆ ˆ Θtk . Θtk+1

(4.31) Notice that we have used here the so-called ‘tower-law’ of conditional expectations. Taking the Θtk -conditional expectation outside the sup operator is legitimate, because (and only because) Θtk is irrelevant to the maximisation conditional on Θtk+1 -information. Finally, invoking the definition (4.5), we translate (4.31) to E

1 Ntk+1

ˆ Vtk+1 (Θˆ tk+1 ) Θˆ tk .

(4.32)

Putting everything together, we can re-write (4.30) as ˆ tk+1 Vtk+1 Fˆtk Xu Fˆtk ˆ ˆ ˆ ˆ ≡ (Θtk ) = E du Θtk + E (Θtk+1 ) Θtk . N tk N tk Nu Ntk+1 tk

(4.33)

Recall that Fˆtk represents the ν’th estimate ‘drawn’ from the time-tk value distribution of P, conditional on exercise not having happened prior to tk and conditional also on it not happening at tk .5 In order to obtain the time-tk value Vˆtk it remains to decide whether exercising at tk results in value larger than Fˆtk , and to set (ν)

Q Vˆtk ≡ Vˆtk (Θˆ tk ) = max Vtk , Fˆtk ,

(4.34)

where Vtk (resp. Fˆtk ) denotes the estimated value of exercising (resp. not exercising) at time tk . This completes the inductive step to be made at time tk . In the case of short optionality, where the holder of the option is the payer and not the receiver of the cashflows X and Y pertaining to P and Q, (4.34) becomes Q

Q Vˆtk ≡ Vˆtk (Θˆ tk ) = min Vtk , Fˆtk .

(4.35)

This inductive step is then repeated until all time points Tk in the partition P are exhausted. One fine point to mention is the following. The value process V P is defined such that VtP is the value of P at time t, conditional on exercise not having happened prior to t. Now suppose that while performing the backward recursion, it is deemed optimal for the ν’th simulated path, to exercise at time tk . It may then happen that at some later step in the recursion (and so at an earlier time tj ), it is also deemed optimal to exercise at tj . By taking the exercise time τE to be the earliest time at which the recursion deems it optimal to exercise, which we assume to be the

5 In

(4.33) we have again suppressed the superscript indices (ν) for readability.

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4 Valuation and Sensitivities

case in the sequel, one ends up with a unique exercise strategy: Q τE ∧TX X VτE u ∗ du Ft + E τˆE = inf t ∈ T |Vˆt = sup E N N u τE t τE ∈Tt

Ft .

(4.36) In principle, computing the second conditional expectation appearing in (4.33) requires one to perform further simulations—effectively to repeat the pricing problem at each time point, and for each of the n sample values of the observables. As already mentioned, this quickly becomes an unfeasible task, and an alternative approach needs to be employed. Different approaches to Monte Carlo pricing hinge on different ways of estimating the second conditional expectation term in (4.33). There are many computational algorithms described in the literature. We move now to describing some of these approaches in details. We will then focus on our specific implementation for credit exposure computation.

4.2 AMC Estimation Algorithms As we concluded in the previous section, it is necessary to find clever ways to estimate the conditional-expectation function Vtk+1 Θtk → E (4.37) Θ t Ntk+1 k appearing in (4.33), where Θ are the so called observables. This is generally done using heuristics which have shown to work well in practice. We now describe some approaches described in the literature that have been used to accomplish this, namely the Tilley [103] and the Longstaff-Schwartz [76] algorithm. The approach we employ to compute counterparty credit exposure is a modification of the regression estimation of Longstaff and Schwartz in combination with Tilley’s bundling algorithm. In all these heuristics the idea is to approximate with simple functions the continuation value and to base the decision algorithm on these approximations. This can be achieved by interpolation methods (as in the Longstaff-Schwarz algorithm) or by splitting (bundling) the domain (as in Tilley algorithm). We will see that these two approaches can be used together to obtain an efficient heuristics which compute exposure for most of the products. In the next sections we will first analyse Tilley’s algorithm, as historically this was one of the first attempts to price American options.

4.2.1 Tilley’s Algorithm Tilley’s algorithm was initially designed for the particular example of American options written on a single underlying stock that pays no dividends. The algorithms

4.2 AMC Estimation Algorithms

89

take into account only one observable Θ ≡ θ , and starts by first sorting the values of the observables {θ (ν) } and then classifying the samples into a chosen number of equally sized bundles. The conditional expectation (4.37) is then estimated as the sample mean for each given bundle, that is, for each sample ν in some chosen bundle B, Tilley sets Fˆt(ν) k (ν)

N tk

=

(ν) 1 Vˆtk+1 , (ν) B ν∈B Ntk+1

(4.38)

to be the estimated value of not exercising at time tk , with B being the number of elements in the bundle B. Thus, Tilley’s algorithm uses the information from θ to partition the estimation set into different bundles. As we will see in the next section, an alternative approach is to regress the continuation value against the observables. Thus, Tilley’s method is akin to fitting a piecewise-linear function to a non-linear data set. We will see that the Longstaff-Schwartz approach fits a non-linear function to the entire data set. Notice also that there is more than one way of choosing the first path ν ∗ for which exercise is optimal; in his original paper, Tilley proposes one particular rule to choose a unique ν ∗ .

4.2.2 Longstaff-Schwartz Regression Longstaff and Schwartz [76] put forward an algorithm that models the continuation (ν) value Fˆtk at each time tk , as a regression on the time-tk value Θtk of the chosen observable of the discounted values computed at time tk+1 . The idea is that, at time tk , the sample conditional expected value of not exercising (that is the continuation value Fˆtk (Θˆ tk ) in (4.31)) can be expressed as a linear combination of basis functions of the time-tk observables. For this reason this algorithm is also called regression algorithm. These basis functions are generally polynomials. Thus, Ftk (Θtk ) := aj Lj (Θtk ). (4.39) j

Here it is supposed that the conditional expectation function is in a space that is spanned by the basis functions {Lj }, j = 1, 2, . . . . Notice that in general, Θ is an nobs -vector so that each Lj maps Rnobs to R. The fit coefficients aj are estimated through regression, as described below. For the Longstaff-Schwartz algorithm, the choice that needs to be made is what number of basis functions, μ say, one should use. Then, at the k’th time step, the regression coefficients (aj ) are estimated by regressing the n sample discounted values (ν)

Ntk

(ν) Ntk+1

(ν) Vˆtk+1 ,

ν = 1, 2, . . . , n,

(4.40)

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on the first μ basis functions evaluated on the sampled observables, L1 (Θˆ tk ), . . . , Lμ (Θˆ tk ), (ν)

(ν)

ν = 1, 2, . . . , n.

(4.41)

The regression carried out by the authors used only those sample values ν which are in the money. In particular, for the American put example considered by Longstaff (ν) and Schwartz, the time-tk regression used only those sample values Stk < K of the stock price for which exercise would yield non-zero payoff.

4.2.3 Biases of Estimates The Tilley estimation algorithm just described yields, of course, only an approximate solution to problem of pricing a product P with Bermudan exercise6 allowed at any of the time points τk ∈ T . The end result obtained is influenced by both upward and downward biases that arise from the necessity to use a finite number of simulations (see also Hyer [65]). (i) Granularity bias is a bias that arises because bundles are ‘too big’. The cause of this bias is that the conditional-expected value of not exercising is identical for all paths in the same bundle. Because the same non-exercise value is used for each path in a given bundle, the estimated exercise rule for paths in that bundle will be sub-optimal, causing a downward bias in estimated price. Granularity bias would be eliminated if each path were itself a bundle. (ii) Small-sample bias is that arising because bundles are ‘too small’. The fact that each bundle contains only a small finite number of paths causes the algorithm to work out a sub-optimal exercise rule, again causing a downward bias in price. (iii) Look-back bias is that arising because the same set of N paths is used in estimating the optimal decision rule as is used to compute the value yielded by that rule. Tilley showed by example that this type of bias results in an upward bias in the estimated price; he did this by comparing the value estimated by his algorithm to the value one obtains when using the same set of paths, but employing the exact known optimal exercise strategy. The presence of an upward bias may at first seem contradictory, as by definition no estimated exercise strategy can dominate the optimal one. Consider, however, a situation in which each bundle consists of a single path, and pick some path (equivalently, bundle) η. Then, the backward induction algorithm would compute (η) (η) VˆT = YˆT 6 If the product P allows American exercise, the need to discretize forces us to model a corresponding product with Bermudan exercise features; we will not discuss inaccuracies arising due to this. What we refer to in this paragraph, rather, are the discrepancies between the true and computed values for the problem of pricing a product P with Bermudan exercise features.

4.2 AMC Estimation Algorithms

91

N tk (η) (η) Fˆtk = E[Vˆtk+1 |Θtk ], Ntk+1

k = K, K − 1, . . . , 0

(η) (η) Q Vˆtk = max{Vtk , Fˆtk }.

(4.42)

Thus, the exercise rule for the path η would depend on the simulated values of that path alone. The time-zero price would then be the expected value over all (η) paths of the terminal payoff on each path, VˆT , discounted back and replaced by the exercise value whenever the latter is larger. Mathematically, the look-back bias arises because of the convexity of the max operator and Jensen’s inequality—the price is estimated as the expectation of maxima of per-path future values, rather than (correctly) as the maximum of expected values over all possible choices of exercise strategy.

4.2.4 An AMC Algorithm to Compute Credit Exposure The drawback of Tilley’s approach is in its use of a single observable to carry out bundling. In practice, transactions will depend on several underlying variables, each of which should be considered in partitioning the sampled paths. On the other hand, the Longstaff and Schwartz method computes a regression to estimate the continuation value Fˆtk , but does not employ bundling. A possible extension is a combination of modified algorithms based on these two basic ideas (see also Hyer [66]). Furthermore, motivated by Sect. 4.2.3, we introduce a bias correction device to improve the quality of estimates Fˆtk .

4.2.4.1 Recursive Bundling Tilley defines bundles by classifying the number of simulation paths according to the level of a single observable. We generalise this notion by bundling recursively on all the nobs observables in the process Θ. To see how this can be accomplished, consider a particular time point tk . Choosing integers m1 , m2 , . . . , mnobs ,

(4.43)

we start by classifying paths into m1 bundles based on the level of the observable θ1,tk . Each of these is then subdivided into m2 bundles using the level of the observable θ2,tk . The procedure is then repeated for all observables, resulting in a total of m1 m2 . . . mnobs bundles. The reason for performing bundling is to classify the simulation paths into subsets such that for any two paths in a particular bundle, all observables have similar values. For observables with continuous distributions, this is accomplished well enough by the above recipe, which allocates paths equally across bundles. However,

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for a discontinuous observable such as, for instance, that corresponding to the default indicator of a reference credit, equal allocation can result in two paths with the same values for an observable being in different bundles. To prevent this, we perform an additional clustering check, and shift paths from one bundle to another, if by doing so the distance between the particular path and its closest neighbour in the bundle is reduced.

4.2.4.2 Regression We adapt the Longstaff Schwartz method by performing regression on each bundle. Thus, for a typical bundle, B, say, we find parameters a and b that fit the model Vˆtk = E

N tk ˆ Vt Ntk+1 k+1

J L j Ft = a θ + bl θl+1,tk + εtk , j 1,tk k j =0

(4.44)

l=0

where εtk ∼ N (0, σt2k ), and εtj is independent of any εtk for j = k. What we are doing here, then, is to estimate the conditional expectation as a polynomial of order J in the first observable and a linear function in the remaining L + 1 observables. Note that cashflows paid at time tk are not included in the regression step, as these are known quantities.

4.2.4.3 Bias Correction We pointed out, in Sect. 4.2.3, that several biases arise in the Tilley estimation algorithm. Fries [46] describes how the look-back bias (which he refers to as foresight bias) can be removed analytically. Consider our regression model (4.44). For each tk , this models the conditional expectation function Vtk+1 Θtk → E Θt (4.45) Ntk+1 k as Θtk → f (Θtk ).

(4.46)

Suppose that ε ∼ N (0, σ 2 ) is the Monte Carlo error in the estimator f , so that Vtk+1 Θ ) = E + ε. (4.47) f (Θtk t Ntk+1 k If K is the intrinsic value of exercising at time tk , then look-back bias arises from Jensen’s inequality, because the value E[max(K, f (Θtk ))]

(4.48)

4.3 Post-Processing of the Price Distribution

93

of the exercise strategy computed by the algorithm differs from the theoretical value Vtk+1 Θt . (4.49) max K, E Ntk+1 k Analytical removal of the bias is possible once we note that for any real a, b, and for ε ∼ N(0, σ 2 ), we have E max(a, b + ε) = σ φ(η) + ησ Φ(η) + a, (4.50) with η ≡ (b − a)/σ . Similarly, for short optionality, the bias correction can be removed using: E min(a, b + ε) = −E max(−a, −b − ε) . (4.51)

4.3 Post-Processing of the Price Distribution The inductive procedure outlined in Sect. 4.1.3 provides, at each valuation / decision time tk , an (estimate) of whether exercise at tk is optimal if it has not happened prior to tk . In order to obtain the correct estimate for the value of the problem, then, one needs to locate the earliest time τE∗ at which exercise has been estimated to be optimal to the alternative of continuing. Symbolically, for each path ν, we set (ν) (ν)∗ Q Q (4.52) τE = inf tk ∈ T Vˆtk = max Fˆtk , Vtk = Vtk . Following this, we then have, for each tk ≥ τE∗ , ⎧ Q ∗ for non-intrinsic cash settlement ⎪ ⎨Vtk 1tk =τE , Q Vˆtk = Vt , for non-intrinsic physical settlement ⎪ ⎩ k πtk = Ytk 1tk =τE∗ , for intrinsic exercise,

(4.53)

where we have, for ease of notation, suppressed the superscript (ν) indexing paths.

4.4 Practical Examples Revisited We revisit here our two illustrative examples from Sect. 4.1.2 in order to show how our algorithm would work in a concrete setting. Cancellable Swap: In this example, we considered a cancellable swap where the holder has the option, at each τk ∈ T , to exit the transaction for a fixed penalty. Clearly, then, at each τk , it would be rational to exercise the option if the estimated value of continuing to receive the swap payments is less than the reward Yτk = −0.01 suffered from cancellation.

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The observables Θ, on which the estimation of the conditional expectation is based, should be chosen while keeping in mind the underlying variable in the trade. In this case, quantities such as the Libor rate or the fair swap rate are valid observables. Using these, the backward induction would, at each time τk estimate the value of the cancellable swap assuming exercise has not previously taken place, and would decide to cancel the swap if its value is lower than the exercise penalty. Physically Settled European Swaption: Recall that for the swaption example, the only allowable exercise time was tk = 5. If we assume that the value of the swap annuity, At , and the fair swap rate, St , are known at each t, then the decision needs to be made only at tk = 5, whether entering into the swap (with value A5 (S5 − 0.05)), has value that exceeds the zero-value of the strategy of allowing the swaption to expire unexercised. At times t prior to tk = 5, the swaption has continuation value Fˆt which will be estimated using regression and the chosen observables. Again in this case, since the trade depends on the fair value swap rate S, it makes sense to choose this and / or related quantities as the observables Θ in the estimation.

4.5 Computing Price Sensitivities No valuation framework would be complete without the capability of computing the sensitivities of a trade to its underlying risk drivers. The usual definition of a price sensitivity is the partial derivative of the price with respect to a given risk driver (keeping all remaining risk drivers constant). Let (ξ (i) ) denote the stochastic processes driving the pricing of a product and for which we want to evaluate the sensitivities. Examples of risk drivers are the stock price, the FX rate, the swap rate, the zero rate, or the volatility level. If V0 = V (Ξ0 ) = V (ξ (i) (0), . . . , ξ (n) (0)) is the time-zero price of the transaction as an explicit function of the underlying risk drivers, then it is usual to define delta of the trade with respect to ξ (i) as ∂V , (4.54) Δ(i) := (i) ∂ξ Ξ =Ξ0 the partial derivative evaluated at the time-zero value of the risk drivers. Similarly, the gamma of the trade with respect to ξ (i) and the cross-gamma of the trade with respect to ξ (i) and ξ (j ) are ∂ 2 V (i) (4.55) Γ := ∂(ξ (i) )2 Ξ =Ξ0 and Γ

(i,j )

∂ 2V := (j ) (j ) . ∂ξ ∂ξ Ξ =Ξ0

(4.56)

4.5 Computing Price Sensitivities

95

4.5.1 The Classical Approach In the banking industry, the standard way of computing price sensitivities is via a finite-difference approximation,7 that is, modifying the set of market data to produce a small change in the value of the risk driver of interest and then re-valuing the trade.8 There are two obvious problems which can arise from this methodology. (i) Computational Speed. Computing finite differences requires a full revaluation of the price distribution for each change in the market data. Thus, the first and second derivative of a product with respect to only one risk driver require already tripling the computation effort. (ii) Flexibility. It may not be possible to perturb a chosen risk driver while keeping all other drivers constant. An example is the swap rate which is the combination of several stochastic quantities. This constrains the set of drivers for which it is possible to specify sensitivities.

4.5.2 Price Sensitivities through Regression Within our framework, we can estimate price sensitivities at almost no extra computational cost. The idea is to regress incremental change in values of the price distribution against corresponding incremental values of the risk driver of interest. In detail, choose ε > 0 to be small and write ΔV = Vε − V0 ≡ V (Ξ (ε)) − V (Ξ (0)).

(4.57)

Assuming that a sample of the price distribution at time ε is available, (which we can ensure at valuation stage), and choosing some ξ (i) to be the risk-driver of interest, we write ΔV as a polynomial in Δξ (i) = ξ (i) (ε) − ξ (i) (0), ΔV =

m

k ak Δξ (i) .

(4.58)

k=0

Regression gives us estimates of the weights ak , which, in turn, enable us to compute the k partial derivatives dkV = k!ak . d(ξ (i) )k

(4.59)

It is important to note that this is the total derivative of the transaction value with respect to the chosen ξ (i) , and that ξ (i) may itself be correlated to other risk factors 7 There are several ways to compute sensitivities. For a survey see for example the book by Glassermann [50]. 8 Practitioners

often refer to this technique as bumping.

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that affect V . Obtaining the true partial derivative of V with respect to a chosen factor, eliminating the effect of correlation between the risk factors themselves, is the subject of the next section. By way of illustration, Table 4.1 compares the prices of European options and their sensitivities—obtained through AMC with 10,000 paths—to their analytical Black-Scholes values. The errors are typically less than the difference in price resulting from a change of one volatility point. Table 4.1 Performance of AMC for price and Greeks on one-year European options. S = 100, r = 2.95%, σ = 20% Type

BSPrice

AMCPrice

BSDelta

AMCDelta

BSGamma

AMCGamma

Call @ 105

7.106

7.100

0.501

0.494

0.0199

0.0198

Call @ 100

9.388

9.386

0.597

0.588

0.0193

0.0193

Call @ 95

12.151

12.153

0.693

0.682

0.0176

0.0176

Put @ 95

4.389

4.366

−0.307

−0.301

0.0176

0.0167

Put @ 100

6.481

6.454

−0.402

−0.394

0.0193

0.0184

Put @ 105

9.054

9.024

−0.499

−0.489

0.0199

0.0189

4.5.3 Removing Correlation As we saw above, sensitivities to a risk driver ξ (i) computed through regression implicitly contain sensitivity also to other risk drivers that are correlated to ξ (i) . While this is useful in the case where we compute only one sensitivity, since it can give more information on the risk of the trade (and therefore better hedges), it is an undesirable effect if we want to use the sensitivities as a tool to explain daily changes of profit and loss (P&L) of a business. It also becomes undesirable as soon as we are interested in sensitivities to more than one risk driver. With some assumptions it is possible, however, to remove this correlation effect and to produce decorrelated sensitivities, that is, sensitivities with respect to a chosen risk driver while keeping all remaining risk drivers constant. To this end, suppose we have m simulated values of each of the n risk drivers ξ (i) , i = 1, . . . , n, ˆ be the m × n matrix defined by9 and let X (i) Xˆ j,i = Δξj ,

i = 1, . . . , n, j = 1, . . . , m,

(4.60)

where the ˆ indicates a sampled (simulated) value and the superscript indicates the ˆ is a sample of size m from the distribusample index. By this, the i’th column of X (i) tion of Δξ , the incremental change in the i’th risk driver. In what follows below, 9 See

Sect. 2.3.1 to clarify notation.

4.5 Computing Price Sensitivities

97

we make the reasonably accurate assumption that all incremental moves X in the risk drivers (which happen over a very short time period ε) are normally distributed with mean zero.10 Now consider the estimation of a linear model for Y ≡ ΔV in terms of the risk drivers Ξ , ˆ = Xα ˆ + εˆ , Y

(ˆε ∼ N (0, Σ)),

(4.61)

ˆ is a vector containing a sample of size m from the distribution of ΔV . The where Y least-squares estimate of α in this model is given by ˆ T X) ˆ −1 X ˆ T Y. ˆ α = (X

(4.62)

Compare this to the corresponding estimate we would get for α if a univariate regression were to be performed on just one of the risk drivers ξ (i) , namely ˆ TX ˆ −1 ˆ T ˆ α˜ j = (X j j ) Xj Y,

j = 1, . . . , n,

(4.63)

ˆ Putting together the estimates for α˜ = ˆ j is the j ’th column of X. where X (α˜ 1 , . . . , α˜ n ), we have ˆ T Y, ˆ α˜ = D−1 X

(4.64)

where D is a diagonal matrix whose (i, i)’th entry is a multiple of an unbiased estimator of the variance of ξ (i) , namely ˆi ·X ˆ i =: (m − 1)Var(ξ (i) ). Di,i = X

(4.65)

All this means that α˜ and α are related by ˆ T X)α. ˆ α˜ = D−1 (X

(4.66)

ˆ TX ˆ has entries In the above note that X ˆ T X] ˆi ·X ˆ j =: (m − 1)Cov(ξ ˆ i,j = X (i) , ξ (j ) ), [X

(4.67)

and is therefore simply a multiple of an unbiased estimate of the covariance matrix ˆ are equal. ˆ TX of Ξ . In particular, the diagonal elements of D and X ˜ as estimates derived from two different linear modThe significance of α and α, els for Y , is that the components of α represent partial derivatives to the ξ (i) while ˜ obtained by regression on ξ (i) alone, represent full derivatives with rethose of α, spect to ξ (i) . The expression (4.66) therefore allows us to obtain the true partial derivatives in terms of the correlated sensitivities computed from regression. To evaluate the accuracy of this methodology and the effect of correlation, consider the following simple example: suppose that we enter into a trade which pays 10 Recall that we have defined our framework so that all risk drivers are derived from simulated Brownian Motions.

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us 100 mEUR on October 20, 2015 and in which we pay 100 mGBP on October 20, 2010. Table 4.2 summarises the deltas11 (in terms of percentage point moves) obtained by bumping, regression and decorrelation, expressed in USD, along with the difference in computational time required for the three methods. We can see that correlated deltas are significantly different from the de-correlated ones. This example also suggests that the accuracy of the decorrelated deltas compared with the deltas obtained via numerical differentiation is acceptable, given the benefit of the higher computational speed achieved. In practice, before using this methodology on a large portfolio, it is necessary to carefully assess its accuracy for different types of products and over different time horizons. One possibility is to predict portfolio movements using sensitivities, and compare the results either with historical valuations, or with full revaluation of the portfolio.12 Table 4.2 Comparison between deltas computed by finite difference and by regression using correlated and decorrelated method. Simulation has been experimented on a desktop Intel Core 2.13 GHz machine Risk Driver

EUR Rates EURUSD FX Rate

Delta Finite Difference

Correlated

Decorrelated

−6,933,027

−1,989,992

−7,177,312

824,200

278,850

892,464

GBP Rates

2,128,061

−1,881,813

2,355,130

GBPUSD FX Rate

−973,972

−620,948

−1,046,550

Computation Time

4.27 s

1.06 s

1.09 s

4.6 Extensions American Monte Carlo valuation techniques have been analysed in various papers. We have already mentioned the Longstaff-Schwartz [76] and the Tilley [103] algorithm. Haugh & Kogan [59] and Rogers [91] introduced a dual method for pricing American options, providing an upper bound for the price of the option. Andersen & Broadie [2], Broadie & Glasserman [20], and Broadie & Cao [19] further develop this methodology obtaining both an upper and lower bound for the Bermudan option price.

11 For 12 In

interest rate deltas, we have considered a parallel-shift type of delta.

the financial industry this procedure is often called P&L explain.

Part II

Architecture and Implementation

Chapter 5

Computational Framework

In Part I we described a general framework that allows the specification of models for different asset classes, and we showed how the AMC valuation technique gives the possibility of computing price distributions, hence estimating counterparty exposure. Our goal is now to show how this mathematical framework can be naturally translated into a computational framework that will enable the computation of exposure in a systematic way for all types of products across the asset classes we provided models for. The basic ideas we highlight in this chapter will lead to the description of a basic software architecture, which can be used to address typical integration problems that large financial institutions face. The motivation for many of the challenges we consider in this and the following chapters, as well as many of the choices we take, will become clearer in Part IV, where the computation, controlling, and hedging of exposure, will be done at counterparty and not just at trade level.

5.1 AMC Implementation and Trade Representation Recall the basic principles of the AMC algorithm. Products are described by defining the cashflows Xt the holder a product P is entitled to, the cashflows Yt of an alternative product Q, which could replace P on a predefined set of exercise dates T (which could include T ∞ ), and the exercise strategy τE . In practice the cashflows X and Y will be defined on a set of dates S = {T1 , T2 , . . . , Tn }, called event dates, which in general contain T . With this information we can fully describe the product and then, using the AMC backward induction process, estimate prices along different scenario paths. To proceed a step further, at this point one remark is key. When we use the AMC algorithm we do not need to explicitly define products. A product is implicitly described via (i) the cashflows X, paid before exercise, and the cashflows Y of the exercise portfolio, paid on the event dates S , G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0_5, © Springer-Verlag Berlin Heidelberg 2009

101

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(ii) the set of exercise dates T , and (iii) the type of exercise. If we find a way to describe and process this information systematically, we have provided a computational framework that gives the capability of computing exposure for all types of products without knowing a priori the product type.1 Consider the cashflows X and Y defined on S . In general they are expressed as functions of financial quantities, such as Libor rates, FX values, or stock prices. These functions can be relatively complex, depending on the nature of the product. To be able to describe them we need to have basic building blocks, which we can combine to obtain the desired results. These building blocks, which we call statistics, are functions of the simulated paths and can be combined together using mathematical functions. We could have for example a statistic that extracts the Libor rate observed at a certain date t over a time interval (T1 , T2 ), or a statistic that computes the swap rate, or the average stock price over a period of time, and functions that e.g. compare values, provide their max or min, and multiply or divide them. With the description of cashflows X and Y , and with a mechanism to choose at predefined dates T if we want to exercise or continue, we can now compute counterparty exposure for generic products. From a computational point of view we simply need to compare values computed via the backward induction steps with predefined cashflows. We have turned the problem from defining individual products, to describing features of products.

5.1.1 Examples To clarify these points consider again the examples we have shown in the previous chapter. Non-exercisable products: To define products that cannot be exercised we need to describe only X. As a concrete example consider an up-and-out option on a stock struck at K and knocking out at H . This product tracks a stock price and its running maximum. If its value does not exceed the barrier H it will pay at maturity the difference between the stock price and the strike K, provided that its value is positive. Mathematically we can describe its payoff as XT = (ST − K)+ 1maxu∈[0,T ] Su ≤H .

(5.1)

To describe (S −K)+ we need a statistic that, given a Brownian path of the stock, extracts the value of the stock price S at maturity T , and two mathematical functions, one performing differences and one finding the max between values. To describe the second part of the payoff (related to the barrier), we need a statistic that computes 1 . . . provided that the underlying model is appropriate to describe the features of that class of products.

5.2 A Portfolio Aggregation Language

103

the extremum value of the stock price between 0 and t , and an indicator function that returns zero or one depending on the extremum and the barrier value. If we implement these statistics we can compute the payoff of this product at maturity for each underlying path of the stock, and then rely on AMC to estimate intermediate prices from maturity to trade inception, and thus to compute counterparty credit exposure. Other common examples of non-exercisable products are vanilla swaps. In this case the value of the cashflows X defined at coupon dates Ti are α(Lt,Ti−1 ,Ti − c), with α being the day count fraction, c the fixed rate, and L the statistic that provides the Libor rate paid over a time interval. The computational mechanism is the same, using a statistic that provides the Libor rate, describes the payoff at each payment date and then uses AMC to evaluate intermediate prices. Cancellable Swaps: As we have seen, cancellable swaps are products with intrinsic optionality. We need to describe at each t in T not only X, but also Y . When we exercise, however, we only need to replace the value of X with Y . Physically Settled European Swaptions: In a physically-settled European swaption, the event dates Ti are the union of the swaption exercise date and swap coupon dates. X is equal to zero and Y is equal to the swap cashflows. The optionality is with physical settlement, meaning that upon exercise we replace the cashflows X with Y , entering into a swap.

5.1.2 Expression Trees We have seen that products can be described via their payoff cashflows X and Y and that their description can be done systematically using statistics as building blocks. We can now consider how to have a mechanism to produce computing code that generates the expressions needed to describe payoffs. The standard way to build and evaluate expressions is to use trees that are generated according to predefined rules, i.e. a predefined grammar. Consider as an example the expression tree of the up-and-out option as described in (5.1). At maturity, i.e. at the event date, we will have to evaluate an expression tree as shown in Fig. 5.1.

5.2 A Portfolio Aggregation Language We can define products via their expression tree and then, using this information, drive the AMC algorithm to compute price distributions. In Chap. 6 we describe some basic implementation principles. Our goal now is to add an abstraction layer in order to (i) easily compute exposure of trades that usually are described via termsheets, (ii) de-couple trade description from implementation of the analytics, and

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Fig. 5.1 An expression tree for an up-and-out option. We have used the following statistics (see Chap. 6 for more details): INDEX to extract stock information, EXTREMUM to obtain the extremum value of a path, INDICATORBELOW to compute the indicator function. In addition we have used mathematical functions to perform differences and multiplication between values and to compute the max between values. The hierarchy of operators is given by the grammar that generates the tree

(iii) bring trades from existing booking systems into a single unified booking representation. The technical solution for these requirements is the definition of a programming language to describe products, which acts as an interface between statistics and analytics. As the main goal is to allow a portfolio view, we have called this language Portfolio Aggregation Language (PAL). By defining an appropriate syntax and grammar, and by using a lexer and parser, we can then generate expression trees, which in turn will call the statistics defined in the analytics. There are many tools available in the market to automatically generate parsers from given grammars. Classical examples are Lex and Yacc [74], Flex and Bison [73], or ANTLR [84]. PAL is designed with two competing goals in mind. It has to be (i) Simple enough to describe different types of trades in a clear and concise way. In other words, the syntax has to allow trade description in a way that is close to business language. (ii) Flexible enough to accommodate various levels of trade complexity and allow translation from other different booking systems across the firm into one single language. To respond to these requirements we have designed PAL with the following main technical features. (i) It has the typical declarative statement of a procedural language. For example it is possible to define numerical or logical (boolean) variables, arithmetic operations between them (with the usual precedence rules), and loops.

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105

(ii) There are some predefined types such as vectors or date schedules. (iii) There are custom and built-in functions: examples are Exp, Max, Log. (iv) It has some object-oriented capabilities to define, for example objects of type ‘Instrument’ (this allows the possibility of having analytical pricing for selected products). (v) It has some typical financial construct. For example you can say that you will Pay or Receive a certain amount at certain dates. (vi) It defines a context that specifies how computation is performed. For example it is possible to define the quoting currency or the fact that a counterparty is collateralised. The following examples clarify these concepts.

5.2.1 PAL Examples The code snippet below shows how a simple interest-rate swap can be described in a succinct way. Table 5.1 Vanilla swap Schedule = From 2009/09/30 to 2019/06/30 every 3 months; Notional = 100 mm EUR; DcFr = DCF(now-3m, now, "ACT/ACT"); // Day Count Fraction Receive Notional * (ir:eur3m on Now - 3m)* DcFr on Schedule; Pay Notional * 3.5%* DcFr on Schedule;

Even if there are some elements typical of programming languages, the syntax is close enough to a typical termsheet description: we can define a schedule using dates, we can specify what parties A and B respectively pay and receive, and we can use typical business terminology. The same swap in arrears will be written as, Table 5.2 Floating leg of a vanilla swap in arrears Receive Notional * (ir:eur3m on Now - 0m)* DcFr on Schedule;

If we want to make our swap callable, we add the following line of code, which defines the cashflows Y and the dates when they need to be paid. Table 5.3 Swap callable at each payment date specified in the swap schedule Long Callable on Schedule into (Receive 0 EUR on Schedule);

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If we have the option to enter into a swap, i.e. if we want to have a trade with physical exercise, we can write the code below, where the non-intrinsic feature of the callability statement is explicit.

Table 5.4 Physically settled swaption Notional = 100mm USD; Schedule = From 2009/09/30 to 2019/06/30 every 6 months; Date = 2014/09/30; DcFr = DCF(now-6m, now, "ACT/365"); Swap = Receive Notional*(IR:USD6M on now-6m)*DcFr on Schedule; Swap = Pay Notional * 3%* DcFr on Schedule; // The settlement is physical Long Callable on Date into Swap with Physical Settlement NonIntrinsic;

Consider now the example of a non-callable product, e.g. the barrier for which we have also shown the expression tree in Fig. 5.1. The PAL code could be written as follows,

Table 5.5 FX up and out option Notional = 100mm EUR; Strike = 1.0; Barrier = 1.5; Receive Notional * max(fx:gbpeur - Strike, 0.0) * (maximum(fx:gbpeur, 2009/10/20, 2010/10/20) < Barrier ? 1.0 : 0.0) on 2010/10/20;

An example of a CDS on a counterparty characterised by a ‘Credit Curve’, is given in the table below. We use here two constructs, ‘creditloss’ and ‘creditevents’. The first corresponds to the loss suffered by the underlying instrument and it is used to describe the protection leg of the CDS. The second indicates simply the event of default and therefore does not take into account recovery rate. It is used to define the payment leg of the CDS. More details are given in Chap. 6, where we describe how these quantities are taken from underlying simulated processes, and in Chap. 10, where we focus on credit derivatives. We have mentioned that PAL allows also to use other typical programming language features, such as loops, matrix operations and some typical objectoriented construction. These features can significantly help the booking of complex

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107

Table 5.6 CDS start = 2004/07/20; end = 2009/09/20; Schedule = From (start+3m) to end every 3 months; Notional = 100 mm EUR; DcFr = DCF(now-3m, now, "ACT/ACT"); // Day Count Fraction Receive Notional * creditloss(cr:"CreditCurve", Now - 3m, now) on Schedule; Pay Notional * 0.0043 * (1.0-creditevents(cr:"CreditCurve", start, now))*DcFr on Schedule;

transactions, described usually in long termsheets. Below is the example of a trade using a ‘for loop’ to describe repetitive payments, and the definition of a new product, a swap, which can be used later within the program. Table 5.7 For Loop example Notional = 100mm USD; DcFr1 = DCF(now-6m, now, "ACT/ACT"); DcFr2 = DCF(now-1Y, now, "ACT/ACT"); ScheduleRec = From 2009/01/01 to 2020/01/01 every 1y; SchedulePay = From 2009/01/01 to 2020/01/01 every 6m; Maturity=2020/01/01; Receive Notional * 0.04 * DcFr2 on ScheduleRec; Pay Notional * (((USD 10y)-(USD 2y))>0 ? ((USD 6M)+0.007)*DcFr1 : 0) on SchedulePay; DD=2005/04/22; S=0; For (i=1;i0 ? ((USD 6M on DD)+0.007)*DcFr1 : 0); } Pay Notional * Max(0,0.3-S) on Maturity;

The code snippet above represents a complex interest-rate trade, where party A pays a fix rate every year, and party B pays Libor plus spread or zero every six months, depending on the difference between two points of the swap curve, the 10 years and the 2 years points. In addition, at maturity, a cumulative coupon is paid. This is computed within the ‘for loop’ depending again on the two points of the curve.

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For very simple products, whose valuation is model-independent (such as for example standard interest-rate swaps), it is possible to use functions, which we call ‘Instruments’ (see below) to define an analytical pricing function. This gives the possibility of combining AMC with other pricing techniques. We will see at the end of this chapter how to use ‘Instruments’ to define more complicated products. Table 5.8 Instrument example Instrument Swap(Notional, Currency, Fix, FromDate, ToDate, Freq) { DcFr = DCF(now-Freq, now, "ACT/ACT"); Float = Libor(Currency, now - Freq, now - Freq, now); Schedule = From (FromDate + Freq) to ToDate every Freq; Receive Notional * Fix * DcFr Currency on Schedule; Pay Notional*(Float on now - Freq)*DcFr Currency on Schedule; } Buy Swap(100 mm, CHF, 3%, 2009/06/30, 2020/06/30, 6 m);

5.3 The Concept of Scenarios We have mentioned on many occasions that the classical Monte Carlo framework used to compute counterparty exposure is via scenario generation and then pricing using analytical formulas or approximations. Sometimes these scenarios are generated in a centralised location and then sent to various engines that perform the pricing step. AMC can also be considered as a pricing approximation, even if sophisticated and thus enabling the valuation of complex transactions. In this sense we could think of scenario generation and AMC valuation as two separated sub-systems. However, because we want simulation and pricing treatment to be generic, it is not possible to know beforehand which financial quantities will need to be extracted from the simulation. Therefore it is essential to be able to retrieve any financial quantity efficiently from the basic stochastic drivers.

5.4 The Concept of Super-Product With the computational framework we have described and the definition of the PAL language, we can now make a step further, and define new types of products, whose payoffs, i.e. X and Y cashflows, depend on the price distribution of an already computed product. We call these products Super-Products to highlight the fact that these are products built with the results of the computation performed for another product.

5.4 The Concept of Super-Product

109

5.4.1 An Example of Super-Products: The C-CDS An example of a super-product is the contingent credit default swap (C-CDS). We will analyse this product in detail in Chap. 14, where we show how to compute credit valuation adjustments and how to hedge counterparty credit exposure. In this context it is sufficient to note that a C-CDS is an OTC derivative between two counterparties, A and B, say. Assume A has a derivative portfolio with a third counterparty C, and that it enters into a C-CDS with B. In case of default of C, under the C-CDS contract B will pay to A the positive value (as seen from A’s perspective), Vt+ , of the portfolio. In other words a C-CDS corresponds to the protection leg of a CDS paying at each point in time the value of an underlying transaction. The construction of the C-CDS is performed as follows: (i) first the price distribution Vt of the derivative portfolio is computed, (ii) then defaults of counterparty C are simulated, and, (iii) finally the cashflows of the C-CDS are created combining the default values with the price distribution. The advantage of a generic computational framework, where only cashflows are relevant and valuation is performed via AMC are clear. We can effortlessly create a new product whose payoff depends on another product. In Table 5.9 we show a PAL example of a C-CDS. We use several concepts described before: object-oriented features, the idea of ‘Instrument’ and the construct used to define CDSs. We can see in particular that the C-CDS corresponds, as mentioned above, to the protection leg of a CDS. Table 5.9 CCDS Instrument ccds(objInstrument) { receive max(0, objInstrument(currentdate)) * creditevents(objInstrument.cpty,previousdate,currentdate)) on objInstrument.schedule); }

Chapter 6

Implementation

The previous chapter introduced a computational framework within which complicated payoffs can be specified and then simulated to obtain the price distributions required for credit exposure estimation. Trade specification is based on quantities we called statistics, which can be thought of as functions that return some financial quantity, given a simulated scenario. We will use these statistics later in Part III to specify various products. This chapter is dedicated to a more detailed analysis of various statistics. We describe their implementation, the practical issues that arise, and the solutions we adopted. Since simulation is at the heart of our framework, we describe also various Monte Carlo schemes for simulating SDEs. We end the chapter by analysing the different types of errors introduced in the various steps of the modelling.

6.1 Spot and Forward Statistics The first type of statistics we consider are those that extract spot and forward values directly from simulated scenarios. These statistics are relatively simple in the sense of being deterministic functions of the simulated Brownian Motion processes. Typical examples are values of stock and foreign exchange rates, bond prices, and Libor rates. Most of these statistics can have a common signature and can be differentiated by the type of underlying they are applied to. Thus, we define a generic statistic (called INDEX), which has as argument the type of underlying (e.g. EQ, FX, or IR), and a symbol identifying a specific instance of the underlying (e.g. the IBM stock, the USDGBP exchange rate, the USD3Y three years swap rate). Having defined the underlying type and its instance, the observation date is expressed as a lag relative to the payoff date. Examples are given in Table 6.1. The generic way we have implemented the INDEX statistic is as follows. Different underlying types will correspond to different martingales, each of which are deterministic functions of simulated Brownian Motion paths. All that the INDEX statistic needs to do is to look up the correct martingale for each underlying type that is asked for. G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0_6, © Springer-Verlag Berlin Heidelberg 2009

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6 Implementation

Table 6.1 At payoff date, return the value of index SYMBOL, of type TYPE, observed at the payoff date minus lag LAG. Examples: (i) The 3-year USD swap rate is defined as USD3Y, and its type, “interest rate”, as IR. In a 3Y USD CMS swap contract, the swap rate is called as INDEX(IR, USD3Y, 0M). (ii) To define a vanilla interest rate swap, paying quarterly, the 3-month libor rate fixed 3 months ago is accessed as INDEX(IR, USD3M, 3M). (iii) The IBM stock is called as INDEX(EQ, IBM, 0M). (iv) USDGBP FX rate is called as INDEX(FX, USDGBP, 0M). (v) USDGBP 1 year forward is called as INDEX(FX, USDGBP, -1Y)

Spot and forward

Date

Payoff

Ti

INDEX(TYPE, SYMBOL, LAG)

The reader may have noticed one practical problem that needs to be dealt with, and will affect computation of all statistics. When simulating the basic martingales for different asset classes, the simulation will need to happen on a finite number of prespecified time-points t ∈ {t0 , t1 , . . . , tn }. Of course the tj cannot be chosen a priori to incorporate all payoff dates of the portfolio that needs to be computed, first because there might be too many such dates, and second because to ensure scenario consistency one needs to be able to compute price distributions of new portfolios starting from the same basic Brownian Paths. We solve this problem by an interpolation method which we describe in a later section in this chapter.

6.1.1 Libor Rates and Bond Prices Libor rates and bond prices occur frequently and play an important role in a large proportion of portfolios. For this reason we have defined specialised statistics to aid in creating payoffs that depend on these quantities. Libor Rates are a specific case of an INDEX in our computational framework, but we have nevertheless defined a specialised statistic that returns simulated Libor rates. Recall that if t < [T1 , T2 ], the Libor rate Lt,[T1 ,T2 ] observed at time t for the period [T1 , T2 ], is a simple function of bond prices, that is, 1 Lt,[T1 ,T2 ] = T2 − T1

Dt,T1 D0,T1 Mt,T1 1 −1 = −1 , Dt,T2 T2 − T1 D0,T2 Mt,T2

(6.1)

where we have used the representation of bond prices D(t, T ) =

D(0, T ) M(t, T ) D(0, t) M(t, t)

(6.2)

in terms of the basic martingales M. Table 6.2 shows the syntax for both Libor and bond price statistics.

6.1 Spot and Forward Statistics

113

Table 6.2 At payoff date, return the Libor rate for a specified currency and tenor observed at payoff date minus lag. Example: the 6-months EUR Libor rate fixed in arrears is called LIBOR(EUR, 6M, 0M). Similarly, ZEROBOND(EUR, 10Y, 1Y) returns the price of a 10-year bond as observed 1 year before the current payoff date Date

Payoff

Libor

Ti

LIBOR(CURRENCY, TENOR, LAG)

ZeroBond

Ti

ZEROBOND(CURRENCY, MATURITY, LAG)

6.1.2 Annuity An annuity pays a unit coupon at regular points in time. In other words, it pays the difference between a coupon bond and a zero bond. The implementation of this statistic is similar to the previous one. At,T1 ,...,Tn =

n

(Ti − Ti−1 ) Dt,Ti =

i=1

n

(Ti − Ti−1 ) D0,Ti

i=1

Mt,Ti . Mt,t

(6.3)

Table 6.3 shows the usage of the ANNUITY function. Table 6.3 At payoff date, return the annuity of a swap with specified tenor and payment frequency. Example: the current annuity of a 3-years USD swap with monthly payments is specified as ANNUITY(USD, 3Y, 12, 0M)

Annuity

Date

Payoff

Ti

ANNUITY(CURRENCY, MATURITY, TIMESPERYEAR, LAG)

6.1.3 Swap Rate The swap rate is the coupon that the fixed-rate leg of a swap would have to pay for the swap to have zero net present value. Said in another way, it is the coupon that an annuity would need to pay in order to have value equal to a floating-rate leg. Thus, the SWAPRATE function can be evaluated as st,T1 ,...,Tn =

Dt,T0 − Dt,Tn D0,T0 Mt,T0 − D0,Tn Mt,Tn = . At,T1 ,...,Tn At,T1 ,...,Tn Mt,t

(6.4)

Again, the right side above involves only quantities that have already been considered. A specific example of usage of SWAPRATE is shown in Table 6.4.

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Table 6.4 At payoff date, return the par rate of a swap with specified tenor and payment frequency. Example: the par rate of a 3-year USD swap with monthly payments is called SWAPRATE(USD, 3Y, 12, 0M)

Swap Rate

Date

Payoff

Ti

SWAPRATE(CURRENCY, TENOR, TIMESPERYEAR, LAG)

6.2 Path Dependent Statistics We now turn to statistics whose evaluation cannot be done in simple deterministic fashion from simulated Brownian Motion paths. Examples of such statistics are the maximum attained by a process, the average value of a process, and the time spent by a process within a given range. In principle, if one could simulate the basic Brownian Motions on a fine-enough grid, such statistics would be a sampling exercise. However, since the size of portfolios and time horizons involved do not allow the luxury of arbitrarily small time steps, we need to find estimators of the above quantities that remain accurate when the simulation time step is relatively large.

6.2.1 Extremum Extremum is the maximum or minimum value reached on a given scenario path between two observable dates. This statistic is typically used to describe barrier features. The parameters we need for its implementation are its type and symbol, and some time parameters to define where to perform the observation. Table 6.5 At payoff date, return either the maximum (ISMAX = true) or the minimum (ISMAX = false) observed value of an index over an observation period starting at T start = Tpay − lag − tenor, and finishing at T end = Tpay − lag

Extremum

Date

Payoff

Ti

EXTREMUM(TYPE, SYMBOL, LAG, TENOR, ISMAX)

The implementation of EXTREMUM can be performed using some properties of Brownian motions. Consider an N-Brownian motion W for which we need to estimate the maximum and minimum values on an interval [t, T ], and suppose that we know the values, Wt = a and WT = b. The idea is to consider maxima and minima as random variables, defined by, Wmax = max Wu u∈[t,T ]

and Wmin = min Wu . u∈[t,T ]

(6.5)

We can write (see the Technical Note 6.4.4), √ √ T − t √ a+b + K + eK πΦ(− 2K) (6.6) E (Wmax | Wt = a, WT = b) = 2 2

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115

a+b − E (Wmin | Wt = a, WT = b) = 2

√ √ T − t √ K + eK π Φ(− 2K) , (6.7) 2

where K=

(b − a)2 . 2(T − t)

(6.8)

Once we have simulated the extremum of the underlying stochastic driver, as long as the quantity X (e.g. stock price, libor rate. . . ) can be expressed as monotonic function f of a single Brownian motion, we can derive the maximum (resp. minimum) of X as being either f (Wmax ) (resp. f (Wmin )) if f > 0 or f (Wmin ) (resp. f (Wmax )) if f < 0.1

6.2.2 Average Knowing Xmax and Xmin on any interval allows also to compute quantities that are path dependent, such as average values, or days within a range. A simple way of doing so for any interval [T1 , T2 ] is – From XT1 and XT2 obtain Xmax and Xmin . – Interpolate the path (T1 → T2 ) by imposing for example that max and min occur at (T2 − T1 )/4 and 3(T2 − T1 )/4, 3(T2 − T1 ) T2 − T 1 = Xmax , and X = Xmin . (6.9) X 4 4 Polynomial interpolation can now be used to obtain the path of X on [T1 , T2 ] from the four values given. Table 6.6 At payoff date, return the average value of any index over an observation period starting at T start = Tpay − lag − tenor, and finishing at T end = Tpay − lag

Average

Date

Payoff

Ti

AVERAGE(TYPE, SYMBOL, LAG, TENOR)

Asian options require to simulate the average value ΠAverage of X on a time interval [T1 , T2 ]: T2 1 ΠAverage = Xu du. (6.10) T2 − T1 T1 1 Note

that this approximation gives a bias due to Jensen’s inequality. If needed, an approximation for E(fmax ) on the Brownian bridge could be implemented. This, however, becomes model dependent.

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6 Implementation

6.2.3 In Range Fraction Range accrual products usually pay an exotic coupon, which will be the product of a fixed coupon c with the proportion of days a given underlying has remained within a range [Klower , Kupper ]. T2 c ΠI nRange = 1Xu ∈[Klower ,Kupper ] du, ∈ [0, c]. (6.11) T2 − T1 T1

Table 6.7 At payoff date, return n/N , where n is the number of days where the index is in the range [LowerRange, UpperRange], and N is the total number of days in the observation period. The observation period starts at T start = Tpay −lag −tenor, and finishes at T end = Tpay −lag

Average

Date

Payoff

Ti

INRANGEFRACTION(TYPE, SYMBOL, LAG, TENOR, LOWERRANGE, UPPERRANGE)

6.2.4 Credit Loss To compute credit derivatives we need to be able to compute credit losses occurring in a given time interval. They are defined as follows, n j =1 Nj (1 − Rj )1Ti 0 between foreign currency bonds and the exchange rate is not respected. To assess the impact of independence, we consider the following example transaction. Fix a time horizon T , say, and consider a contingent claim CT = χT ,

(6.59)

that simply pays one unit of the foreign currency at T . At each t , CT should have value D˜ t,T χt , the time-t price of a foreign T -bond expressed in the reference currency. Now consider the option to enter, at time s < T , a portfolio that is long CT and short D˜ s,T χs . Because D˜ s,T χs is the no-arbitrage time-s value of the claim, the option should be worthless. If we assume independence, this is not guaranteed, because the FX rate simulation happens independently of the individual bonds (and also because of Monte Carlo and AMC regression error). As an illustrating example, we fixed s to be the end of the year 2015,2 and priced the option described above for values T shown in Table 6.10, and for four different currencies. We first computed the prices of the option using the independence assumption—results of this are shown in the third column in Table 6.10. For comparison, we re-did the computation by performing the simulation differently ensuring the interest-rate parity (which entails lifting the independence assumption) that 2 Time

of computation is April 09.

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6 Implementation

Table 6.10 Value, as a percentage of notional, of an option to enter into a portfolio that is long an FX forward and short a bond in the foreign currency. The theoretical value of the option is zero. The table compares AMC values of the option under the assumption of independence between FX rates and interest rates, to the values obtained when that assumption is lifted. The time T is the maturity of the bond and forward. The option’s exercise time is fixed at 2015. In the last column we also report the estimated value of the underlying at time of expiry of the option, computed under the assumption of independence between rates and FX. The underlying has theoretical value zero by construction. The notional amount is 100 million units of the payment currency (computation performed April 09) T

USD

GBP

EUR

JPY

FX and Rates Independent

FX and Rates not Independent

Value of Option Underlying

2020

0, then the time-zero value of the close-out risk per share of stock faced at default is given by E Nt−1 (St − St−δ )+ , (14.20) therefore hedging the close-out risk on a stock forward involves hedging the cliquettype payoff appearing in the expectation in the equation written above.

14.9 Case Study To highlight the importance of dynamically hedging CVA, consider the following stylised example. Suppose that in June 2008 we had entered into a 20-year EUR/USD forward contract of 500 million EUR notional, in which we receive USD

14.9 Case Study

227

and pay EUR. To value the trade at par, the contract would have been struck at around 1.65 USD per EUR. Assume now that the counterparty we are facing had at inception a CDS curve trading at 300 bps flat. The initial CVA for this transaction would have been in the order of 10 mUSD. Leaping one year forward to June 2009, because of the fall in the EUR/USD exchange rate, the forward contract is now worth about 47 mUSD in our favour. Figure 14.3 shows the difference in exposure profiles generated on both dates. As we can see, the EPE profile computed in June 2009 is substantially higher than that computed a year before.

Fig. 14.3 PFE and EPE profiles computed June 08 and June 09. The two set of profiles are superimposed and the time axis is referred to the 2008 computation. The 2009 EPE and PFE profile start at the one-year point. Note that their initial value is within the PFE profile computed one year earlier, but due to the new market condition, the new profiles are outside the bounds computed in 2008

It is also interesting to note that the present value of the trade in June 2009 lies within the PFE confidence level computed in 2008.6 Assuming that the counterparty spread stayed at 300 bps, the resulting CVA would have gone up to roughly 24 mUSD. Had we only hedged the risk relating to the counterparty’s CDS curve, we would therefore face a loss of about 14 mUSD. Figure 14.4 shows the difference in CCDS profiles generated on both dates. Assume now that at inception of the trade in 2008, we had decided to also hedge the EUR/USD risk. To do so, we would have needed to choose an instrument which is liquidly traded and that does not add additional counterparty risk, such as one-year EUR/USD futures. In June 2008, the EUR/USD delta for the CVA of our fictional 6 Note that the EUR/USD exchange rate saw its greatest historical absolute fall between 2008 and 2009.

228

14 Pricing Counterparty Credit Risk

Fig. 14.4 CVA EPE and PFE profiles computed in June 08 and 09. The initial points of the two sets of profile corresponds to the CVA computed at inception and one year later

transaction was of the order of −37 mUSD, meaning that for every 0.01 move in the EUR/USD exchange rate, the CVA would increase by 370 kUSD. Hence, being long 37 mEUR notional worth of one-year futures should in theory eliminate the currency risk. The table below summarises the result of our hedging strategy from inception until June 2009. As we can see, the hedging strategy would have resulted in a profit of roughly 1 mUSD, as opposed to an un-hedged loss of 14 mUSD.7 Table 14.1 Summary of hedging strategy including CDS and FX hedges CVA

June 2008

June 2009

−9,983,526

−24,061,324

0

14,870,065

9,983,526

10,185,207

0

993,948

Hedge Cash Net

Of course, it is unrealistic to assume that the counterparty spread would have remained unchanged during a one year period. Assume now that in fact the CDS spread would have increased from 300 bps to 400 bps. In this case, the CVA computed in 2009 would no longer be 24 mUSD, but 27.7 mUSD. The credit delta8 computed in 2008 was of the order of 20 kUSD per basis point. While the maturity of the underlying portfolio is 20 years, assume that the only liquid maturity for the counterparty CDS is five years, for which the credit delta would be of the order of 7 Note 8 In

that we have assumed that the deposit rate at which the cash grows is 2% per annum.

other words, the counterparty spread delta of the CVA.

14.9 Case Study

229

400 USD per basis point on a one million USD notional, meaning that in order to hedge the counterparty spread risk we should enter into a 5 year CDS of roughly 50 mUSD notional. In June 2009, this position would have yielded a profit in the region of 1.6 mUSD, partially offsetting the loss. We can see now how, even in this stylised example, a simple market hedge can considerably improve P&L resulting from changes in CVA. An un-hedged position would have resulted in a net loss of almost 18m USD, while the full hedge we described would have reduced this loss to about 1 mUSD. It is worth noting that a static hedge involving solely the counterparty CDS would have resulted in a loss of roughly 16 mUSD. From this and the previous examples we can see that replicating a C-CDS involves hedging a hybrid product, which has market, (e.g. FX or interest rate), and credit components. Ignoring for example the FX risk would clearly undermine any hedging strategy. It is interesting to note that in a classical set-up, where the CVA is computed statically using simply the EPE profile and the spread of the counterparties, the market risk components of the hedge are difficult to compute, as they require the EPE sensitivities to market risk factors. They involve, however, the usage of instruments, which are in general traded on exchanges. The credit component can be computed more easily, but on the other side it involves CDS products, which are still mainly traded as OTC transactions and are not available for all names and all maturities.9 This involve the usage of credit indices and the finding of curves which can be used as proxies for illiquid names. In addition to the risks we have already mentioned, we need to consider the so called vega-risk deriving from movements of implied volatility. This can be substantial especially when the portfolio is dominated by FX positions. To appropriately hedge this risk we need to include in the C-CDS computation volatility as a stochastic driver. This can be performed by implementing a stochastic volatility model, as described in Chap. 3.

9 To reduce the counterparty risk in general CDS hedges are traded with fully collateralised counterparties. There are extensive discussions to standardise CDSs and trade them on exchanges.

Concluding Remarks

Our goal in this book was to model counterparty credit exposure for all types of transactions. We saw that by appropriately choosing the fundamental quantities to model we can approach the problem in a modular way, dividing features and conquering products. Price distributions are obtained using American Monte Carlo (AMC) techniques, allowing a valuation framework where modularity and flexibility are key. With the introduction of a booking language, PAL, we added a further layer of de-coupling and abstraction, enabling a system architecture that could address most of the problems faced by a counterparty exposure system dealing with large diverse portfolios. The natural next step was to investigate how to manage counterparty exposure, both in static and dynamic ways. This led to the introduction of the so-called contingent credit default swap product, C-CDS, which replicates the cost of protection. We now summarise the steps needed to compute and hedge credit exposure. (i) Translation. First of all, all trades within the portfolio should be understood by the valuation engine. This means that each trade needs to be translated into the common trade representation language. (ii) Portfolio Valuation. Once the first stage is completed, it is possible to model the underlying risk drivers, which have been recognised via the common trade representation language, and value each trade, along with its future price distributions. All trades are then aggregated together, including possible netting rules or break clauses, to finally arrive at the future distributions of the portfolio. If a collateral agreement exists, its logic should then be applied to the portfolio distributions. (iii) C-CDS Valuation. The credit valuation adjustment, CVA, can be valued using the modified EPE profile of the portfolio and the counterparty credit spread curve. Using C-CDSs, however, we can compute not only the value of CVA, but also the CVA future price distribution. (iv) Sensitivities Computation and Replication. As a final step, sensitivities can be computed from the C-CDS distribution, using either a regression-based approach, or a full revaluation (known in the industry as ‘bumping’ method), starting the process again from step (ii). G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0, © Springer-Verlag Berlin Heidelberg 2009

231

232

Concluding Remarks

(v) Post-processing. For purposes of risk control (e.g. to compute regulatory capital or compare PFE with limits), a post-processing of the price distribution may be needed. Examples of this include stress-testing and accounting for rightway/wrong-way risk. The techniques we described can also be applied to other problems that large financial companies need to address. Examples are (i) computing the value of the so called own credit of a company, (ii) valuing debt valuation adjustments (DVA) of portfolios of transactions, (iii) addressing the problem of valuing the cost of funding and cost of collateral, (iv) computing potential values of transactions in different scenarios, (v) determining the value of risk weighted assets and of regulatory capital, or (vi) investigating various hedging strategies. All these problems deserve a thorough analysis which could be the subject of further research. It is interesting to note here that any solution to these questions will require, as fundamental feature, the capability of computing future distributions of prices. This is the feature at the heart of our work. A final remark to conclude. What we described in this work is only a brief overview of the problem we try to solve. As we highlighted throughout this book, in many occasions we accepted compromises in our implementation and highlighted shortcuts. Many points can be improved, further explored and changed. We think, however, that at a general level, the framework and the ideas we provide are a viable solution to the modelling, pricing and hedging of counterparty credit exposure for large portfolios of different products.

Appendix A

Approximations

We summarise here some useful approximations of counterparty exposure computation, often used by practitioners. While they cannot provide satisfactory results in general, they may serve as a sanity check for more complex computations, and to help intuition. In some cases in the computation of Expected Positive Exposure (EPE) for some types of products, they are based on pricing information and give exact valuation. Some of the formulae we present are general and others can be used only for specific products. We consider here what we found useful in our day-to-day work.

A.1 Maximum Likely Exposure In general, the Potential Future Exposure profile (PFE) of a given product is a function of time. We call its maximum value Maximum Likely Exposure (MLE). In the following sections we provide some MLE estimate for simple products.

A.1.1 MLE of Equity and FX Products MLE values can be easily approximated in the case of options or forwards on assets that can be modelled as Geometric Brownian Motions assuming constant volatility and interest rate. Under these assumptions in fact the exposure profile reaches its maximum at maturity of the trade, where its value coincides with its intrinsic value. Thus, to compute the MLE, what is necessary is to estimate the potential value of the asset at maturity. Consider for example an option on a stock S with Black-Scholes volatility σ , interest rate r, and strike K. The maximum value of the exposure at maturity T (within a 97.5% confidence interval) is given by 1

MLE = Se(r− 2 σ

2 )T +1.96σ

√

T

− K.

G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0, © Springer-Verlag Berlin Heidelberg 2009

(A.1) 233

234

A Approximations

If we assume zero interest rate, stock returns normally distributed, and at the money products (S = K), we can simplify this formula as follows, √ MLE = 1.96Sσ T . (A.2) The main problem in these valuations is the choice of volatility. If the volatility is assumed to be constant, it is necessary to estimate the value that will best fit the terminal asset distribution. If the choice is to use implied volatilities, the atthe-money volatility is often the most suitable one to use. In practice if implied volatilities are not available historical volatilities are used.

A.1.2 MLE of Swaps Throughout this book we have seen several PFE profiles of interest-rate swaps. In general, when the product is vanilla, they show a typical bell shape, which starts from zero, increases over time and then decreases to reach zero again at maturity. This shape is driven by two factors, the declining duration (time to maturity) and the increasing variance of the swap. Assume that, at any time t, the duration is proportional to the remaining life of the swap via a constant A0√< 1, and that the interest-rate volatility increases with the square root of time, σN t.1 We can write the volatility of the swap as √ VolSwap = A0 (T − t)σN t. (A.3) The peak exposure, i.e. the MLE, is reached at about one third of the life of the trade. We can see this by simply taking the first derivative of the volatility with respect to time, and imposing its value to be zero. ∂VolSwap T 1 = 0 ⇐⇒ −A0 + A0 (T − t) = 0 ⇐⇒ t = . ∂t 2t 3

(A.4)

Using this result and assuming that the price distribution of an at-the-money swap is normally distributed, we can estimate the price distribution of a swap at time T /3, 2 T Z, (A.5) SwapDistributiont=T /3 ≈ A0 T σN 3 3 where Z ∼ N(0, 1). If we want to value the MLE, i.e the peak PFE exposure at 97.5% confidence interval, we need to substitute Z with 1.96. The present value of EPE can be computed by taking the expectation of the positive part of this distribution. Doing this we obtain √ √ 1 EPEPt V ≈ √ A0 (T − t)σN t ≈ 0.4A0 (T − t)σN t. 2π 1σ N

(A.6)

is the volatility of a normal distribution. It is related to the log-normal (Black-Scholes) volatility σ of the swap rate via the level of interest rate, σN ≈ rσ .

A.2 Expected Positive Exposure

235

A.2 Expected Positive Exposure The Expected Positive Exposure (EPE) computation is strongly related to pricing. In general, under pricing measure assumptions, the EPE of a transaction at time t is the price of an option to enter in the transaction at time t. This is a very useful result, as it allows to approximate EPE computations using price information.

A.2.1 EPE and CVA of Equity Options As a first example consider an option on a stock or an FX currency. Under simplified assumptions, EPE can be written as EPEt = E[Vt+ ],

(A.7)

where Vt is the price distribution at time t. In the case where Vt is always nonnegative, as for example for options, this equation becomes EPEt = E[Vt+ ] = E[Vt ] = E[E[e−r(T −t) (ST − K)+ |Ft ]] = V0 ert ,

(A.8)

where we have assumed constant interest rates and volatility. Thus, EPEt = V0 ert .

(A.9)

In other words the EPE of an option at time t is the option premium increased at the risk-free rate. The CVA can be computed as the discounted EPE multiplied by the spread (assumed to be constant) multiplied by time to maturity, CVA ≈ V0 s0 T .

(A.10)

This formula holds for any product whose price distribution is non-negative and which does not pay intermediate cashflows. For example it can be used to compute CVA of a cash-settled swaption, while it cannot be applied in the case of a physically-settled swaption.

A.2.2 Relation between MLE, EPE If we assume zero interest rate we can approximate the price V0 of an at-the-money (S = K) option as, V0 = SΦ(d1 ) − KΦ(d2 ),

(A.11)

236

A Approximations

where Φ is the cumulative normal distribution, and √ √ ln(S/K) σ T σ T ± d1/2 = =± . √ 2 2 σ T

(A.12)

Using the following approximation Φ(x) =

1 1 + √ x + O(x 3 ), 2 2π

√ and assuming σ T 1 the above equation becomes, √ √ √ √ σ T σ T 1 − KΦ − ≈ √ Sσ T ≈ 0.4Sσ T . V0 = SΦ 2 2 2π

(A.13)

(A.14)

We have seen that the EPE of an option can be computed as the option premium growing at risk free rate. Thus √ EPE ≈ 0.4σ S T . (A.15) We can now compute a relation between EPE and the 97.5% MLE. Recall that, √ MLE ≈ 1.96σ S T . (A.16) Thus, we obtain, EPE ≈ 0.2. (A.17) MLE In other words, if the distribution of the portfolio is normal and centered around zero, then the 97.5% MLE is roughly five time larger than the EPE.

A.3 CVA of Swaps The EPE value at time t of a swaps portfolio is often computed by practitioners as the value of a swaption, i.e. the value of an option to enter into a (portfolio of) swaps. This valuation is correct, however, only if the modified value of the EPE, as defined in Chaps. 12 and 14, is used. Often this valuation methodology is called swaption approach. We can evaluate approximation of the CVA of a swap as follows. CVA

swap

≈ 0

T

EPEPu V s0 du ≈ s0

4 = s0 0.4A0 σN T 5/2 , 15

T

√ 0.4A0 (T − u)σN udu

0

(A.18)

A.3 CVA of Swaps

237

where s0 is the CDS spread and EPEP V is the present value of the EPE. Recalling (A.5) we can approximate the peak value of the discounted EPE profile as 2 V ≈ 0.4A0 σN T 3/2 √ , EPEPmax 3 3 and thus,

(A.19)

√ 6 3 V EPEPmax . (A.20) CVA ≈ s0 T 15 Noting that the maximum value of the EPE profile of an at-the-money swap occurs at t = T /3 and using the ‘swaption approach’ we defined earlier, we get, T 2 ,T , (A.21) CVA ≈ s0 T Swaption 3 3 √ where we have approximated 6 3/15 with 2/3, and Swaption( T3 , T ) is the value of an option to enter at time T /3 into a swap of maturity T .

Appendix B

Results from Stochastic Calculus and Finance

This book is concerned with the pricing and hedging of risk borne by financial institutions when entering into transactions with other counterparties. Such risk arises from the random nature of the prices of products transacted as well as the possibility that the counterparty defaults, but its pricing and replication uses the same concepts as for other kinds of financial derivatives. This appendix collects a few technical results that we will need throughout. We start by giving definitions for the basic stochastic processes we use, and then recall the concept of change of measure. We give also a brief overview of the fundamental theorem of asset pricing, which allows us to characterise the hedging portfolio for a traded derivative from martingale representation. Derivation and analysis of these results can be found in standard finance books, such as Baxter & Rennie [10], Hunt & Kennedy [64], Karatzas & Shreve [68], Rogers & Williams [93, 94], Shreve [98], and Williams [106].

B.1 Brownian Motion and Martingales All our processes are defined relative to a filtered probability space (Ω, F , (Ft )t≥0 , P), where (Ft )t≥0 is a filtration in F . The basic process we work with is Brownian Motion. Definition 1 A process W ≡ (Wt )t≥0 on (Ω, F , P) is called Brownian Motion if (i) W0 (ω) = 0, for all paths ω ∈ Ω; (ii) for each ω ∈ Ω, Wt (ω) is a continuous function of t; (iii) for each t, h ≥ 0, Wt+h − Wt is independent of Wt , and has a Gaussian distribution with mean 0 and variance h. Brownian Motion is an example of a martingale, the most important class of processes. Definition 2 A process M is called a martingale with respect to (Ft )t≥0 if G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0, © Springer-Verlag Berlin Heidelberg 2009

239

240

B Results from Stochastic Calculus and Finance

(i) M is adapted, that is Mt is Ft -measurable; (ii) E[|Mt |] < ∞; (iii) if s ≤ t, then E[Mt | Fs ] = Ms . M is a supermartingale (resp. submartingale) if we replace equality in (iii) above by ≤ (resp. ≥). For proving general results, the class of martingales is not the right notion to work with, and one needs to consider local martingales. While all martingales are also local martingales, the converse is true only if certain conditions hold. The distinction will not be important for our purposes in this book. At the heart of most of what we do is the idea of looking at various processes in a measure different from that of the given probability triple (Ω, F , P). Indeed, if Z is a non-negative random variable (that is, F -measurable) then ˜ ) := E[Z1F ]/E[Z], P(F

F ∈F

(B.1)

defines a new probability measure P˜ on F for which ˜ ] = 0. P[F ] = 0 ⇒ P[F

(B.2)

The last implication allows us to make the following definition: Definition 3 A probability measure P˜ on (Ω, F ) is said to be absolutely continuous with respect to P, denoted P˜ P, if for all F ∈ F , (B.2) is true. If both P˜ P and P P˜ are true, then P and P˜ are said to be equivalent. In this case, P and P˜ have the same sets of measure zero. The converse to (B.1) is given by the Radon-Nikodym theorem. Theorem 1 Let P˜ P be a probability measure that is absolutely continuous with respect to P. Then P˜ can be characterised as in (B.1) for some non-negative random variable Z, which is then called the Radon-Nikodym derivative of P˜ with respect to P, and we write Z≡

d P˜ . dP

(B.3)

The context in which we will most often see measure-change at work is when changing the drift of a Brownian Motion process. Given a P-Brownian Motion W , if the process γ ≡ (γt )t≥0 is such that t 1 t 2 γs dWs − γs ds (B.4) ζt := exp 2 0 0 is a martingale, then there exists a unique probability measure P˜ such that t Wt − γs ds 0

(B.5)

B.2 Replication of Contingent Claims: Martingale Representation

241

˜ is a P-Brownian Motion. Moreover, the Radon-Nikodym derivative of P˜ relative to P is given on every Ft by d P˜ = ζt . (B.6) dP Ft ˜ W has a drift of γ . Equivalently, The above says that under P, ˜ W˜ is a P-martingale ⇐⇒ ζ W˜ is a P-martingale.

(B.7)

The change-of-measure technique is an indispensable device for simplifying calculations by removing from a process an unwanted drift term. We use it also to study price distributions under probability measures different to the ones in which they are simulated (see also Chap. 13).

B.2 Replication of Contingent Claims: Martingale Representation Consider an economy that puts at our disposal a number of assets St = (1) (n) (i) (St , . . . , St ), so that St is the time-t price of the i’th asset. There is a market for trading these assets. Thus, at any time t , a market participant, of wealth Vt (1) (n) say, will have a proportion of wealth allocated to a portfolio θ t = (θt , . . . , θt ), with the remainder held in some deposit account, so that Vt = ϕt Bt + θ t · St ,

(B.8)

where Bt is the value at t of one unit invested in the deposit account at time zero, and ϕt Bt is the wealth not invested in S. Because any value kept in the deposit account grows at some positive rate, it is more useful to express asset prices in terms of B, writing V˜t ≡ Bt−1 Vt , S˜ t ≡ Bt−1 St . The wealth equation (B.8) then becomes V˜t = ϕt + θ t · S˜ t ,

(B.9)

so that, as we expect, in any time interval where the holdings ϕ and θ are kept constant, the growth in discounted wealth V˜ derives only from growth in the discounted ˜ assets S. Of course, funds may be switched between the holdings in S and the deposit account, but it is natural to suppose that no new wealth can be injected, in which case the portfolio of holdings (ϕ, θ ) is said to be self-financing. The consequence of V being self-financing is then that t ˜ ˜ θ u · d S˜ u , (B.10) Vt = V0 + 0

so that the discounted wealth is the integral of the portfolio holdings against the discounted asset price process.

242

B Results from Stochastic Calculus and Finance

The fundamental theorem of asset pricing, formalised by Harrison & Kreps [57] and Harrison & Pliska [58], and formulated in more general setting in the work of Delbaen & Schachermayer (for example, [34] and [35]), states that arbitrage is excluded if and only if there is some equivalent martingale measure under which discounted asset price processes are martingales. This implies that the price of a contingent claim can be computed as the expectation in the martingale measure of the discounted payoff of that claim. If the market is also complete, so that all claims can be replicated perfectly,1 then the martingale measure (and hence the market price for any claim) is unique. Now if P˜ is a measure under which S˜ is a martingale, and Y = f (S˜ T ) is a con˜ the discounted time-t price of Y , π˜ t,T , say, being the price of a tingent claim on S, traded asset, is itself a P˜ martingale. It follows that π˜ has a representation as ˜ −1 Y |Ft ]. π˜ t,T = Bt−1 πt,T = E[B T

(B.11)

In the absence of any other condition enforcing a unique price for the claim Y , there will be potentially as many prices π˜ for Y as there are market agents, each price reflecting that agent’s own risk aversion. If the market is complete, however, there is a price-enforcing mechanism: the price of Y will be the cost V0Y of setting up a portfolio worth V Y (0) = ϕ0Y + θ 0 · S0

(B.12)

V Y (T ) = Y

(B.13)

at time zero and

at time T . The existence of a unique process θ Y that makes the wealth equation (B.10) true is a consequence of the martingale property of the price processes π˜ t,T = V˜ Y (t) and St and the martingale representation theorem (see Rogers & Williams [94]). Theorem 2 Let X be a local martingale on the filtered probability space (Ω, F , (Ft ), P), and assume that (Ft ) is the filtration generated by X. Then, any local martingale M adapted to (Ft ) has a representation as t Mt = M0 + Hu dXu (B.14) 0

where H is previsible with respect to (Ft ). Moreover, H is unique up to sets of measure zero. ˜ Because the claim price process π˜ t,T and the asset price process S are both Pmartingales, the martingale representation theorem shows the existence of a strategy with which to hedge the claim Y by trading in the assets S. this we mean that for every time-T claim Y one can find a portfolio V˜tY = V˜0 + such that VT = Y . 1 By

t 0

θ u · d S˜ u

B.3 Change of Numeraire

243

B.3 Change of Numeraire In writing the wealth equation (B.10) we defined S˜ t ≡ Bt−1 St and V˜ ≡ Bt−1 Vt by expressing the prices of assets and the wealth V in units of the deposit account. One says that the deposit account is being used as numeraire. There is nothing that keeps us from using as numeraire the value of a different asset, and in fact changing numeraire is a powerful modelling and computational technique. Geman and Jamshidian were the first to employ this idea. Suppose X is the price of any traded asset (scaled by its time-zero value); for reasons that will soon become obvious, we need to assume Xt > 0 for each t. Then, because by ˜ definition of P˜ all discounted assets are P-martingales, we have that Xt ˜ is a P-martingale. Bt

(B.15)

This allows us to define a new measure, PX say, whose Radon-Nikodym derivative is given for every t by Xt dPX ζt = = (B.16) . Bt d P˜ Ft It then follows, for any given process M, that ˜ Bt−1 Mt is a P-martingale ⇐⇒ Xt−1 Mt is a PX -martingale, so for any claim Y maturing at T we can write the equivalent expressions

−1

−1 B (T ) X X (T ) F Ft , πt,T = E˜ = E Y Y t −1 −1 B (t) X (t)

(B.17)

(B.18)

where the first expectation happens under P˜ and the second under PX . For example, if one takes for X the price process of the bond maturing at time T , the price of any claim Y received at T is

−1 B (T ) T ˜ F = D Y Y Ft , (B.19) E πt,T = E t t,T B −1 (t) where Dt,T is the observed time-t price of the T -bond, so that DT ,T ≡ 1, and where the expectation is now in the T -forward measure in which asset prices discounted by the T -bond are martingales. The price of Y can now be computed as the expectation of Y in the T -forward measure. An in-depth account of martingale theory and stochastic processes, which we have used here, is Rogers & Williams [94]. Our description of self-financing portfolios closely follows the article of Rogers [92], which shows how ideas of economic equilibrium lead directly to the existence of equivalent martingale measures.

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Index

A ABS, see Asset backed securities Absolute return swap, see Asset swap, absolute return ABX, 13, 215 Accelerated shares re-purchase (ASR), 176, 177 Advance, see Interest-rate swap, in advance Aggregation, see Risk aggregation AMC, see American Monte Carlo American Monte Carlo (AMC), 19, 20, 79, 80, 88–94, 101, 135 backward induction, 20, 85–88, 90, 94, 101 bias correction, 92 bundling, 88–92, 130 continuation value, 86, 88–91 error, 91, 125, 129 granularity bias, 90 observables, 82, 129, 218 regression, 92, 93, 130 Architecture, 18, 20, 21, 101, 131 component, 139 conceptual view, 137 design principles, 136 logical view, 137, 139 physical view, 137, 142 requirements, 136 Arrears, see Interest-rate swap, in arrears ASR, see Accelerated shares re-purchase Asset backed securities, 215 Asset swap, 166 absolute return, 166 relative return, 167

B Backtest, 128, 129, 187 Backward induction, see American Monte Carlo (AMC), backward induction Barrier option, see Equity-FX option, barrier Base correlation, 65 Basel II accord, 128, 186, 201 Bermudan option, see Interest rate option and equity-FX option, Bermudan Bernoulli distribution, 72, 74 BGM, see Libor market model Bi-modal price distribution, 172 Black model, 53 Black-Scholes formula, 16 Black-Scholes model, 16, 53 Booking language, 20 Booking system, 21, 139 Break clause, 150, 189, 223 Brownian Motion, 28–30, 239–241 Bullet bond, 175, 176 Bundling, see American Monte Carlo (AMC), bundling Business requirements, 136 C C-CDS, see Contingent credit default swap Calibration, 45, 47 correlation, 69, 75 to caps, 47 to CDO tranches, 69 to floors, 47 to options on CDSs, 65, 66 to swaptions, 48 to variance swaps, 54

G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0, © Springer-Verlag Berlin Heidelberg 2009

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250

Call option, see Equity-FX option, call Call spread overlay (CSO), 180 Callability, 5, 24 Callable daily range accrual (CDRAN), 178 Cancellability, 150, 189 Cancellable swap, see Interest-rate swap, cancellable Cap, see Interest-rate option, cap Capital, 10, 175, 185, 186, 232 Caplet, see Interest-rate option, caplet Cash bond, 11 Cash settlement, 81, 156 CDO, see Collateral debt obligation CDRAN, see Callable daily range accrual CDS, see Credit default swap CDS curve, 39 CDX, 215 CEV, see Constant Elasticity of Variance Change of measure, 26, 27, 34, 202, 241 Characteristic function, 59, 60, 63 Cliquet, 168 Close-out risk, 8, 13, 183, 184, 194, 195 CMBX, 215 Coherent measure, 204 Collateral, 8, 183, 184, 188, 191, 192, 194, 195, 199 Collateral agreement, 13, 184, 190–194, 223 Collateral debt obligation (CDO), 3, 43, 126, 172, 173 attachment / detachment point, 43, 172 tranche, 43, 65, 69, 73, 171–173 Commodities, 159 Conservatorship, 189 Constant Elasticity of Variance (CEV) model, 55 Constant maturity swap, see Interest-rate swap, constant maturity Contingent claim, 241 Contingent credit default swap (C-CDS), 109, 135, 216–226 PAL example, 109 Continuation value, see American Monte Carlo (AMC), continuation value Contribution, 196, 197, 199 Convertible bond, 179 Convexity, see Volatility, convexity Copula, 39, 42 Gaussian, 42, 69, 73

Index

Corporate bond, 42 Correlation between Brownian Motions, 62 between drivers, 36 between reference names, 42 between volatility and asset price, 58 calibration, 75, 126 Gaussian copula, 42, 69 instantaneous, 37, 38, 58 product, 65 structure, 65 Cost of funding, 232 Counterparty credit exposure approximations, 233 basic concepts, 3 cross-gammas, 94, 220 definition, 10, 23–25, 80 hedging, 10, 216–220 profile, 5 sensitivities, 4, 9, 79, 94–97, 221–223 simulation, 17 Credit charge, see Credit valuation adjustment Credit crisis, 173, 184 Credit default swap (CDS), 3, 8, 39, 171, 172, 215, 220, 225 PAL example, 106 payment leg, 106, 171, 172 protection leg, 106, 109, 171, 216, 217 Credit derivative, 3, 27, 38, 65, 116, 171– 173, 185, 195 Credit exposure, see Counterparty credit exposure Credit loss, 42, 65, 116, 171 Credit model, 38–41, 65, 66 Credit Officer, 184 Credit protection, 15 Credit quantification, vii, 15 Credit risk, 8 Credit spread, 8, 15, 38, 65 Credit spread overlays (CSO), 179, 180 Credit support annex (CSA), 14, 188, 192– 194 Credit valuation adjustment (CVA), 4, 8, 14, 109, 190, 195, 213, 216, 217 approximation, 235 sensitivities, 221–223 Cross-currency swap, 153, 188, 212

Index

Cross-gammas, see Counterparty credit exposure, cross-gammas CSA, see Credit support annex CSO, see Call spread overlay CVA, see Credit valuation adjustment D Debt valuation adjustment (DVA), 186, 190, 232 Default correlation, 42 dependence, 69 probability, 38–42 stochastic probability, 41 time, 38–42, 66–69, 216 Delta, see Counterparty credit exposure, sensitivities Digital option, see Equity-FX option, digital Disaggregation, see Risk allocation Distance-to-default, 206, 212 DJ Euro Stoxx, 167–169 Domestic currency, see Reference currency Dupire formula, 63 DVA, see Debt valuation adjustment E EAD, see Exposure at default EPE, see Expected positive exposure Equity model, 37, 38, 52–64 Equity tranche, see Collateral debt obligation (CDO), tranche Equity-FX option American, 159, 160, 162 Asian, 159, 160, 164 barrier, 160, 164 Bermudan, 159, 160, 162 call, 15, 160, 162, 187, 210 digital, 160, 162, 178 European, 159, 160 PAL examples, 106 put, 160, 162, 187, 206 vanilla, 162 Equivalent martingale measure, 242 Error analysis, see American Monte Carlo (AMC), error ES, see Expected shortfall Euler allocation, see Risk allocation, Euler Euler scheme, 117–119

251

European option, see Interest rate option, European Expected positive exposure (EPE), 4, 12, 185 approximation, 235 modified, 127, 186, 216, 225 reverse, 186 Expected shortfall (ES), 9, 12, 162, 172, 185 Expected tail loss, see Expected shortfall Exposure at default (EAD), 186 Exposure profile, see Counterparty credit exposure Expression tree, 103 F Fannie Mae, 188 Fast Fourier Transform (FFT), 59, 62 Fat tail, 52 FFT, see Fast Fourier Transform Filtration, 11, 24, 239 Flesaker-Hughston framework, 27 Floorlet, see Interest-rate option, floorlet Foreign currency, 33–37 Forward contract, 159, 160, 169 strip, 159 Freddie Mac, 188 Functional requirements, 136 FX model, 33–37, 52–64 G Gamma, see Counterparty credit exposure, sensitivities Gas price, 206 Gaussian copula, see Copula, Gaussian Gaussian model, 39 Geometric Brownian Motion, 161, 233 Gold price, 204, 205 Grace period, 195 Grammar, 103, 104 Granularity bias, see American Monte Carlo (AMC), granularity bias Grid, 131, 142 H Haircut, 192, 195 Heath-Jarrow-Morton framework (HJM), 31, 126 Hedging, see Counterparty credit exposure, hedging

252

Heston model, 58–60, 62 HJM, see Heath-Jarrow-Morton framework Hull-White model, 31, 45, 50–52 Hybrid model, 75 Hybrid product, 20, 75, 183, 229 I Implied volatility, see Volatility, implied Inflation index, 159 Inflation model, 37, 63 Initial margin, 13, 191, 195 Instantaneous forward rate, 50 Intensity process, 41 Interest-rate model, 27–33, 45–52 Interest-rate option, 156 Bermudan, 8, 83 cap, 47 caplet, 47, 156, 225 digital, 156 European, 8 floorlet, 156 Interest-rate swap, 149–152, 234 cancellable, 5, 84, 93, 103, 150, 152 capped, 150, 152 constant maturity, 154 floored, 150, 152 in advance, 150 in arrears, 105, 150 PAL examples, 105 range accrual, 155 steepener, 45, 126, 154 vanilla, 4, 5, 151 International Swap Dealer Association (ISDA), 188 Intrinsic exercise, 81 ITraxx, 3, 215, 222 J Jump model, 60 K Kurtosis, see Volatility, kurtosis L Laplace transform, 60 Large homogeneous portfolio, 73 Lévy process, 52, 60 Lexer, 104 Libor market model (BGM), 32, 126

Index

Libor rate, 26, 29 Limits, 183, 188, 190 Local currency, see Reference currency Local volatility model, 52–56, 60 Longstaff-Schwartz algorithm, 89 Loss distribution, 73 Loss product, see Credit derivative Loss simulation, 42 M Margin call, 14, 184, 191 Markov process, 27 Martingale, 17, 26, 239–243 Martingale interpolation, 122 Maximum likely exposure (MLE), 233 MBS, see Mortgage backed securities Mean, 4 Measure, see Pricing measure and physical measure Merton model, 206 Mezzanine tranche, see Collateral debt obligation (CDO), tranche Milstein scheme, 118, 119 Minimum transfer amount, 14, 191 MLE, see Maximum likely exposure Model risk, 125 Modified EPE, see Expected positive exposure, modified Monoline, 3, 14 Monte Carlo, 11, 17–20, 108, 111, 117–121, 183, 186, 196 Mortgage backed securities (MBS), 13 Municipality, 175 N Nested simulations, 8, 144 Netting, 12 Netting / no-netting agreement, 188 Nominal rate, 159 Number of paths, 131 Numeraire, 11, 24–26, 243 Numeraire measure, 25 O Object-oriented, 105, 106 Observables, see American Monte Carlo (AMC), observables Oil price, 203–206, 212

Index

Option, see Equity-FX option and interest rate option OTC, see Over the counter Over the counter (OTC), 3, 109, 229 Own credit, 232 P P&L, see Profit and loss (P&L) PAL, see Portfolio aggregation language Parser, 104 Partial differential equation (PDE), 19 Payment leg, see Credit default swap (CDS), payment leg Payoff language, 173 PDE, see Partial differential equation Percentile, see Quantile PFE, see Potential future exposure Physical measure, 25, 201, 202 Physical settlement, 81, 156 Portfolio aggregation language (PAL), 20, 21, 103–108, 135 Portfolio manager, 139 Potential future exposure (PFE), 4, 12, 184 approximation, 234 Price distribution, 4, 5, 23, 101, 125 Pricing function, 19 Pricing measure, 11, 185–187, 201 Probability measure, 240 Probability space, 24, 239 Procedural language, 104 Profit and loss (P&L), 18, 96, 98, 221 volatility, 216, 217 Programming language, 104–106 Protection, see Credit protection Protection leg, see Credit default swap (CDS), protection leg Put option, see Equity-FX option, put Q Quantification unit, 141 Quantile, 4, 184, 195 Quanto adjustment, 38 R Radon-Nikodym derivative, 34, 202, 203, 208, 240 Range accrual, see Interest-rate swap, range accrual Rating, 173

253

Rating migration, 65 Rational lognormal model, 27 Real measure, see Physical measure, 11 Real rate, 64, 159 Real-world measure, see Physical measure Recovery rate, 9, 171–173, 215, 216 Recursion, 72 Reference currency, 27, 188 Reference measure, 27 Rehypotecation, 190 Relative return swap, see Asset swap, relative return Repudiation risk, 189 Retail price index (RPI), 37, 159 Right-way/wrong-way exposure, 14, 180, 201, 205–213, 224 Risk aggregation, 12, 187–190 Risk allocation, 13, 195–198 Euler, 196–198 naive, 196 Risk Control, 184, 187 Risk measure, 12, 184–187, 196 Risk mitigation, 190–194 Risk weighted assets (RWA), 10, 185, 232 Risk-neutral measure, see Pricing measure Risk-on-risk, see Right-way/wrong-way exposure RPI, see Retail price index RWA, see Risk weighted assets S Scenario, 17, 18, 23, 108, 125 consistency, 13, 19, 23, 75, 183, 187, 189 generation, 11, 141 SDE, see Stochastic differential equation Senior tranche, see Collateral debt obligation (CDO), tranche Sensitivities, see Counterparty credit exposure, sensitivities Separable volatility, see Volatility, separable Sharpe ratio, 203, 221 Shifted Libor rate, 33 Short rate, 30, 50 Simulation, 17 Sinking fund, 175, 176 Skew, see Volatility, skew Smile, see Volatility, smile State of the world, 141 State-price density, 27

254

Statistics, 102, 116 Steepener, see Interest-rate swap, steepener Stochastic differential equation (SDE), 28, 31, 117–121 Stochastic volatility model, 52, 53, 58, 60, 62, 63 Straddle, 169 Stress scenario, 201 Stress test, 10, 204, 205 Structured products, 175 Submartingale, 240 Subprime mortgage, 13 Super-product, 108, 135, 218 Super-senior tranche, see Collateral debt obligation (CDO), tranche Supermartingale, 27, 240 Survival probability, 41, 66, 67 Swap, see Interest-rate swap Swaption, 48, 156, 236 cash settled, 81, 156 European, 84, 94, 103 PAL example, 106 physically settled, 81, 84, 103, 156 Swaption approach, 236 T Tail risk, 185 Target redemption swap, 169 TCP-IP, 142 Threshold, 13, 191 Tilley algorithm, 88 Tranche, see Collateral debt obligation (CDO), tranche Translator, 21, 139

Index

external, 139 internal, 141 U UML, see Unified modeling language Unified modeling language (UML), 139 V Value-at-risk, 10 Variance swap, see Calibration, to variance swaps Vasicek model, 50 Vega risk, 229 Volatility, 17 at-the-money, 234 convexity, 55, 58 historical, 234 implied, 47, 53, 229, 234 instantaneous, 38 kurtosis, 63 separable, 30, 31, 45–47 skew, 52, 54–56, 58, 60, 63 smile, 54, 63 Volume weighted average price, 177 VWAP, see Volume weighted average price W West Texas Intermediate (WTI), 204, 212 Wrong way risk, see Right-way/wrong-way exposure WTI, see West Texas Intermediate Y Yield curve, 27, 29, 45

Editorial Board M. Avellaneda G. Barone-Adesi M. Broadie M.H.A. Davis E. Derman C. Klüppelberg W. Schachermayer

Springer Finance Springer Finance is a programme of books addressing students, academics and practitioners working on increasingly technical approaches to the analysis of financial markets. It aims to cover a variety of topics, not only mathematical finance but foreign exchanges, term structure, risk management, portfolio theory, equity derivatives, and financial economics. Ammann M., Credit Risk Valuation: Methods, Models, and Application (2001) Back K., A Course in Derivative Securities: Introduction to Theory and Computation (2005) Barucci E., Financial Markets Theory. Equilibrium, Efficiency and Information (2003) Bielecki T.R. and Rutkowski M., Credit Risk: Modeling, Valuation and Hedging (2002) Bingham N.H. and Kiesel R., Risk-Neutral Valuation: Pricing and Hedging of Financial Derivatives (1998, 2nd ed. 2004) Brigo D. and Mercurio F., Interest Rate Models: Theory and Practice (2001, 2nd ed. 2006) Buff R., Uncertain Volatility Models – Theory and Application (2002) Carmona R.A. and Tehranchi M.R., Interest Rate Models: An Infinite Dimensional Stochastic Analysis Perspective (2006) Dana R.-A. and Jeanblanc M., Financial Markets in Continuous Time (2003) Deboeck G. and Kohonen T. (Editors), Visual Explorations in Finance with Self-Organizing Maps (1998) Delbaen F. and Schachermayer W., The Mathematics of Arbitrage (2005) Elliott R.J. and Kopp P.E., Mathematics of Financial Markets (1999, 2nd ed. 2005) Fengler M.R., Semiparametric Modeling of Implied Volatility (2005) Filipovi´c D., Term-Structure Models (2009) Fusai G. and Roncoroni A., Implementing Models in Quantitative Finance (2008) Geman H., Madan D., Pliska S.R. and Vorst T. (Editors), Mathematical Finance – Bachelier Congress 2000 (2001) Gundlach M. and Lehrbass F. (Editors), CreditRisk+ in the Banking Industry (2004) Jeanblanc M., Yor M., Chesney M., Mathematical Methods for Financial Markets (2009) Jondeau E., Financial Modeling Under Non-Gaussian Distributions (2007) Kabanov Y.A. and Safarian M., Markets with Transaction Costs (2009) Kellerhals B.P., Asset Pricing (2004) Külpmann M., Irrational Exuberance Reconsidered (2004) Kwok Y.-K., Mathematical Models of Financial Derivatives (1998, 2nd ed. 2008) Malliavin P. and Thalmaier A., Stochastic Calculus of Variations in Mathematical Finance (2005) Meucci A., Risk and Asset Allocation (2005, corr. 2nd printing 2007, Softcover 2009) Pelsser A., Efficient Methods for Valuing Interest Rate Derivatives (2000) Platen E. and Heath D., A Benchmark Approach to Quantitative Finance (2006, corr. printing 2010) Prigent J.-L., Weak Convergence of Financial Markets (2003) Schmid B., Credit Risk Pricing Models (2004) Shreve S.E., Stochastic Calculus for Finance I (2004) Shreve S.E., Stochastic Calculus for Finance II (2004) Yor M., Exponential Functionals of Brownian Motion and Related Processes (2001) Zagst R., Interest-Rate Management (2002) Zhu Y.-L., Wu X., Chern I.-L., Derivative Securities and Difference Methods (2004) Ziegler A., Incomplete Information and Heterogeneous Beliefs in Continuous-time Finance (2003) Ziegler A., A Game Theory Analysis of Options (2004)

Giovanni Cesari John Aquilina Niels Charpillon Zlatko Filipovi´c Gordon Lee Ion Manda

Modelling, Pricing, and Hedging Counterparty Credit Exposure A Technical Guide

Giovanni Cesari UBS AG 100 Liverpool Street London UK EC2M 2RH [email protected]

Zlatko Filipovi´c UBS AG 100 Liverpool Street London UK EC2M 2RH [email protected]

John Aquilina UBS AG 100 Liverpool Street London UK EC2M 2RH [email protected]

Gordon Lee UBS AG 100 Liverpool Street London UK EC2M 2RH [email protected]

Niels Charpillon UBS AG 100 Liverpool Street London UK EC2M 2RH [email protected]

Ion Manda UBS AG 100 Liverpool Street London UK EC2M 2RH [email protected]

ISBN 978-3-642-04453-3 e-ISBN 978-3-642-04454-0 DOI 10.1007/978-3-642-04454-0 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2009942279 Mathematics Subject Classification (2000): 60H10, 60H20, 60H35, 62P05, 65C05, 65C20, 91Bxx, 91-08 JEL Classification: C02, C61, C63, E43, E47, G12, G13, G32, G33 © Springer-Verlag Berlin Heidelberg 2009 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: WMXDesign GmbH Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Preface

It was the end of 2005 when our employer, a major European Investment Bank, gave our team the mandate to compute in an accurate way the counterparty credit exposure arising from exotic derivatives traded by the firm. As often happens, exposure of products such as, for example, exotic interest-rate, or credit derivatives were modelled under conservative assumptions and credit officers were struggling to assess the real risk. We started with a few models written on spreadsheets, tailored to very specific instruments, and soon it became clear that a more systematic approach was needed. So we wrote some tools that could be used for some classes of relatively simple products. A couple of years later we are now in the process of building a system that will be used to trade and hedge counterparty credit exposure in an accurate way, for all types of derivative products in all asset classes. We had to overcome problems ranging from modelling in a consistent manner different products booked in different systems and building the appropriate architecture that would allow the computation and pricing of credit exposure for all types of products, to finding the appropriate management structure across Business, Risk, and IT divisions of the firm. In this book we describe some of our experience in modelling counterparty credit exposure, computing credit valuation adjustments, determining appropriate hedges, and building a reliable system. What do we mean by all of this? Counterparty credit exposure is the amount a company could potentially lose in the event of one of its counterparties defaulting. At a general level, computing credit exposure entails simulating in different scenarios and at different times in the future, prices of transactions, and then using one of several statistical quantities to characterise the price distributions that has been generated. Typical statistics used in practice are (i) the mean, (ii) a high-level quantile such as the 97.5% or 99%, usually called Potential Future Exposure (PFE), and (iii) the mean of the positive part of the distribution, usually referred to as Expected Positive Exposure (EPE). With these measures and default probability information or counterparty CDS premia, it is then possible to price counterparty risk. In the financial industry, the economic value of this risk is generally called Credit Valuation Adjustment (CVA). v

vi

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Preface

vii

As we will have occasion to see later in this book, it can be computed as the price of a Credit Default Swap paying the Expected Positive Exposure. Equivalently expressed, CVA is the price of a new type of hybrid product, the so-called Contingent Credit Default Swap (C-CDS), which pays the value of the exposure (floored at zero) upon default of the counterparty. The credit crisis which started in 2007 clearly shows why it is of crucial importance for any company entering a derivative business to (i) measure counterparty exposure, (ii) compute capital requirements, and (iii) hedge counterparty risk. Measuring counterparty exposure is important for setting limits on the amount of business a firm is prepared to do with a given counterparty; hedging it gives a possibility of mitigating it and transferring risk; and from a regulatory perspective there is significant pressure on financial institutions to have the capability of producing accurate risk measures to compute capital. In addition, computing counterparty exposure can also give insights into prices of complex products in potential future scenarios. It seems that what was until recently a Risk Control function attracting relatively limited attention, is now becoming a central activity of all major financial institutions, requiring significant resources from all parties. Our approach to counterparty credit exposure analysis is quantitative. The focus is on mathematical modelling, simulation techniques using various Monte Carlo approaches, and pricing. In contrast, we are only marginally interested in assessing the quality of counterparties or in analysing historical market data in order possibly to forecast future behaviours of the economy, or in risk and regulatory aspects of the problem. We consider derivative products and complex structures which are usually traded in Investment Banks, and focus on practical aspects of the problems at hand. All models used in our analyses are tested with practical data and real transactions. Given this quantitative focus, we sometimes refer to our work as Credit Quantification. The book is divided into four parts, (I) Methodology, (II) Architecture and Implementation, (III) Products, and (IV) Hedging and Managing Counterparty Risk. In Part I we present a generic simulation framework, which can be used to compute counterparty exposure for both vanilla and exotic products. We show how the classical Monte Carlo framework, where price distributions are computed by generating thousands of scenarios and by explicitly pricing the product at each point in time and at each scenario, is a special case of our more general framework. The classical Monte Carlo approach works well only for products that can be priced in analytical or quasi-analytical form. It is not practical for products that cannot be priced in closed form and require, for instance, a Monte Carlo or lattice pricing approach. Typical examples are products with callability features or exotic interestrate transactions. We show how in these cases American Monte Carlo techniques used generally for pricing can also be applied efficiently to compute exposure, as they provide intermediate valuations over time and scenarios. Part II shows how our simulation framework naturally leads to the implementation of a software architecture and the definition of a programming language that allows the computation of both vanilla and exotic products in a scenario-consistent way. In practice, in a large financial institution one of the main problems in building

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Preface

counterparty exposure systems, is to integrate different products, booked in different systems and priced using libraries written in different languages and with different technologies, in order to compute portfolio exposure across different businesses. We show that our approach leads to an architecture that can integrate other systems in a natural way. In Part III we consider how to compute exposure for different products. We show how the general techniques and models described in Part I and the architecture described in Part II can be used in practice. Finally in Part IV things are put together. We consider how to perform risk management and risk control of counterparty exposure on a portfolio basis. We describe different aggregation techniques and a standard set-up that uses collateral to mitigate exposure. We also analyse how to model wrong-way/right-way exposure, where transaction price fluctuations and quality of the counterparty are correlated and we address the problem of changing the reference probability measure after the simulation has been performed. The final chapter is dedicated to pricing counterparty credit exposure and to computing CVA and CVA sensitivities not only to credit spread, but also to market risk factors. The whole book can be seen as a roadmap to achieve this goal. One note to conclude: in our work we describe and use well-established simulation and pricing techniques. Our goal is not to suggest new or more sophisticated algorithms. It is rather to show how well-known algorithms can be put together and used to compute counterparty credit exposure and which limitations have to be taken into consideration if we want to move from vanilla products to complex exotic transactions. London, September 2009

Giovanni Cesari John Aquilina Niels Charpillon Zlatko Filipovi´c Gordon Lee Ion Manda

Acknowledgements

This book developed from the experience gained during a long-term project within the Investment Bank we work for. As such it would have never been written without the support, advice and encouragement of several people to whom we would like to express our gratitude. Duncan Rodgers and Myles Wright gave us full support in the development of the ideas described in this book. Darryll Hendricks and Tom Daula helped us to have a global approach to credit exposure computation and inspired us in the early stages of this project. From Thomas Hyer and Trevor Chilton we gained a better understanding of the challenges of the American Monte Carlo (AMC) algorithms, while Yuan Gao engineered an early prototype that used AMC not just for pricing but also to estimate credit exposure. Helmut Glemser kept us on our toes by insisting on explanations (or corrections!) whenever our calculations gave results that puzzled him. Sincere thanks go to our colleagues Rong Fan and Yi Yuan, who, from across the Atlantic, contributed to the writing and testing of a significant part of our code. We would like also to thank Mark Davis and Martijn Pistorius for their very useful comments, and Catriona Byrne and her team for their continuous support during the publishing phase of this book. Thanks are also due to the following people for useful discussions during the course of the project: Richard Adams, Annette Alford, Rowan Alston, Philip Anderson, Ashima Bhalla, Rajinder Basra, Marc Baumslag, Alan Baxter, Lucia Bonilla, Gareth Campbell, Denton Capp, Ben Cassie, Dean Charette, Paul Charles, Dipak Chotai, Jack Chung, Mark Dahl, Valdemar Dallagnol, Gerald Elflein, Jesus Fernandez, Alex Ginzburg, Alasdair Gray, Lionel Guerraz, Stephen Johnston, Jeffrey Lin, Catarina Lopes, Felix Matschke, David Matthews, Peta McRae, Sourav Mishra, Andrew Morgan, Bruno Mugica, Logan Nerio, James Ntuk-Idem, Sarah Peplow, Tom Prangley, David Shieff, Andrew Tseng, and Winnie Zheng.

ix

Disclaimer

The views and opinions expressed in this book are those of the authors and are not those of UBS AG, its subsidiaries or affiliate companies (“UBS”). Accordingly, UBS does not accept any liability over the content of this book or any claims, losses or damages arising from the use of, or reliance on, all or any part of this book. Nothing in this book is or should be taken as information regarding, or a description of, any operations or business practice of UBS. Similarly, nothing in this book should be taken as information regarding any failure or shortcomings within the business, credit or risk or other control, or assessment procedures within UBS.

xi

About the Authors

Giovanni Cesari is Managing Director at UBS. He has more than 10 years’ experience in modelling and pricing counterparty credit exposure. Before moving to finance, Giovanni worked for several years in particle physics and in theoretical computer science. Giovanni holds a Laurea in Electrical Engineering from the University of Trieste, a Perfezionamento in Physics from the University of Padova, and a Ph.D. from ETH, Zurich. John Aquilina holds an M.Phil. in Statistical Science from the University of Cambridge and a Ph.D. in Mathematical Finance from the University of Bath. He has worked on modelling counterparty credit exposure at UBS since 2005. Niels Charpillon holds a Diplôme d’Ingénieur from Ecole des Mines, an M.Sc. in Financial Mathematics from Warwick Business School, and a Licence in Economics from University of St. Etienne. He joined the counterparty exposure team at UBS in 2006. Zlatko Filipovi´c started working for UBS in 2005 as a Quantitative Analyst in the counterparty exposure team. Before joining UBS, Zlatko had been working for Mako Global Derivatives, London, as a Financial Engineer. Zlatko obtained a Ph.D. in Quantitative Finance from Imperial College, London, after graduating from the Faculty of Mathematics, University of Belgrade. Gordon Lee joined the counterparty exposure team at UBS in 2006. Prior to UBS, he was a Senior Associate in quantitative risk and performance analysis at Wilshire Associates. Gordon holds an M.A. in Mathematics from Churchill College, University of Cambridge. Ion Manda holds a Diploma de Inginer from the University of Bucharest and a M.Sc. in Financial Engineering from University of London. He has been working in the credit exposure team at UBS since 2006. Ion has about 10 years’ experience as a software engineer. xiii

Contents

Part I

Methodology

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Preliminary Examples . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Vanilla Interest-Rate Swap . . . . . . . . . . . . . . . 1.2.2 Cancellable Swap . . . . . . . . . . . . . . . . . . . . 1.2.3 Managing Credit Risk—Collateral, Credit Default Swap 1.3 Why Compute Counterparty Credit Exposure? . . . . . . . . . 1.4 Modelling Counterparty Credit Exposure . . . . . . . . . . . . 1.4.1 Definition . . . . . . . . . . . . . . . . . . . . . . . . 1.4.2 Risk Measures . . . . . . . . . . . . . . . . . . . . . . 1.4.3 Netting and Aggregation . . . . . . . . . . . . . . . . . 1.4.4 Close-Out Risk . . . . . . . . . . . . . . . . . . . . . . 1.4.5 Right-Way/Wrong-Way Exposure . . . . . . . . . . . . 1.4.6 Credit Valuation Adjustment: CVA . . . . . . . . . . . 1.4.7 A Simple Credit Quantification Example . . . . . . . . 1.4.8 Computing Credit Exposure by Simulation . . . . . . . 1.4.9 Implementation Challenges . . . . . . . . . . . . . . . 1.4.10 An Alternative Approach: The AMC Algorithm . . . . 1.5 Which Architecture? . . . . . . . . . . . . . . . . . . . . . . . 1.6 What Next? . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Modelling Framework . . . . . . . . . . . . . 2.1 Counterparty Credit Exposure Definition . 2.2 Process Dynamics . . . . . . . . . . . . . 2.3 Interest Rate: Single Currency . . . . . . . 2.3.1 Simple Specifications . . . . . . . 2.3.2 HJM Framework . . . . . . . . . . 2.3.3 Libor Market Models . . . . . . . 2.4 Multiple Currencies and Foreign Exchange 2.5 Inflation . . . . . . . . . . . . . . . . . .

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Contents

2.6 Equity . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Credit . . . . . . . . . . . . . . . . . . . . . . . . 2.7.1 Default Probabilities from par CDS Spreads 2.7.2 Stochastic Default Probabilities . . . . . . . 2.7.3 Loss Simulation . . . . . . . . . . . . . . . 3

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Simulation Models . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Interest-Rate Models . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Separable Volatility . . . . . . . . . . . . . . . . . . . . 3.1.2 Example: Hull-White (Extended Vasicek) . . . . . . . . . 3.2 Equity and FX Models . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Black Model . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Local Volatility . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Stochastic Volatility . . . . . . . . . . . . . . . . . . . . 3.2.4 Jump Models . . . . . . . . . . . . . . . . . . . . . . . . 3.2.5 Extension to Stochastic Interest Rates . . . . . . . . . . . 3.2.6 A Simpler Approach: Independent Interest Rates . . . . . 3.2.7 Different Models for Different Markets . . . . . . . . . . 3.3 Credit Models . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Simulation of Single-Name Default Probabilities and Default Times . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Inter-Name Default Dependence . . . . . . . . . . . . . 3.3.3 Technical Note: Recursion . . . . . . . . . . . . . . . . . 3.3.4 Properties of the Loss Distribution: Large Homogeneous Portfolio . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.5 Calibration of Correlation . . . . . . . . . . . . . . . . . 3.4 Choice of Model . . . . . . . . . . . . . . . . . . . . . . . . . .

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Valuation and Sensitivities . . . . . . . . . . . . . . . . . . . . . . 4.1 American Monte Carlo: Mathematical Notation and Description 4.1.1 Mathematical Formulation . . . . . . . . . . . . . . . . 4.1.2 Practical Examples . . . . . . . . . . . . . . . . . . . . 4.1.3 Backward Induction Algorithm . . . . . . . . . . . . . 4.2 AMC Estimation Algorithms . . . . . . . . . . . . . . . . . . 4.2.1 Tilley’s Algorithm . . . . . . . . . . . . . . . . . . . . 4.2.2 Longstaff-Schwartz Regression . . . . . . . . . . . . . 4.2.3 Biases of Estimates . . . . . . . . . . . . . . . . . . . 4.2.4 An AMC Algorithm to Compute Credit Exposure . . . 4.3 Post-Processing of the Price Distribution . . . . . . . . . . . . 4.4 Practical Examples Revisited . . . . . . . . . . . . . . . . . . 4.5 Computing Price Sensitivities . . . . . . . . . . . . . . . . . . 4.5.1 The Classical Approach . . . . . . . . . . . . . . . . . 4.5.2 Price Sensitivities through Regression . . . . . . . . . 4.5.3 Removing Correlation . . . . . . . . . . . . . . . . . . 4.6 Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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79 80 80 83 85 88 88 89 90 91 93 93 94 95 95 96 98

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Contents

Part II

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Architecture and Implementation

5

Computational Framework . . . . . . . . . . . . . . . 5.1 AMC Implementation and Trade Representation . . 5.1.1 Examples . . . . . . . . . . . . . . . . . . . 5.1.2 Expression Trees . . . . . . . . . . . . . . . 5.2 A Portfolio Aggregation Language . . . . . . . . . 5.2.1 PAL Examples . . . . . . . . . . . . . . . . 5.3 The Concept of Scenarios . . . . . . . . . . . . . . 5.4 The Concept of Super-Product . . . . . . . . . . . . 5.4.1 An Example of Super-Products: The C-CDS

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6

Implementation . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Spot and Forward Statistics . . . . . . . . . . . . . . . . 6.1.1 Libor Rates and Bond Prices . . . . . . . . . . . 6.1.2 Annuity . . . . . . . . . . . . . . . . . . . . . . 6.1.3 Swap Rate . . . . . . . . . . . . . . . . . . . . . 6.2 Path Dependent Statistics . . . . . . . . . . . . . . . . . 6.2.1 Extremum . . . . . . . . . . . . . . . . . . . . . 6.2.2 Average . . . . . . . . . . . . . . . . . . . . . . 6.2.3 In Range Fraction . . . . . . . . . . . . . . . . . 6.2.4 Credit Loss . . . . . . . . . . . . . . . . . . . . . 6.3 Monte Carlo Stepping . . . . . . . . . . . . . . . . . . . 6.4 Technical Notes . . . . . . . . . . . . . . . . . . . . . . 6.4.1 SDE Integration Schemes . . . . . . . . . . . . . 6.4.2 Milstein 2 Scheme . . . . . . . . . . . . . . . . . 6.4.3 Martingale Interpolation . . . . . . . . . . . . . . 6.4.4 Distribution of Maxima and Minima . . . . . . . 6.5 Error Analysis . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Choice of Model: Scenario and Exposure Analysis 6.5.2 AMC Error . . . . . . . . . . . . . . . . . . . . . 6.5.3 Numerical Errors . . . . . . . . . . . . . . . . . 6.5.4 Approximations: Arbitrage Conditions . . . . . .

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7

Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.1 Functional, Non-Functional Requirements, and Design Principles . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Conceptual View: Methodology . . . . . . . . . . . . . . . . . 7.3 Logical View . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 Portfolio Manager Components . . . . . . . . . . . . . 7.3.2 State of the World Components . . . . . . . . . . . . . 7.3.3 Quantification Components . . . . . . . . . . . . . . . 7.4 Physical View . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Alternative Approaches . . . . . . . . . . . . . . . . . . . . .

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xviii

Contents

Part III Products 8

Interest-Rate Products . . . . . . . . . . . . 8.1 Interest-Rate Swaps . . . . . . . . . . . 8.1.1 Swaps in Advance and in Arrears 8.1.2 Capped and Floored Swaps . . . 8.1.3 Cancellable Swaps . . . . . . . . 8.1.4 Cross-Currency Swaps . . . . . . 8.2 Constant-Maturity Swaps and Steepeners 8.3 Range Accruals . . . . . . . . . . . . . 8.4 Interest-Rate Options . . . . . . . . . .

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9

Equity, Commodity, Inflation and FX Products 9.1 Forwards and Options . . . . . . . . . . . . 9.1.1 Forwards Contracts . . . . . . . . . 9.1.2 Vanilla and Digital Options . . . . . 9.1.3 Bermudan and American Options . . 9.1.4 Asian Options . . . . . . . . . . . . 9.1.5 Barrier Options . . . . . . . . . . . 9.2 Asset Swaps . . . . . . . . . . . . . . . . . 9.2.1 Absolute Return Swaps . . . . . . . 9.2.2 Relative Return Swaps . . . . . . . . 9.2.3 Cliquets . . . . . . . . . . . . . . . 9.2.4 Target Redemption Swaps . . . . . .

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10 Credit Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 10.1 Credit Default Swaps . . . . . . . . . . . . . . . . . . . . . . . . 171 10.2 Collateral Debt Obligations . . . . . . . . . . . . . . . . . . . . . 172 11 Structures . . . . . . . . . . . . . . 11.1 Sinking Funds . . . . . . . . . 11.2 Accelerated Share Re-Purchase 11.3 Callable Daily Accrual Notes . 11.4 Call Spread Overlays . . . . .

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12 Counterparty Risk Aggregation and Risk Mitigation . . . . . . . 12.1 Risk Measures . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Choice of Measure . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Portfolio Risk Aggregation . . . . . . . . . . . . . . . . . . . 12.3.1 Reference Currency . . . . . . . . . . . . . . . . . . . 12.3.2 Netting and No-Netting Agreements . . . . . . . . . . 12.3.3 Break Clauses . . . . . . . . . . . . . . . . . . . . . . 12.4 Collateral Agreements . . . . . . . . . . . . . . . . . . . . . . 12.4.1 Counterparty Exposure of Collateralised Counterparties

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Part IV Hedging and Managing Counterparty Risk

Contents

xix

12.4.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 12.5 Close-Out Risk . . . . . . . . . . . . . . . . . . . . . . . . . . 12.6 Risk Allocation . . . . . . . . . . . . . . . . . . . . . . . . . 12.6.1 Naive Allocation . . . . . . . . . . . . . . . . . . . . . 12.6.2 Euler Allocation . . . . . . . . . . . . . . . . . . . . . 12.6.3 Comparison with Naive Allocation . . . . . . . . . . . 12.6.4 Contribution Calculation of Collateralised Transactions

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13 Combining Market and Credit Risk . . . . . . . . . . . . . . . 13.1 Change of Measure: Practical Implementation . . . . . . . . 13.2 Exposure under Real-World Measure . . . . . . . . . . . . . 13.3 Stress Testing . . . . . . . . . . . . . . . . . . . . . . . . . 13.4 Right-Way/Wrong-Way Exposure . . . . . . . . . . . . . . . 13.4.1 Right-Way/Wrong-Way Exposure: Merton Approach 13.4.2 The Inverse Problem . . . . . . . . . . . . . . . . . . 13.4.3 Example 1: Call Option on Stock . . . . . . . . . . . 13.4.4 Example 2: Call Put Structure on Oil . . . . . . . . . 13.4.5 Example 3: Cross-Currency Swap on USD-GBP . . . 13.4.6 Comparison with the Change of Measure Approach .

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14 Pricing Counterparty Credit Risk . . . . . . . . . . 14.1 Credit Valuation Adjustment and Static Hedging 14.2 Contingent Credit Default Swap . . . . . . . . . 14.2.1 American Monte Carlo Valuation . . . . 14.2.2 Example . . . . . . . . . . . . . . . . . 14.3 Dynamic Hedging of Counterparty Risk . . . . 14.4 Optimal Static Hedging . . . . . . . . . . . . . 14.5 CVA Sensitivities . . . . . . . . . . . . . . . . 14.6 Collateral Agreements . . . . . . . . . . . . . . 14.7 Right-Way/Wrong-Way Risk . . . . . . . . . . 14.8 Examples . . . . . . . . . . . . . . . . . . . . . 14.8.1 C-CDS on a Vanilla Interest-Rate Swap . 14.8.2 Impact of Discretization Schedule . . . . 14.8.3 Collateralised Equity Swap . . . . . . . 14.9 Case Study . . . . . . . . . . . . . . . . . . . .

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Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 A

Approximations . . . . . . . . . . . . . . . . A.1 Maximum Likely Exposure . . . . . . . A.1.1 MLE of Equity and FX Products A.1.2 MLE of Swaps . . . . . . . . . . A.2 Expected Positive Exposure . . . . . . . A.2.1 EPE and CVA of Equity Options A.2.2 Relation between MLE, EPE . . A.3 CVA of Swaps . . . . . . . . . . . . . .

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B

Contents

Results from Stochastic Calculus and Finance . . . . . . . . . . B.1 Brownian Motion and Martingales . . . . . . . . . . . . . . B.2 Replication of Contingent Claims: Martingale Representation B.3 Change of Numeraire . . . . . . . . . . . . . . . . . . . . .

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249

Part I

Methodology

Chapter 1

Introduction

The aim of this first chapter is to introduce basic notions of counterparty credit exposure, and to motivate with a few simple examples the problems and concepts we will be considering in more detail later in this book.

1.1 Basic Concepts Consider two parties, A and B say, who enter into an OTC transactions portfolio.1 This portfolio could consist of products ranging from interest-rate and crosscurrency swaps in different currencies with various exotic features, to exotic options on equity, foreign exchange and commodity underlyings. It could also include various types of credit derivatives contracts, such as credit default swaps (CDS) on single names or collateral debt obligations (CDO) tranches in swap form on portfolios of reference entities, or credit indices.2 In general a given company, say a financial institution A, will have portfolios with many other counterparties, varying among sovereign entities, corporates, hedge funds, insurance companies (including for examples monolines3 ). It may also happen that the credit quality of the counterparty is not independent of the performance of the transaction entered into, such as what happens for example, when an electricity generating oil-powered plant bets on the price of oil. Counterparty credit exposure is the amount a company, say A, could potentially lose in the event of one of its counterparties defaulting. It can be computed by simulating in different scenarios and at different times in the future, the price of the 1 An

OTC (Over The Counter) transaction is a transaction that is not traded through an exchange.

2A

typical credit index is for example iTraxx; it is composed of the 125 most liquid CDS names referencing European investment grade credits. 3 Monoline

insurance are companies that guarantee to bond investors the payment of coupon and notional. They insure different type of securities, such as CDO, structured products and municipal bonds. Monolines have been affected in the recent credit crunch, raising counterparty risk issues. G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0_1, © Springer-Verlag Berlin Heidelberg 2009

3

4

1 Introduction

transactions with the given counterparty, and then by using some chosen statistic to characterise the price distributions that have been generated. Typical statistics used in the industry are (i) the mean, (ii) the 97.5% or 99% quantile, called Potential Future Exposure (PFE), and (iii) the mean of the positive part of the distribution, referred to as the Expected Positive Exposure (EPE). We will also have occasion to speak about less commonly used statistical measures that can be more appropriate for certain products. As important as measuring counterparty exposure, via PFE or EPE, is the computation of the cost of hedging it, and the capability of having a dynamic hedging strategy, i.e. the computation of exposure sensitivities. In the financial industry the price of hedging is generally called Credit Valuation Adjustment (CVA). We will see that there are strong links between EPE and CVA computation.

1.2 Preliminary Examples Some simple examples will help clarifying these points.

1.2.1 Vanilla Interest-Rate Swap Consider counterparties A and B who enter into an interest-rate swap where A receives every six months the 6-month Libor rate on a notional of $100 million, while paying to B a fixed amount equal to the par 10-year swap rate on the same notional observed at inception. This is a typical swap contract with value zero at inception. As time passes and market conditions change, the value of the swap changes accordingly. Thus, if the

Fig. 1.1 Exposure profile for a typical USD 10-years swap contract, paying fix and receiving floating on a notional of 100 mUSD. The full distribution is shown in Fig. 1.2

1.2 Preliminary Examples

5

swap rate decreases (resp. increases), the transaction will be out of the money (resp. in the money) as seen from A’s point of view. Therefore, if B were to default at a point in the life of the trade when swap rates had increased, then A would need to replace in the market—at higher cost than the fixed amount being paid to B—the floating cashflows promised and not delivered by B. To compute the credit exposure for the swap, we would need to estimate the values the swap could take in different market scenarios at points in the future. Figure 1.1 shows the 97.5%, the 2.5% quantiles and the EPE of the swap price distribution, over its entire life, as seen from party A’s point of view. A plot like this is usually referred to as the exposure profile. Note that the 2.5% quantile seen from A’s perspective, corresponds to the 97.5% quantile seen from B’s perspective. Figure 1.2 shows in the top panel the full price distribution over time. The bottom panel shows three slices of this distribution at three different points in time. For this example, the 97.5% PFE quantile is a function that starts at zero, peaks at around the 4-year point and then decreases to zero. First, by definition, the fixed payment in the trade is the fair value for the swap, and this must therefore have value (and hence exposure level) identically equal to zero at inception. Similarly, towards the end of the transaction, when all payments but one due under the swap have been paid, the exposure remaining is that from only a single coupon exchange. This explains what happens at the right end of the profile. At intermediate times, the shape of the profile is the result of opposing effects. On the one hand, as the interest rates underlying the swap diffuse, there is more variability in the realised Libor rates, potentially leading to higher exposure. On the other hand, as time evolves there are fewer payments remaining under the swap, and this mitigates the effect of diffusing rates. The profile therefore tells us that with 97.5% probability, the loss of A due to default of B will not exceed roughly $28 million. Of course, this estimate is based on market information at inception of the swap, and would change if it were to be repeated at a different time.

1.2.2 Cancellable Swap We can make our example slightly more interesting. It is common for swaps to trade with an additional callability feature, whereby one counterparty would have the option, at certain times in the life of the swap, to cancel (“call”) the transaction for a fixed fee (which may be zero).4 Suppose that party A, from whose perspective we look at exposure, also holds the option to cancel the trade; one says that A is long callability. Assuming A behaves 4 We define “cancellable swap” a swap which has an embedded option to terminate it at zero cost (or at a given predetermined fee). Sometimes these swaps are also called “callable”. We use the term callable swap in a more generic way, considering the possibility that the swap is “called” into a new product. In this sense a cancellable swap is a simple example of a callable swap.

6

1 Introduction

Fig. 1.2 Future value distribution for a typical USD 10-years swap contract, paying fix and receiving floating. The PFE and EPE are shown in Fig. 1.1

1.2 Preliminary Examples

7

Fig. 1.3 Exposure of a typical cancellable 10-years swap, paying yearly the fair swap rate fixed at inception and receiving semi-annually the 6-month Libor rate on a notional of 100 mUSD. On the left (resp. right), the exposure represents long (resp. short) optionality to cancel the swap every year. The value of the swap at time-zero corresponds to the value of the option, which is assumed to be paid up-front by the counterparty

rationally, it would never decide to walk away from the swap in those scenarios where the swap has a high value (because the swap rate has increased and future receivables are worth more than at inception). This means that having the option to cancel, at zero cost, should not affect materially A’s exposure. On the other hand, suppose A is short callability, meaning that it is B who has the option to walk away from the swap. Rational behaviour on B’s part implies that B would cancel the swap when they are making a loss on the transaction, which is exactly when A would be in the money. Thus we would expect that with A being short callability, A’s PFE (and EPE) to B is reduced to zero at each date where B has the option to cancel the transaction. Figure 1.3 shows all this happening. In the left panel, we see that with A having the option to cancel the trade, the PFE profile is similar to that of a vanilla swap, with the exception of the time-zero level, which equates to the value of the cancellation option. On the right we see that A’s exposure is reduced to zero at dates where B can cancel; on remaining dates, the PFE is reduced to that arising from coupons due until the next allowed cancellation date. Note that the time-zero point is not zero but negative from A’s point of view, since it is B who holds the option in this case. Note that in practice the value of the option is often embedded in the fixed coupon of the swap, which has then value zero at inception. From the computational point of view, there is a fundamental difference between the vanilla swap example of the previous section and the cancellable swap we have just described. A vanilla swap can in fact be priced analytically and in a model independent way, and therefore, as we will see, exposure could be computed in a classical Monte Carlo framework, where scenarios are generated and then products are priced at each scenario and each time step. On the other hand, a cancellable swap is priced using a lattice or Monte Carlo simulation, making therefore impractical the

8

1 Introduction

computation of credit exposure itself by Monte Carlo simulation.5 This would entail in fact a Monte Carlo of Monte Carlo approach (with nested simulations), where one set of simulation is used for scenarios and one set, at each time step and scenario, for pricing. We will analyse this aspect in more details in the next chapters.

1.2.3 Managing Credit Risk—Collateral, Credit Default Swap When structuring a new transaction (or portfolio of transactions), one of the criteria is the amount of acceptable credit exposure. This will depend on several factors including risk appetite and quality of the counterparty. The most common way to reduce counterparty exposure is to set up a collateral agreement, whereby the client is required to deposit collateral into a separate account at regular time intervals. A collateral agreement between counterparties can take one of several forms. For instance, it can be in the form of cash or securities, can be called daily or at other regular intervals, and there can be thresholds and minimum transfer amounts. In addition, since the point of holding collateral is to be able to liquidate it in case of the counterparty defaulting, market liquidity plays an important role in determining the amount of collateral needed. When collateral agreements are in place, therefore, credit exposure computation has to take into account features of that agreement together with the dynamics of the trade itself, in order to compute so-called close-out risk. Close-out risk measures the amount by which the value of a transaction could change during the period from when the counterparty is deemed to have defaulted, until the collateral has been liquidated and used to fund, at current market conditions, the replacement of the defaulted counterparty in the transaction. In general this computation should also include change in value of the collateral, possibly taking into account the correlation between collateral and transaction value. A further possibility for A to manage counterparty credit exposure to B is to buy Credit Default Swap (CDS) protection on B from another counterparty C. The transaction between A and C is typically fully collateralised. This will transfer the risk of B to C. In case of default of B, the CDS would ensure that C will step in and make good any payments that were originally promised by B, or simply pay the value of the transaction. This should cover the value of the products (e.g. the interestrate swap we described before) as calculated at the time of B’s default. The value of the protection is called Credit Valuation Adjustment (CVA) and in principle should be charged to the client (in our case counterparty B) in order to reflect its credit risk. For instance, suppose the credit spread of B is 100 bps,6 the amount to protect $100 million, the trade maturity 10 years. Under these market conditions the price 5 Note that a cancellable swap is the combination of a vanilla swap and a Bermudan option. If the option is European (i.e. the swap can be called only on one date), the cancellable swap can be priced in closed form. 6 bps:

basis points, a hundredth of a percent.

1.2 Preliminary Examples

9

of buying protection on B will be in the order of $10 million.7 Such protection is sufficient only at the time of calculation, and one would need to compute exposure sensitivities to the underlying factors in order to dynamically hedge the required amount of protection on B, as the future exposure to B will evolve with market conditions. As we mentioned, the usage of CDS transfers the credit exposure from B to C. So, even if one assumes the exposure to B to be perfectly hedged via the CDS, there will be counterparty exposure to C, which offers protection.8 Consider again the example above where A buys CDS protection on B on a notional amount of $100 million. Figure 1.4 shows a typical profile for such a transaction, assuming it is un-collateralised. For such a default protection product, the exposure one observes Fig. 1.4 Exposure of a typical credit default swap on a notional of 100 mUSD and spread about 100 bps. PFE (on different quantiles), EPE and ES are shown

results from the effects of (i) movements in the simulated credit spread of B and (ii) defaults. Clearly, the payment triggered by B’s default, equal to about 1 − R = 60% of notional, would imply that in a default scenario, A would have an exposure to C of $60 million. R is the recovery rate, i.e. the amount which can be recovered upon default of the counterparty. Now in Fig. 1.4, the PFE profile (which we recall is the 97.5% quantile of the distribution) does not show such high levels of exposure. This must mean that in the simulated scenarios, fewer than 2.5% of the scenarios involve B defaulting. Or in other words that the event of B defaulting is a rare event. To take into account this event one could display higher quantiles of the distribution, say the 99.9% quantile. Alternatively, one can calculate the Expected Shortfall (ES) of the distribution, which is simply the expected value of the tail of the distribution (see Chap. 12 for more details); this measure will uncover any large outliers in the distribution (such as the rare event of default of B, and hence large payment by C, in this case). Figure 1.4 displays this quantity, and clearly shows that defaults are indeed occurring even if they are not frequent enough to affect the 97.5% PFE. 7 This 8 This

is roughly equal to the 10-years duration multiplied by the 10-years spread.

is one of the reasons why, after the 2007–08 credit events, it is under discussion to use clearing houses when dealing with credit default swaps.

10

1 Introduction

This example shows that with credit products, where events of small probability can lead to large payments, the PFE might not be the appropriate exposure measure to consider. We will have more to say on this in due course.

1.3 Why Compute Counterparty Credit Exposure? Counterparty risk is at the root of traditional banking. Historically, the first form of financial instruments were bonds, and their value was mainly driven by the market’s view of how creditworthy the issuers of these bonds were. However, today’s financial world is much more complex, and the process of estimating counterparty risk much more challenging. While for loans and other traditional products the focus is mainly on estimating the capability of the borrower to repay its obligation, for derivative transactions, estimating accurately the future value of the transaction is as important and challenging as having a view on the ability of the counterparty to honour its obligations. Accuracy is important because credit exposure models are used for several purposes in financial institutions, such as (i) Setting limits on the amount of business allowed with a particular counterparty. (ii) Dynamic hedging of counterparty risk, by buying credit protection on the counterparty. This in effect allows one to trade away counterparty credit risk. (iii) Computation of risk weighted assets and capital requirements. (iv) Obtaining insight about prices of complex transactions in potential future scenarios. For example, while counterparty risk is concerned with measuring how high the value of a transaction can go (and therefore how much a counterparty would owe), there are similarities between this and computing Value at Risk, or stress testing, where one would be interested in how much the value of a transaction could drop.

1.4 Modelling Counterparty Credit Exposure In the previous sections we have introduced the concept of counterparty exposure and have provided some simple examples. We focus now on a more formal approach which will give a flavour of the mathematical tools we will need in the next chapters.

1.4.1 Definition Given a portfolio of positions traded with a counterparty, the main quantity we need to model to compute the counterparty credit exposure at time t, is the distribution Vt of the portfolio prices, computed at time t > 0 and seen from today. We will see in the next chapter how Vt can be described in its full generality. For the moment

1.4 Modelling Counterparty Credit Exposure

11

we consider the case of products without callability features and where cashflow payments are performed at discrete time points (Ti ), i = 1, . . . , n, with Tn being the maturity of the trade. Define Xt to be the (generally stochastic) payment made by the portfolio at any time t (Xt = 0 if t is not a member of (Ti ), i = 1, . . . , n). Then at any time t ≥ 0, Vt can be expressed as: Tn XTi Vt = Nt E (1.1) Ft , NTi Ti >t

where Nt indicates the numeraire, E is the expectation in the numeraire measure and Ft the usual filtration. More details of the concept of numeraire, pricing measure, and filtration can be found in the literature (see for example Baxter & Rennie [10] for an intuitive description, or Rogers & Williams [93, 94] and Shreve [98] for a more formal approach) and will also be given later in this book (see Appendix B). For our purposes here it is enough to think of the numeraire as being the cash account, used to discount cashflows, and the filtration as the information available at time t . At time t = 0 the distribution degenerates into the current price of the portfolio. We are interested in the distribution of Vt under either the real or the pricing (called also risk-neutral) measure. In general the price distribution Vt will change with time due to changes in market conditions, portfolio composition (for example due to payment of cashflows), and time value. If the portfolio is collateralised, it can be extended to take into account additional positions representing the collateral value. The computation of the price distribution Vt depends also on specific contractual features with the counterparty, e.g. netting agreements between short and long positions in the portfolio, or break clauses held by the counterparties. The industry practice to compute exposure is to use a simple Monte Carlo framework implemented in three steps: (i) scenario generation, (ii) pricing, and (iii) aggregation. The first step involves generating scenarios of the underlying risk factors at future points in time. Simple products can then be priced on each scenario and each time step, therefore generating empirical price distributions. From the price distribution at each time it is then possible to extract convenient statistical quantities. Exposure of portfolios can be computed by consistently pricing different products on the same underlying scenarios and aggregating the results taking into account possible netting and collateral agreement with the counterparty. If taken literally, this approach works only for relatively simple products which can be priced analytically, or which can be approximated in analytical form, and which do not need complex calibrations depending on market scenarios. More exotic products requiring relatively complex pricing, cannot be treated in this way. As already mentioned, even a cancellable swap, which is a relatively simple product, cannot be computed easily in this framework. In the next chapters we will show how (1.1) can be generalised and which algorithms can be implemented to compute exposure for more exotic products. We will also challenge the simple Monte Carlo approach we have just described, and see how more sophisticated modelling frameworks can provide answers to some of the common problems faced when building a counterparty exposure system.

12

1 Introduction

1.4.2 Risk Measures For practical reasons it can be useful to characterise the distribution Vt with some statistical quantities which can then be used for various risk controlling or risk management purposes. The Potential Future Exposure (PFE), computed at time t is defined as PFEα,t = qα,t = inf{x : P(Vt ≤ x) ≥ α},

(1.2)

where α is the given confidence level, and P indicates the probability distribution of Vt . Note that this is a function of time t and is the price of the obligation in the future given a set of scenarios. This pricing is called sometimes Mark-to-Future. The graph of PFEα,t as a function of t is known as the exposure profile of the trade. Similarly the Expected Positive Exposure (EPE) will be computed as9 EPEt = E Vt+ ,

(1.3)

where the expectation can be taken under the real or pricing measure depending on the usage of EPE. An alternative measure to the quantile is the Expected Shortfall, called also Expected Tail Loss, defined as ESα,t = E Vt | Vt > PFEα,t . (1.4) Expected shortfall is used especially when it is convenient to have a measure which takes into account events of significant magnitude, which, however, can occur with only very small probability. As we have shown above, typical examples are credit derivatives, where the default of the reference entity protected by the derivative is a low probability event, which, however has significant impact.

1.4.3 Netting and Aggregation In general, the credit exposure to a particular counterparty arises not from a single transaction but several ones. For any particular market scenario, some of these transactions will have positive, and others negative value. Consider, for example, a long and a short position on an option on highly correlated stocks, a portfolio of payers and receivers swaps10 in different currencies, or, as a more sophisticated example, 9 We will see later in Chap. 12 and Chap. 14 that other definitions of EPE are more appropriate to compute CVA. 10 A

payer (resp. receiver) swap, is a swap that pays (resp. receives) a fixed rate and receives (resp. pays) a floating Libor rate.

1.4 Modelling Counterparty Credit Exposure

13

a long position on ABX11 and a short position on a tranche of pool of MBS. One would expect that, at a given time as one position increases in value, the value of the other position decreases. Since both transactions are facing the same counterparty, it is natural to think about the possibility of netting these positive and negative values together, in order to reduce the overall exposure. The possibility of treating risk in this way will depend on the legal agreement in place. Netting agreements can have different flavours. For example for a given counterparty it could be possible to net together interest-rate swaps, but not swaps with e.g. equity transactions. From the quantitative and computational perspective netting and no-netting agreements will determine how aggregation is performed within a pool of transactions. The main challenge is the requirement of being scenario consistent across trades. This means that the price distributions of all transactions have to be computed together in order to choose the correct risk measure of the whole portfolio together with the correct netting agreements. This can pose significant constraints on the software architecture as well as on the computational capacity. Once counterparty exposure is computed at portfolio level, one can be interested in assigning a portion of the exposure to each single transaction. It is interesting to note that this is not equivalent to computing exposure for each single transaction separately. This process of redistributing exposure is often called exposure allocation or disaggregation and can be performed in different ways leading to different results. We will analyse quantitative aspects of both aggregation and allocation in Part IV where we discuss hedging and managing counterparty risk.

1.4.4 Close-Out Risk Close-out risk refers to the possibility of loss during the time period between when a counterparty is deemed to be in default and when the transaction with that counterparty has been wound down or replaced in the market. The length of this time period, referred to as the close-out period, is typically assumed to be ten business days. In practice it may be shorter for liquid transactions or longer for specialised and bespoke transactions. To mitigate close-out risk, a collateral agreement is often included in the transaction. Under such an agreement, the counterparty would have a commitment to post assets (be they in form of cash or other highly-rated assets) whenever the exposure from the transaction is observed to increase. There are several components that may be specified in a collateral agreement, such as (i) an initial upfront collateral amount called the initial margin, (ii) the threshold exposure above which extra collateral 11 The

ABX Index is a series of credit-default swaps based on 20 MBSs that relate to subprime mortgages.

14

1 Introduction

would need to be posted, (iii) the minimum amount of collateral that may be posted on each collateral exchange date, and (iv) the frequency of the margin calls. The collateral agreement is a legal agreement also referred to as the CSA (Credit Support Annex). Typically the terms of this agreement will depend on the jurisdiction where it applies. In Part IV we will analyse some quantitative and modelling aspects of close-out risk, without addressing all the intricacies of the legal aspects.

1.4.5 Right-Way/Wrong-Way Exposure In all the examples we have analysed previously, we did not consider the quality of the counterparty, assuming in effect that counterparty exposure is equivalent to the future replacement value of the trade at time of counterparty default. In general, however, the level of exposure caused by the trade and the quality of the counterparty are not independent of each other. Information about one would force us to re-evaluate information we have about the other. We refer to such dependence as right-way or wrong-way exposure. The question is how to factor this effect into a credit quantification computation. Typical examples where such considerations are called for are when call or put options are written on the counterparty’s own stock. These are limiting cases with practically no need of accurate modelling. The problem becomes more interesting when the product is complex and the correlation between counterparty quality and level of exposure cannot be clearly determined. Consider for example an energy producer which swaps energy futures for a stream of coupons. In general, increases in energy prices could be beneficial to the company, therefore reducing its probability of default. One way of taking into account the Right Way risk is to measure correlation between energy prices and company credit spread. Another interesting example12 of wrong way risk are negative basis swaps performed with monoline insurance companies. Typically in this case the insurance company receives a premium and pays default protection on missing payments from a pool of mortgages. These swaps are called negative basis as, in normal market conditions, the price for the protection is lower than the value implied by the spread paid by the mortgages.

1.4.6 Credit Valuation Adjustment: CVA Once counterparty exposure has been computed it is necessary to find ways of mitigating it. The simplest way is to compare the portfolio PFE with pre-defined limits and constrain the amount of transacted notional or, as we have seen previously, enter 12 . . . especially

in light of the 2007/2008 credit events. . . .

1.4 Modelling Counterparty Credit Exposure

15

into a collateral agreement. A possible alternative consists in buying credit protection on the counterparty. Its price corresponds to the value of the protection leg of a CDS that pays the exposure amount in case of default of the counterparty. This value is called in the industry credit valuation adjustment, CVA. Intuitively we can see this as follows. Within a pricing framework the value of credit exposure can be seen as the expected value of the positive part of the price distribution weighted by the default probability. Assuming that prices are independent from defaults, we can separate expectations, obtaining that CVA is the value of a CDS with the notional being the EPE profile of the underlying transaction. Suppose for simplicity that the EPE profile is a piece-wise constant function over a time interval (Ti − Ti−1 ). CVA = EPEi (Ti − Ti−1 )D0,Ti si , (1.5) i

where si is the spread corresponding to the time interval Ti − Ti−1 and D0,Ti the discount bond maturing at time Ti . We can see that the CVA corresponds to a portfolio of forward starting CDSs (or equivalently long and short CDS positions) with piecewise constant notional. The availability of CDSs of different maturities will dictate how the EPE profile is discretized. The CVA depends on the level of exposure as well on the credit spread of the counterparty. As counterparty exposure and spread change with time, the amount of credit protection needs to be adjusted accordingly. The process of balancing of exposure with CDSs and other instruments sensitive to market parameters corresponds to dynamically hedging counterparty credit exposure. More details on how to compute and hedge CVA are given in Chap. 14.

1.4.7 A Simple Credit Quantification Example We will discuss in detail in Chap. 9 the computation of credit exposure for equity products. We consider here a very simple example where the form of the exposure profile and the maximum values of the PFE can already be deduced from an approximation. Suppose company A has bought from counterparty B a call option of strike K on a stock S. Our goal is to compute the credit exposure and close-out risk company A is facing. As mentioned in the previous section we need to calculate the price distribution Vt . In the case of the call option, in a simplified context where rates are deterministic, (1.1) becomes (ST − K)+ −r(T −t) (1.6) E (ST − K)+ | Ft , Vt = N t E Ft = e NT where S is the stock price, K the strike, and r the interest rate assumed to be constant. The notional (number of options) has been assumed equal to one. As mentioned previously, Ft is the usual filtration, Nt the numeraire, and the expectation is taken in the measure N.

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1 Introduction

To solve this equation we need for the stock price S a model, with which simulate the stock value till maturity T . A simple model, which is often used in credit, is the geometric Brownian motion with constant volatility σ , interest rate r and dividend yield d. dS = (r − d)dt + σ dWt , S

(1.7)

where Wt is a standard Brownian motion. As it is well known this stochastic differential equation can be solved analytically. Thus, to compute exposure we need to simulate the stock with (1.7), and then, using the Black and Scholes formula [15] we can price the option at each time step and in each scenario. As the exposure of an equity option is generally monotonic in the underlying and is growing with time, and a vanilla stock option depends only on the current stock value (the product is not path dependent), the max PFE will be in general at maturity T .13 We can compute it at let’s say 97.5% confidence level as (see also Appendix A) √ σ2 (1.8) PFET = S0 e(r−d− 2 )T +1.965σ T − K, assuming it is a positive quantity. The expected exposure can be computed in this simple case as EPEt = V0 ert ,

(1.9)

where V0 is the option premium. We can see this as following. Given that the value of a call option is always positive, we can write (in our simplified set-up), EPEt = E[Vt+ ] = E[Vt ] = E[E[e−r(T −t) (ST − K)+ |Ft ]] = V0 ert .

(1.10)

As for approximating the close-out exposure for a short close-out period, one can use a first-order Taylor approximation. CloseOutt ≈ V0 + Δ(St − S0 ),

(1.11)

where Δ is the first order derivative of the call option price with respect to the stock and St is the value of the stock during the close-out period. This is the close-out risk for the initial period, i.e. for the time between time-zero and t . We will see later in this book that the computation of close-out risk presents subtleties which go far beyond this simple computation.

13 The

exact shape of the PFE curve will depend on the interest rate, dividend curve, and option characteristics.

1.4 Modelling Counterparty Credit Exposure

17

1.4.8 Computing Credit Exposure by Simulation Within a Monte Carlo framework, to compute exposure we could simulate the stock price from today to maturity using (1.7), and then price the option on each path and each time step using Black and Scholes, again with constant rate, dividend, and volatility. As we will see in the next chapters, it is more convenient to simulate martingale processes, for which only the volatility structure is relevant, while the drift (and thus the dividends) does not need to be specified (for a definition of martingale see Appendix B and for more details see for example the books by Baxter & Rennie [10], Rogers & Williams [93, 94] and Shreve [98]). In practice a convenient quantity we can simulate are forward prices. By considering our example in these terms, we can write (1.6) as (FT ,T − K)+ Vt = Nt E (1.12) Ft , NT where Ft,T is the t-value of the T -forward. The link with the notation in the previous section is, Ft,T = St e(r−d)(T −t) .

(1.13)

As before, assume for simplicity that the numeraire N is independent from the stock price, and impose also the simple specification dFt,T = F0,T σ dWt ,

(1.14)

with the volatility being a constant σ > 0 and with W being a standard N-Brownian Motion. This is the quantity we simulate in t. The paths generated by integrating this SDE are our scenarios which we show in Fig. 1.5 in a stylised representation. Note that (1.14) can be integrated analytically at each time step, thus avoiding discretisation errors, σ2 Ft,T = F0,T exp − t + σ Wt . (1.15) 2 We can then price at each scenario and each time step the stock option using again Black and Scholes, expressed in terms of the forward price Ft,T . PFE can be computed analytically at maturity, where the option price is given by the stock price minus the strike.

+ √ 1 PFET = F0,T exp σ T q˜α,T − σ 2 T − K , (1.16) 2 where q˜α = Φ −1 (α) is the α-quantile of the standard normal distribution. This is the equivalent of (1.8) generalised for any quantile. We have also floored at zero the exposure, as in some cases one is interested only at the amount the counterparty should pay. Negative exposure represents the amount we owe to the counterparty.

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1 Introduction

Fig. 1.5 Computing exposure by Monte Carlo simulation. The paths on the left panel represent stock prices. At each scenario and each time step, the price of the option is computed using the analytical Black and Scholes formula. Resulting prices are represented on the right panel. From the price distributions generated in this way at each time step, various statistical quantities (e.g. PFE and EPE) can be extracted. The bigger circles indicate a mean

1.4.9 Implementation Challenges The Monte Carlo framework we have shown in the previous section, seems to give a good implementation recipe. For a given portfolio of transactions we could (i) identify the underlying risk factors and simulate forward (or spot) prices, taking into account correlations if required, (ii) use functions already implemented to price each product, and then (iii) derive statistical quantities. As we have mentioned already, this could be the approach followed by a financial institution to assess the counterparty credit risk of its OTC derivatives portfolios. In the implementation phase, however, there can be issues which need to be addressed. (i) The generation of correlated scenarios is not trivial, as there can be thousands of different risk factors driving the dynamics of products in the portfolio. Consider for example an equity portfolio, where each underlying stock needs, at least in principle, a specific simulation. (ii) The scenarios have to be consistent across systems to build a counterparty view. This is a requirement which is much more stringent than what is generally specified in the design of a Front Office system used for pricing or a Risk system used to monitor the Profit and Loss (P&L) of a bank. Basically what we need here is the same underlying models, or the same family of models, for all types of products. In fact, even if the correlation between asset classes can be in some cases ignored (e.g. equity could be considered not correlated

1.4 Modelling Counterparty Credit Exposure

19

with interest rate), still all these models need to be expressed using the same numeraire (the discount factors in equity have to be consistent with the discount factors used in FX or rates). This consistency can be difficult to achieve, as often large financial companies have different systems to book and value, for example, interest-rate, equity, or FX products. (iii) Pricing functions developed in various libraries are not necessarily designed to be integrated in a counterparty exposure framework. This has implications from both a software and architecture, as well as from a methodological point of view. Consider for example path dependent products. Counterparty exposure depends on the whole scenario history, which could be in different formats across different pricing systems. (iv) Not all products can be computed in analytical form. Most exotics are priced on grids using PDEs or using Monte Carlo approaches. In these cases the exposure computation would require a Monte Carlo simulation for scenarios and a Monte Carlo simulation, or a PDE computation, for each scenario and time step to price the instrument. This becomes quickly unfeasible from a computational point of view. In addition, depending on the model used for pricing, calibration could also become problematic, as it has to be performed at each scenario. In practice credit systems based on the classical Monte Carlo scheme approximate products using a simplified representation. While these approximations could have their justification in a risk environment, they are difficult to use when counterparty risk has to be priced and hedged.

1.4.10 An Alternative Approach: The AMC Algorithm The points highlighted in the previous section clearly show that the classical Monte Carlo scheme has intrinsic limitations and that we need an alternative approach. As we will see at length in the rest of the book, there are possibilities to circumvent in a systematic way some of the problems related to valuation and architecture. The basic idea is to approach the counterparty exposure problem as a pricing problem, and thus to use pricing algorithms, which generate not just the value of a trade at inception, but rather a price distribution at predetermined time steps. One possibility is to use the so called American Monte Carlo algorithm, which we will refer to as, simply, the AMC algorithm. The main feature of this algorithm is that, instead of building a price moving forward in time, it starts from maturity, where the value of the transaction is known, and goes backwards, till the inception. In general AMC is used for pricing products with callability, i.e. products whose values depend on a strategy which can be determined by only knowing future states of the world. From a counterparty exposure perspective, the benefit of this approach is that, not only a price at time-zero is provided, but also the price distribution at each time step. In addition, the algorithm is generic, in the sense that using simply a payoff description, we can obtain the information needed to compute counterparty

20

1 Introduction

Fig. 1.6 A simplified graphical representation of the AMC algorithm. In the left hand panel we show the scenarios generated according to some underlying model. At maturity the payoff of the trade is known. To estimate the value at intermediate scenarios we need to proceed with a backward induction step

exposure. This suggests the possibility of having a generic trade representation and thus the possibility of having a modular software architecture that incorporates trade descriptions without explicit knowledge of each type of product. The challenge is that we need to develop an underlying model capable of pricing a hybrid product, consisting potentially of a large portfolio of transactions. This hybrid model will need to take into account all stochastic drivers of the portfolio in a consistent, arbitrage free way. It is natural to ask what is the performance of the AMC algorithm for vanilla products. We will see that by a careful implementation the prices computed via AMC are very close to those computed using for example closed form formulae.

1.5 Which Architecture? Building a system that computes credit valuation adjustments and counterparty exposure for the book of a large financial firm is a very challenging task, not only from the modelling and algorithmic perspective, but also from the technical and IT point of view. One of the problems is that often in large institutions such as Investment Banks, products are not booked on one system. They are in general recorded on several systems, which do not necessarily communicate between each other. To overcome this situation we suggest developing not only a common modelling platform, but also a programming language, which allows the representation of different types of products. As we will see later in this book, we have called our language

1.6 What Next?

21

Fig. 1.7 High level architecture description

PAL, Portfolio Aggregation Language, to highlight the fact that we need to aggregate trades at counterparty (or at netting pool) level. Once we have this common booking language, we can translate bookings made in other systems into PAL, bridging the difference between these systems. In the figure below we show how the system architecture could be implemented.

1.6 What Next? We have introduced all basic concepts needed to understand counterparty credit exposure. We have now to analyse in detail the steps necessary to build a system designed to compute and hedge counterparty risk for large portfolios of exotic transactions. This is what we do in the rest of this book. We start from a generic modelling

22

1 Introduction

and simulation framework based on American Monte Carlo techniques, and then we present a software architecture, which, with its modular design, allows the computation of credit exposure in a portfolio-aggregated and scenario-consistent way.

Chapter 2

Modelling Framework

Our goal is to define a general framework which can be used to compute counterparty credit exposure for all types of transactions. As highlighted in the Introduction, computing counterparty exposure consists of computing distributions of prices at future times. For simple products this can be achieved by scenario simulation, followed by pricing on each scenario, at each time step. However, in the case where no analytical form is known for the price of the product, this approach is not practical and a different approach is required. The framework we define is intended to cater for the need to estimate price distributions in a consistent way, not just for a single product, but for complex portfolios of products admitting no closed-form pricing. Price distributions will be obtained through simulating underlying processes specified in a generic way, and this chapter is dedicated to defining the problem at hand and describing the various processes to be simulated. There are several ways to model the stochastic processes driving the price of derivatives. We start from a generic representation and show how to adapt it to cope with practical constraints. By making certain independence assumptions, we then analyse how to modify the model so that new asset classes can be introduced in a modular way that maintains scenario consistency across products but does not require the re-evaluation of models already in place. Such assumptions have consequences for pricing, and these are assessed in Chap. 6. After having prescribed the theoretical framework, the next chapters will be focused on implementation. In Chap. 3 we will specify simulation models. For vanilla trades, which can be priced analytically, this will be the main step to compute credit exposure, as there is a direct mapping between scenarios of the underlying risk factors and price distribution. In Chap. 4 we will analyse valuation techniques that can be applied when this mapping is not obvious, as, for example, with products having callability features and for which no closed-form valuation is available.

2.1 Counterparty Credit Exposure Definition Consider a financial product, which we denote generically by P , and denote by X ≡ (Xt )0≤t≤TX the cashflows which the holder of the product is entitled to. In G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0_2, © Springer-Verlag Berlin Heidelberg 2009

23

24

2 Modelling Framework

general, P might entitle the holder to not just the cashflow X, but also the option to replace, at predetermined points in time, the cashflow X with an alternative product, Q say. Associated with the product Q are cashflows Y . We also generally assume in the following that the option allows exercise only once. Now let τE∗ denote the optimal (stochastic) time at which the option should be exercised to replace the cashflows X with the product Q. The basic problem at hand is to compute at all times t the value of the product P whose payoff may depend on decisions made after time t . The fair value for product P should be that attainable by employing the optimal among all possible exercise strategies τE . Solving the pricing problem entails solving for the best possible exercise strategy. We now formalise what we have said above with a definition, and introduce the required notation that will serve us in Chap. 4 when we describe valuation techniques in detail. First, denote by T the set T := {τ1 , τ2 , . . . , τnE } ∪ {∞}

(2.1)

of points where the holder of the option is allowed to exercise from P into Q. Even if the optionality is of American type and exercise is allowed at any t within a time interval, any attempt at numerical solution would need to approximate the problem with one where exercise is allowed at a discrete set of points. The point at infinity is included in T , as this corresponds to the possibility of the holder never deeming it optimal to exercise into Q. Q Write V Q ≡ (Vt )t≥0 for the value process1 of the product Q, and let N denote our chosen numeraire process. At any time t before exercise, the holder of the product P chooses the optimal exercise time τE∗ ∈ T so as to attain, VtP = Nt sup

τE ∈Tt

E t

τE ∧TX

Q Xu VτE du Ft + E Nu NτE

Ft

(t < τE∗ ),

(2.2)

where Ft is the usual filtration at time t , Tt = {τ ∈ T | τ ≥ t} is a subset of T , and where the expectation is taken in the pricing measure relating to the numeraire N . For vanilla products without callability features, we have T = {∞}, so that the question of optimising the time of exercise does not arise. Here on the left hand side of (2.2), VtP is the time-t value of P conditional on exercise not having happened prior to t . This is the pre-exercise value of product P . On the right-hand side, the two terms, τE ∧TX Xu no Πt := Nt E du Ft Nu t (2.3) Q VτE Q Vt = Nt E Ft , NτE we work with processes defined on a filtered probability space (Ω, F , (Ft )t≥0 ). More details are given in Appendix B.

1 Throughout

2.1 Counterparty Credit Exposure Definition

25

represent, respectively, the time-t value of the non-exercise portfolio containing the X cashflows,2 and the time-t value of entering into the exercise portfolio containing the Y cashflows at time τE . Q Upon exercise at τE∗ , the product will have value Vτ ∗ . The credit exposure at E times t > τE∗ will depend on the way in which the option inherent in P may be exercised. We will discuss in Chap. 4 several types of exercise that are possible. Here it suffices to point out that exercising into Q might give rise to credit exposure for the lifetime of the product Q, which may exceed the maturity TX of the preexercise flows X. Putting everything together, we may write the price of product P as

Vt =

VtP , Q

Vt ,

t < τE∗ t ≥ τE∗ .

(2.4)

In a complete market, there will be unique prices for any derivative that can be replicated. In particular, in a simulation framework, the price calculated will not depend on the numeraire (and therefore the numeraire measure) chosen for the simulation. However, the distribution of prices that drives the credit exposures will depend on the simulated processes, which in turn depend on the measure chosen for the simulation. Thus, for instance, there could be good reasons for generating scenarios in the physical measure even if pricing is accomplished by taking expectations in the numeraire measure corresponding to numeraire N . We will analyse this point in more detail in Part IV, where we discuss Hedging and Managing Counterparty Risk. From (2.2), it is apparent that the elements required to simulate values of the distribution Vt are (i) Simulations of the numeraire process N . (ii) The ability to simulate the cashflows X provided by P , and similarly the cashflows offered by Q. Since all these cashflows are usually stochastic, we need to specify a simulation model tailored to the particular products P and Q. This is what we deal with in Chap. 3. (iii) Estimation of the price of the no-exercise portfolio Π no and that of the product Q for each point in time at which we are interested in having a simulated value available. Similarly, we need to account for the sup operator numerically within the simulation. We will consider this aspect in Chap. 4. The rest of this chapter is dedicated to describing our general approach to modelling the dynamics of the numeraire and the cashflows of the generic products P and Q mentioned in (i) and (ii) above. that X in general can be decomposed as Xt = Xtc + δtj xj , where X c is a continuous process, xj is a cashflow at time tj and δt is the Dirac delta function. 2 Note

26

2 Modelling Framework

2.2 Process Dynamics Most approaches to simulating the financial quantities required for credit exposure estimation choose to model stochastic variables that are directly observable, e.g. Libor rates, swap rates, stock prices, foreign exchange rates. While specific payoffs can usually suggest which observable quantity it is more sensible to simulate, there is the inherent drawback that a new payoff may require simulation of new quantities, which the framework in place had not been designed to handle. We take an alternative approach, for which (i) Extraction of all stochastic quantities of interest (for all possible payoffs) is done from the same basic set of martingale3 processes. This set does not need to be augmented in order to tackle a new payoff type. (ii) Time-zero term structures are reproduced exactly by construction. The specification of a new model is therefore reduced to specifying the volatility structure. Such specification must allow for fast and accurate calibration to vanilla option market prices. The basic idea is as follows. Suppose we have a contingent claim XT paying off at time T . The risk-neutral price at t < T of XT is given by4

p0 = EN (NT /N0 )−1 XT , (2.5) the expectation being in a measure that makes martingales out of (non-dividendbearing) tradable assets when expressed in units of the numeraire N . We can equivalently use the T -bond as numeraire, so that T p0 = EN (DT ,T /D0,T )−1 XT = D0,T EN [XT ] T

= D0,T EN [MT ,T XT ],

(2.6)

where the expectation is now in a measure where prices of (non-dividend bearing) assets expressed in units of the T -bond are martingales. The usefulness of this second representation is that by modelling the change-of-measure process5 dNT Dt,T /D0,T Mt,T ≡ = , (2.7) dN Ft Nt /N0 3 A martingale is a stochastic process whose expected future value, conditional on present information, is its current value. In mathematical terms this can be expressed as E[Mt | Fs ] = Ms , for all s ≤ t (see Appendix B for definitions and, for example, Williams [106], or Shreve [98] for a detailed treatment of martingale theory). 4 For clarity we explicitly indicate the measure in which expectation is taken; in the rest of the book we will ignore the superscript N when expectation is taken in the numeraire measure N. 5 See

Appendix B for more details.

2.3 Interest Rate: Single Currency

27

the time-zero price of XT is a multiple of time-zero observed bond prices. That is to say, the yield curve at time zero is replicated by construction. Comparing (2.5) and (2.6), we see moreover specifying the martingale M·,T forces the numeraire to take the form NT−1 = D0,T MT ,T .

(2.8)

At a general level, our approach is similar to that followed in Constantinides [28], Flesaker [45], and Rogers [90]. These papers model directly (in some fixed reference measure P∗ ) the state-price density process ζ , which has the property that for any non-dividend-bearing asset with price process S, ζt St is a P∗ -martingale.

(2.9)

If the numeraire process is increasing, then ζ is a positive supermartingale. It is this property that Flesaker in [45] and Rogers in [90] exploit to come up with a surprising variety of interest-rate models, essentially writing ζt = e−αt f (Xt )/f (X0 )

(2.10)

for a given Markov process X and inspired choices of the function f . We choose to work in the reference measure P∗ = N and to model the numeraire directly. The recipe for modelling assets other than the chosen reference currency (foreign currencies, equity and inflation) is similar and will be elaborated on further below. Conceptually, there will be two new sources of randomness introduced with each new asset class. The first of these is the output derived from owning one unit of the asset (bond yield volatility in the case of a currency, and stock dividend volatility in the case of equity), and is modelled by introducing for each asset i a martingale (i) process M·,T , analogous to (2.7) above. The second source of randomness is the fluctuation of the asset price itself (that is to say, the exchange rate process in the case of a currency foreign to the reference one, and the stock price process in the case of equity) when expressed in some existing unit of value; this will be modelled through a second process, which we typically denote Y (i) for the i’th asset. Credit derivatives present different challenges; nevertheless, the idea there will still be to find the relevant martingale and to directly specify dynamics for it in our chosen reference measure.

2.3 Interest Rate: Single Currency We start by considering the modelling of the reference currency itself. All the ingredients for this have been given by way of motivation in Sect. 2.2, namely the change-of-measure martingales (2.7) Mt,T =

Dt,T /D0,T Nt /N0

(t ≤ T )

(2.11)

28

2 Modelling Framework

which in fact can also be interpreted as the price process of the T -bond, expressed in units of the numeraire N (for t > T we assume Mt,T = MT ,T ). We will take Mt,T as the fundamental quantity to be simulated and from which the other interest rates-related stochastic quantities we need can be extracted. Table 2.1 Single currency interest-rate product notation Dt,T

T-bond in the local (domestic) currency

Nt

Numeraire in the local currency

Mt,T

Martingale used to simulate the T -bond in the local measure

N

Measure in which prices of non-dividend-bearing assets expressed in units of N are martingales

NT

Measure in which prices of non-dividend-bearing assets expressed in units of the T -bond are martingales

While, for each t, Nt depends only on the “diagonal” values Mt,t of the martingale M·,t , it is clear that simulated values of Mt,T will be required for all t ∈ [0, T ], and for each T ≥ 0. Indeed, from (2.11) it follows that D0,t Mt,T = Dt,T , Mt,t D0,T

(2.12)

which means that the time-t price of a T -bond is Dt,T =

D0,T Mt,T . D0,t Mt,t

(2.13)

Other quantities of interest, such as lending rates, can similarly be written in terms of the martingales Mt,T . All that is left now is to choose an SDE for the strictly positive martingale M. Consider, for example, for each T ≥ 0 the SDE dMt,T = Mt,T σt,T dWt,T ,

(2.14)

where W·,T is a Brownian Motion process and σt,T is a volatility term. For deterministic σ , (2.14) can be written in integral form as t 1 t 2 σu,T dWu,T − σu,T du , (2.15) Mt,T = M0,T exp 2 0 0 where

2 tΣt,T

:= 0

t

2 σu,T du

(2.16)

is the total variance of ln Mt,T . The framework described by (2.15) gives the simulation recipe: first choose a volatility structure σ , then simulate the martingale Mt,T and finally derive the

2.3 Interest Rate: Single Currency

29

stochastic quantities which are needed to estimate price distributions of the relevant product. For example, the numeraire can be written in integral form (for deterministic volatilities) as Nt −1 = D0,t Mt,t

t 1 2 = D0,t M0,t exp σu,t dWu,t − Σt,t t . 2 0

(2.17)

Similarly, the Libor rate of interest Lt,[T1 ,T2 ] observed at t for the period [T1 , T2 ], is (with α ≡ T2 − T1 ) −1 Dt,T1 Lt,[T1 ,T2 ] = α −1 Dt,T2 −1 D0,T1 Mt,T1 −1 =α D0,T2 Mt,T2 M t,T1 − α −1 , (2.18) = L0,[T1 ,T2 ] + α −1 Mt,T2 or, in other words, Lt,[T1 ,T2 ] + α −1 Mt,T2 = L0,[T1 ,T2 ] + α −1 Mt,T1

(2.19)

showing that simulation of the martingale process and the knowledge of the initial yield curve allows simulation of Libor rates.

2.3.1 Simple Specifications We have chosen our simulation approach to best serve our goal of computing exposure for all types of products across all asset classes in a consistent way. More familiar models from the literature usually take a different approach, and it is instructive to see how these models can be represented within our framework. This is what we do in this section. The first practical problem we need to tackle is of course that of dimensionality. In general, the cashflows that need to be simulated for a particular product will depend on values of the martingale Mt,T for arbitrary values of T . For example, we see from (2.18) that the Libor rate for two different periods will depend on two different martingales. This means that all martingales for all values of T are needed if one is to ensure that cashflows of any product can be extracted. In practice, of course, only finitely many stochastic drivers can be simulated, so one needs to find ways of projecting the richness provided by the infinite family of martingales M·,T onto a finite set of Brownian Motions which can be simulated. The richness retained by such a dimensional reduction will depend on how many different Brownian Motion processes one chooses. In particular, one can force the

30

2 Modelling Framework

martingales M·,T to all depend on the same Brownian Motion, as would be the case in any one-factor short rate model. In this case (2.14) becomes, dMt,T = Mt,T σt,T dWt ,

0 ≤ t ≤ T.

(2.20)

Note that the Mt,T depends on t but also, thanks to σt,T , on T . In fact, if σ (and therefore M) were to not depend on T , then bond prices in the model would be deterministic, because the bond price Dt,T =

D0,T Mt,T D0,T = , D0,t Mt,t D0,t

(2.21)

would be determined at time zero. Similarly, for the forward rate one would get f (t, T ) = −

D˙ 0,T ∂ log Dt,T =− , ∂T D0,T

(2.22)

where D˙ represents derivative with respect to maturity of the bond. From the point of view of implementation, a separable specification of the volatility term σt,T in (2.20) turns out to be very useful. In detail, write σt,T = ft gT

(2.23)

for positive deterministic functions f and g. The integral form of Mt,T then becomes, t 1 t 2 Mt,T = M0,T exp σu,T dWu,T − σu,T du 2 0 0 1 2 = M0,T exp gT Xt − Σt,T t , (2.24) 2 where

Xt :=

t

fu dWu 0

2 Σt,T

:= t

−1

0

t

(2.25) fu2 gT2 du ≡ Ft2 gT2 .

This specification is simple to extend to a multi-factor setting as follows. Starting from an Rn -Brownian Motion W, we write6 dMt,T = Mt,T gT · (ft RdWt ), 6 Throughout

(2.26)

this work we use boldface to indicate vectors and matrices. The dot (scalar) product between two vectors a and b is indicated by a · b, and the row by column product between two matrices (or a matrix and a vector) X and Y, by XY. The transpose of matrix X is XT . We indicate with X(i) (or Xi ) the i-th column (vector) of matrix X, and with aj the j -th element (scalar) of vector a. Xi,j is the i, j element of matrix X.

2.3 Interest Rate: Single Currency

31

where now (1)

(n)

ft = Diag(ft , . . . , ft )

(2.27)

is a diagonal matrix with the ft(i) on the diagonal, (1)

(n)

gT = (gT , . . . , gT )

(2.28)

(i)

is the vector of the gT , and R is such that RT R is a positive semi-definite matrix making the i’th component of RW a standard Brownian Motion, which then gets time-changed by ft(i) . We will see in Chap. 3, for instance, that the Hull-White model is a special case of a parametrization of this type. The impact of separability is that simulation of a small number of Brownian Motions t (j ) (i) fu(i) Ri,j dWu (2.29) X = j

0

(i) together with parametrization of the volatility function Σt,T = Ft(i) gT(i) , allows simulation of Mt,T for all T . Without a separable specification for σ , the integral t 0 σu,T dWu would need to be computed on the fly, or stored, for each maturity T . Because of its simplicity, we will employ the separable volatility specification in most of what follows in this book.

2.3.2 HJM Framework The well-known HJM framework [60] can also be accommodated within our framework, as we will see in the following. In the standard way of specifying an HJM model with d > 0 factors, one takes the forward rates ft,T defined via (2.22) and writes down the SDE dft,T = αt,T dt + st,T · dWt ,

(2.30)

where W is an Rd -Brownian Motion and st,T ∈ Rd is a vector specifying the instantaneous volatility at time t of the T -forward rate. Given that, in our framework, we model directly the bond price Dt,T , it is natural to compare the bond price dynamics in our framework to those implied by the HJM dynamics (2.30). Indeed, using Itô calculus and (2.30), we can see that in the HJM parametrization the discounted log-bond price satisfies T αt,u du dt, (2.31) d[log Dt,T Nt−1 ] = St,T · dWt − t

with

St,T = −

T

st,u du. t

(2.32)

32

2 Modelling Framework

The drift term α in (2.30) cannot be chosen without restriction, as doing so might introduce arbitrage between bonds of different maturities. Arbitrage will be excluded, however, if there exists a process γ ≡ (γ t )t≥0 in Rd such that (2.33) αt,T = st,T · γ t − St,T , as this will guarantee that bond prices discounted by the numeraire are martingales. Notice the key fact that the process γ should make (2.33) true for all T simultaneously. In our framework, the drift condition (2.33) is in fact satisfied automatically. With hindsight this should be expected, since our model laid down at the outset that the discounted bond prices are martingales. To see this, recall that Dt,T =

D0,T Mt,T . D0,t Mt,t

(2.34)

Hence, d[ln Dt,T Nt−1 ] = d[ln Mt,T ], and 1 d[log Dt,T Nt−1 ] = σ t,T · dWt − σ t,T · σ t,T dt, 2

(2.35)

showing that the volatilities of forward rates within our framework satisfy st,T = −

∂ σ t,T . ∂T

(2.36)

2.3.3 Libor Market Models Libor Market Models, also referred to as BGM models, choose to model Libor rates directly, Lt,[T1 ,T2 ] (for the original description of the model see [17]; for a comprehensive Libor market model analysis see for example Rebonato [89]). In essence, BGM models require a discretization of the set {Ti } of Libor fixing dates. Recall that, at time t < Ti−1 , the Libor rate for time period [Ti−1 , Ti ] is defined by −1 Lt,[Ti−1 ,Ti ] = αi−1 Dt,Ti−1 − Dt,Ti Dt,T , (2.37) i where αi = Ti −Ti−1 . Since the term in brackets is the difference of two bond prices, Lt,[Ti−1 ,Ti ] Dt,Ti

(2.38)

is a tradable process. Consequently, the process Lt,[Ti−1 ,Ti ] will be a martingale under NTi , which is the measure specified by taking the Ti -bond as numeraire. This is the cornerstone of the Libor Market Models, which allows the application of the Black formula for pricing caplets by modelling Libor rates as log-normal stochastic processes.

2.4 Multiple Currencies and Foreign Exchange

33

In our framework, the SDE satisfied by the Libor rate is cumbersome because our starting point is martingale modelling of the bond prices. Nevertheless, the SDE can be written down, as we do now. Let us consider, instead the shifted Libor rate L¯ t,[Ti−1 ,Ti ] := Lt,[Ti−1 ,Ti ] + αi−1 .

(2.39)

From (2.19), we can express Mt,Ti in terms of the Libor rates as Mt,Ti =

i ¯j i ¯j L0 L0 M = . t,T0 j ¯ ¯j j =1 Lt j =1 Lt

(2.40)

Therefore, if we are to model directly Libor rates as log-normally distributed satisfying the SDE dLit = Lit μit dt + σti dWti , (2.41) then we need to derive the drift μit of each rate under N. To this end, we apply Ito’s formula to (2.40) to obtain the finite-variation part of Mt,Ti as i i i ∂ 2M i j ∂M j j j j t t dMti L Lk σ σ k dWt dWtk + μ L dt j t t ¯ jt ∂ L¯ kt t t t t ∂ L ∂ L¯ t j =1 k=1 i i j k L L j j t t Mti σt σtk ρi,j − μt dt, j L¯ kt L¯ t j =1

(2.42)

k=1

where signifies that the two sides differ by a martingale term. Since M i is an Nmartingale, the drift μit of the Libor rate Lt,[Ti−1 ,Ti ] under N must therefore satisfy μit = σti

i j Lt

¯j j =1 Lt

j

ρi,j σt .

(2.43)

While the BGM approach is appealing because it allows easy calibration to caplet prices, its application to the purpose of exposure computation is hindered by the fact that not all Libor rates are directly simulated, and therefore interpolation across tenors is required. More importantly, the simulated Libor rates are not directly obtainable from a common stochastic process, making any simulation computationally intensive.

2.4 Multiple Currencies and Foreign Exchange We now turn to modelling of interest rates in a currency other than the reference one, leading naturally to modelling the exchange rate between the two currencies.

34

2 Modelling Framework

Table 2.2 Cross currency interest-rate products notation Dt,T D˜ t,T

T-bond in the local (domestic) currency

Nt N˜ t

Numeraire in the local currency

Mt,T t,T M

Martingale used to simulate T-bonds in the Nt (local) measure t (foreign) measure Martingale used to simulate T-bonds in the N

χt

Spot FX rate from foreign to reference currency

Yt

˜ Change of measure from the reference measure N to the foreign measure N

T-bond in the foreign currency Numeraire in the foreign currency

The basic tradable asset in the reference currency remains the bond, and we have modelled this in the previous section through the N-martingale M: Mt,T =

Dt,T /D0,T . Nt /N0

(2.44)

Whatever holds for the reference bond in the reference currency holds for the foreign one in the foreign currency, so in analogous manner we define another process (in terms of the foreign bond and foreign numeraire) D˜ t,T /D˜ 0,T , M˜ t,T = N˜ t /N˜ 0

(2.45)

˜ ˜ being the measure that makes martingales out of N which is now an N-martingale, all foreign tradable assets expressed in units of foreign numeraire. Now, some relation is going to have to hold between the change-of-measure induced by M and M˜ and the exchange rate linking the foreign and reference currencies. Indeed, if χ ≡ (χt )t≥0 is the FX process representing the value in reference currency of one foreign currency unit, then it is a standard no-arbitrage argument that the process Y ≡ (Yt )t≥0 defined by ˜ χt N˜ t dN = Yt = χ0 Nt dN Ft

(2.46)

is the Radon-Nikodym derivative7 for changing from the measure N to the mea˜ By this, Yt is an N-martingale. sure N. Inserting Nt = D0,t Mt,t and N˜ t = D˜ 0,t M˜ t,t in (2.46), we get Yt =

7 See

Appendix B.

χt D0,t Mt,t . χ0 D˜ 0,t M˜ t,t

(2.47)

2.4 Multiple Currencies and Foreign Exchange

35

It turns out to be more convenient to express Y in terms of the basic FX forwards that one would observe in the market. To this end, define F¯t,T to be χt D˜ t,T /Dt,T , F¯t,T := Ft,T /F0,T = χ0 D˜ 0,T /D0,T

(2.48)

the time-t FX forward normalised by its time-zero value. In terms of this, Y has the simple representation ˜ t,T /χ0 D˜ 0,T D χ t Yt M˜ t,T = F¯t,T Mt,T = (2.49) Nt /N0 which is in fact an identity involving measure changes, namely T ˜ dN ˜T ˜T ˜T dN = d N dN ≡ d N , ˜ Ft dNT Ft dN Ft dN Ft d N dN Ft

(2.50)

corresponding to changing from the reference numeraire measure to the foreign T forward measure. It will prove useful to read off from (2.49) the facts that Yt M˜ t,T ,

F¯t,T Mt,T ,

Yt ,

and Mt,T

(2.51)

are all N-martingales. Intuitively, in (2.49) the volatility structure for F¯ will relate to that of foreign exchange options,8 while the volatility of M and M˜ arises from stochasticity of interest rates. The only component in (2.49) that is free to model is Y , and one would then obtain the FX process from this as χt = F0,t F¯t,t = F0,t Yt M˜ t,t /Mt,t .

(2.52)

The dynamics of Y have therefore to be chosen in such a way that market-observed FX option prices can be reproduced, given the dynamics of M and M˜ which would have already been calibrated to their respective interest-rate markets. In particular, the volatility structure for Y will depend on the volatilities and covariances of M, M˜ and F¯ . In practice, doing this becomes very tedious especially when a large number of currencies are involved, so it is useful to look at a less exact but simpler approach. 8 The volatility for the FX forwards F¯ would be implied from market-observed option prices so that the price of an FX call struck at K, of maturity T , is

C(K, T ) = EN NT−1 (χT − K)+

˜ T [χ −1 ≤ K −1 ] − D0,T KNT [χT ≥ K], = D0,T F0,T N T where we recall F0,T D0,T = χ0 D˜ 0,T and where we have used the identities (2.50) to switch from one measure to another. Note in the last equality that F¯ (resp. F¯ −1 ) is a martingale in the forward ˜ T ). measure NT (resp. N

36

2 Modelling Framework

We go about this by modelling F¯ (and therefore χ ) as if it were independent of the ˜ In effect, we mimic (2.49) by interest-rate martingales M and M. χt D˜ t,T /χ0 D˜ 0,T Yt M˜ t,T = ≈ Fˆt,T Mt,T Nt /N0

(2.53)

with Fˆ independent of M and M˜ but having the same marginal distributions as F¯ . The reason this helps is that the market prices for FX options tell us directly what the volatility structure for F¯ must be. The immediate consequence of this independence assumption9 is that Fˆt,T is an N-martingale. The cost of this assumption is of course that while M F¯ /M˜ and M F¯ are Nmartingales, M Fˆ /M˜ and M Fˆ are not; to mitigate this we impose a drift correction on M˜ t,T so as to ensure at least that their expected values stay constant (and equal to one), that is E Mt,T Fˆt,T /M˜ t,T = 1 = E Mt,T F¯t,T /M˜ t,T = E(Yt ) ˜ χt Dt,T /χ0 D˜ 0,T . E Mt,T Fˆt,T = 1 = E Nt /N0

(2.54)

˜ it can be seen that Using the independence between Fˆ and the pair (M, M), −1 ˜ E M˜ t,T = 1 − Cov Mt,t M˜ t,t , Mt,T .

(2.55)

We re-iterate here that the above expectations are taken on time-zero information, that is, in the filtration F0 . Thus, the drift correction that we derive for M˜ is the expected drift seen at time zero. Attempting similar calculations for arbitrary Ft fail because the processes Fˆ M and Fˆ M/M˜ are not bona-fide martingales. This arises as a direct consequence of the independence assumption for Fˆ ; this independence is of course inconsistent with the fact that the conditional expectations of the FX forwards do depend on observed bond prices. In practice, the input to the model is the correlation between the drivers of M and M˜ in the N-measure, and in concrete examples, the covariances above need to be expressed in terms of this correlation. In particular, when M and M˜ are modelled with deterministic volatilities (see (2.15)), the condition (2.55) is expressed by saying that the Brownian Motion X˜ t driving the martingale M˜ t,T (see (2.24)) has, in the N-measure, the drift αt,T := σ˜ t,t − ρσt,t ≡ α(t),

(2.56)

9 . . . it also implies that while rates in different currencies can have co-dependence, all other asset classes are independent of interest rates. . . .

2.5 Inflation

37

where ρ is the instantaneous correlation between the Brownians X˜ and X that drive ˜ respectively. Note that (2.56) is a function only of t, so the martingale M and M, that the Brownian Motion X˜ t looks like t α(u)du, (2.57) Bt + 0

in terms of an N-Brownian Motion B having correlation ρ with X.

2.5 Inflation The basic inflation product, an inflation-linked bond, is designed to preserve the purchasing power of money in some given currency. Modelling of inflation products is generally similar to modelling of foreign exchange since for each currency, one can consider the nominal and real rates to be like a conventional (local and foreign, respectively) currency pair. Inflation yield curves (arising from inflation-linked bonds) and volatility term structures (deduced from prices of inflation-linked options) serve to calibrate the inflation model, in the same way that foreign currency yield curves and FX options allow calibration of foreign exchange models. In concrete terms, we let the exchange rate χ represent the inflation index denominated in the reference currency, with M˜ being calibrated to the volatility of real rates. For example, if the reference currency is GBP, the UK RPI inflation rate (denominated in GBP) would be modelled as some process χ , as if the RPI index were a foreign currency.

2.6 Equity Equity can be tackled within our framework by treating it as an asset foreign to the reference currency, similar to what we do for foreign currencies. In this way, one (i) lets the ‘exchange rate’ χ represent the value in its denomination currency of one unit of stock and (ii) uses M˜ to control the volatility of the stock dividend yields. In more detail, recall the identity (2.49) χt D˜ t,T /χ0 D˜ 0,T Yt M˜ t,T = F¯t,T Mt,T = . Nt /N0

(2.58)

The third term here represents the tradable asset for the foreign currency, namely the price of the foreign bond expressed in the reference currency. Now consider a stock denominated in a foreign currency. The stock forward price, St,T , discounted by the foreign bond and expressed in units of the foreign numeraire, is the relevant ˜ traded asset for the stock and has the representation as the N-martingale St,T D˜ t,T /S0,T D˜ 0,T (S) (S) Y˜t Mt,T := S¯t,T M˜ t,T = , N˜ t /N˜ 0

(2.59)

38

2 Modelling Framework

¯ being the stock forwards (respectively, forwards normalised with S (respectively, S) by their initial values), denominated in their own currency, and with Y˜ (S) and M˜ ˜ being N-martingales. The martingale M (S) will control the volatility of the stock dividends, so the element in (2.59) that is free to model is now Y˜ (S) , exactly as what happens for Y ˜ when modelling the FX rate. What remains is to ask what the N-martingale Y˜ (S) looks like in N. (S) is an N-martingale, ˜ ˜ But because Yt = d N/dN| we have that Y Y˜ (S) Ft , and Y˜ is an N-martingale. Thus, for each s < t, (S) Y˜s(S) Ys = E Y˜t Yt Fs (S) (S) (2.60) = Ys E Y˜t Fs + Cov Yt , Y˜t Fs , where we have used the N-martingale property of Y . Dividing out Ys we get that (S) (S) (2.61) Y˜s(S) = E Y˜t Fs + Ys−1 Cov Yt , Y˜t Fs . The second term on the right here is commonly referred to as a quanto adjustment. In the case where Y and Y (S) are both log-normally distributed with deterministic volatility, the equation above will result in a drift term for Y (S) of the form (S)

μt

(S)

= −ρt σt σt ,

(2.62)

(S)

where ρt , σt and σt are respectively the instantaneous correlation linking Y and Y˜ (S) , the instantaneous volatility of Y and the instantaneous volatility of Y˜ (S) . Table 2.3 Notation for equity products denominated in a foreign currency

St,T S¯t,T Y˜ (S)

Forward stock price in the stock’s currency Normalised stock forward price, St,T /S0,T ˜ Change of measure dN(S) /d N

2.7 Credit For credit products, exposure is driven by (i) the likelihood and occurrence of defaults, (ii) the dynamics of credit spreads, and (iii) the inter-dependence between defaults of different entities. Often the fundamental quantities chosen to be modelled are the credit spreads, as they are directly observable in the market (see for example the books by Bielecki & Rutkowski [14], Duffie & Singleton [39], Lando [72] or Schönbucher [95] for a discussion about credit models). We opt for a model whereby stochastic default probabilities are simulated directly. As we will see this allows us

2.7 Credit

39

Fig. 2.1 Change of measures between the local measure N, the T-forward local measure NT , the ˜ and the T-forward foreign measure N ˜T foreign measure N

to work in a framework similar to what we have introduced in the previous sections. Section 2.7.1 below describes how the par CDS spreads observed in the market can be used to obtain initial default probabilities. After that, we propose a method for evolving default probabilities of single entities, simulating default times, and modelling inter-dependence of credit spreads and default times of different entities. The model as we present it imposes a Gaussian dependence structure, which is used to achieve dependence between default probabilities and simulated default times of any pair of reference entities. Such Gaussian models are widely used in practice, even if it is well known that the Gaussian copula does not build dependence in the tail of the distribution. Other, more realistic, ways of introducing dependence can be used (for a discussion of other types of copula see, for example, [78]), but the mathematics is bound to become much more tedious.

2.7.1 Default Probabilities from par CDS Spreads At any time t, the observed CDS curve consists of a set of spreads, st,Ti for a set of maturities Ti , i = 1, 2, . . . , n. The spreads indicate, for a given entity which is not yet in default at time t, the market view on the propensity of that entity to default in the future. By writing down the value of the CDS contract,10 we can express default probabilities for the horizons Ti in terms of the set of spreads st,Ti observed at time t. 10 A

CDS (credit default swap) is a product entitling its holder to receive, at time of default and in return for a bond issued by an entity that has just defaulted, the face value of that bond. A common convention when pricing CDSs is to assume that the value of a defaulted bond is a fraction R of its face value. Under such a convention, the net value of the CDS at the time of default is then R¯ := (1 − R) per unit of face value. We will describe CDSs in detail in Chap. 10.

40

2 Modelling Framework

Table 2.4 Notation for credit products τ, τ (i)

Generic default time, default time for reference name i

qt,T ≡ 1 − pt,T

Probability of surviving beyond T , conditional on t-information and survival until t

M¯ t,T

The N martingale E[1τ >T | Ft ]

To see this, consider a CDS contract of maturity Tn , for which the observed spread is st,Tn , and write T1 , . . . , Tn for the fee payment dates of the CDS. The distance between payment dates is αi = Ti − Ti−1 , with t = T0 < T1 . Set pt,T := N (τ ∈ (t, T ) | Ft , τ > t)

(2.63)

to be the probability in the time-t filtration that the default time τ of the entity underlying the CDS lies in (t, T ] (given survival until t). Then the fee leg of the CDS has value n At := st,Tn Dt,Ti αi (1 − pt,Ti ). (2.64) i=1

Similarly, if R¯ ≡ 1 − R represents the payment per unit notional made by the CDS upon default, the value of protection offered by the CDS is given by Bt := R¯

n

Dt,Ti (pt,Ti − pt,Ti−1 ),

(2.65)

i=1

where we have assumed that default payments are made at time-points in a discretized grid. The observed (fair) CDS spread st,Tn is that which makes At and Bt equal. That is to say, the unknown probabilities pt,T1 , . . . , pt,Tn satisfy n

¯ i−1 − αi st,Tn + R¯ pi = 0, Dt,Ti αi st,Tn + Rp

(2.66)

i=1

where we have abbreviated pj ≡ pt,Tj . For n = 1, (2.66) is an expression in p1 and p0 ≡ 0, and therefore gives us the value of p1 as α1 st,T1 + R¯ . (2.67) p1 = α1 st,T1 Similarly, given values of p1 , . . . , pj −1 , we obtain the value of pj in terms of pi , i = 1, . . . , j − 1 and st,Tj . Explicitly, pj =

−1 Dt,T j

j −1 i=1

¯ i−1 − (αi st,Tn + R)p ¯ i} Dt,Ti {αi st,Tn + Rp

αj st,Tj + R¯ +

¯ j −1 αj st,Tj + Rp , αj st,Tj + R¯

j = 2, 3, . . . , n.

(2.68)

2.7 Credit

41

The inductive recipe (2.68) gives us the per-period default probabilities pt,Ti , which at time t are consistent with CDS market prices observed as the spreads st,Ti . In particular, at t = 0, we can obtain probabilities p0,Ti which are consistent with timezero observed CDS spreads.

2.7.2 Stochastic Default Probabilities The time-zero default probabilities (2.68) serve as the starting point for the simulation of stochastic default probabilities for a given entity. Our modelling hinges on specifying dynamics for the N-martingale (2.69) M¯ t,T = E 1τ >T | Ft . Here, expectation is taken in the martingale measure corresponding to numeraire N , where we will assume that credit quantities and interest rates are independent.11 The initial values M¯ 0,T in (2.69) are chosen to replicate exactly the initial term structure of survival probabilities, namely M¯ 0,T = (1 − p0,T ) ≡ q0,T .

(2.70)

The filtration F = (Ft )t≥0 contains information about the underlying drivers of the economy, but not about actual defaults. In this filtration, the martingale M is related to survival probabilities via the intensity process, λ = (λt )t≥0 , for τ , that is, M¯ t,T = E 1τ >T | Ft t T = exp − λu du E exp − λu du Ft 0

t

=: M¯ t,t qt,T ,

(2.71)

where the last equality serves to define qt,T as the probability that conditional on having survived until time t, and conditional on the information in Ft , the reference name does not default before T . From (2.71), default probabilities qt,T can be obtained from values of martingales M¯ ·,T . This approach is in a sense akin to those followed by Sidenius [99] and Bennani [12]. The form of M¯ t,t also points to how one can simulate values of the default time for any given reference name: (i) draw a random uniform U ; (ii) set τ to be the least member of the set {t ≥ 0 : M¯ t,t ≤ U }. 11 The

implication of this is that M¯ t,T is a martingale under any rate-based numeraire measure.

42

2 Modelling Framework

Looking at the recipe above, correlation between different reference names can be introduced by (i) imposing dependence between U (i) and U (j ) for any i, j , and/or (j ) (i) (ii) setting up diffusion dynamics for the martingales M¯ t,T and M¯ t,T and then allowing the driving Brownian Motions to be correlated. The question of how to introduce dependence between default times has been approached in several ways in the literature, as any standard book on credit derivatives will reveal (see for example Lando [72] or Schoenbucher [95]). We approach the problem using a copula method. One of several copulas can be used, each type of copula building dependence in a different way (see, for example Madan et al. [77]). In our exposition below, we use the example of the well-known Gaussian copula, for which dependence is equivalent to a linear correlation parameter. Thus, we specify for each reference name i, U (i) = 1 − Φ ρ (i) · M + ρ¯ (i) M (i) (2.72) dW (i) = η(i) · dZ + η¯ (i) dZ (i) . Here, M and M (i) are standard normally distributed random variables (the former of dimension possibly larger than one) driving dependence between default times. Similarly, Z and Z (i) are standard Brownian Motions (with the former again of dimension possibly larger than one) driving thestochasticity of survival probabilities. The reals ρ¯ (i) = 1 − ρ (i) · ρ (i) and η¯ (i) = 1 − η(i) · η(i) ensure that U (i) and dW (i) have uniform and normal distributions, respectively. For each pair (i, j ), Z, Z (i) , Z (j ) , dW (i) and dW (j ) are all independent; expressions (2.72) build up dependence between U (i) , U (j ) , and between dW (i) , dW (j ) , because defaults of all entities depend on Z and on M. Elements in M and Z are thought of as market factors impacting defaults and spread evolutions for different entities, while M (i) and Z (i) affect only the spread evolution and default time of reference name (i) . We will see in Chap. 3 the specific form the inter-name dependence takes for a ¯ particular choice of dynamics for the martingale M.

2.7.3 Loss Simulation For credit products that depend on defaults of several entities, the simulation of defaults for individual names implies, assuming an appropriate dependence structure has been imposed, a corresponding simulation of losses suffered by a chosen portfolio of names. Consider an investor who holds several corporate bonds B¯ (i) , i = 1, 2, . . . , n. The loss li incurred by bond i upon default is li = (1 − Ri )Ai ≡ R¯ i Ai ,

(2.73)

2.7 Credit

43

where Ai is the nominal amount on bond i and Ri is the fraction of face value that is retained by the bond upon default. Given this, the fractional total loss suffered by the portfolio of bonds in the time interval [0, t] is defined to be Lt =

n

n

R¯ i Ai 1{τ (i) ≤t}

i=1

Ai .

(2.74)

i=1

The law of Lt depends of course on the dependence between default times of different entities. The typical observable quantity that provides information on such dependence is the market price of a CDO tranche, which is a product whose payoff at time t is of the form n + n + Lt − ka − Lt − kd , (2.75) Πt = i=1

i=1

where ka (respectively, kd ), satisfying 0 ≤ ka ≤ kd ≤ 1, are referred to as the attachment (respectively, detachment) point.12 We will see in Chap. 3 how market quotes for different tranches can be used to calibrate the dependence coefficients ρ and η appearing in (2.72).

12 CDO

(Collateral Debt Obligation) products will be described in details in Chap. 10.

Chapter 3

Simulation Models

In Chap. 2 we defined a general framework to enable estimation of counterparty exposure for different product classes. Throughout, we highlighted the importance of being able to simulate price processes of different asset classes simultaneously and in consistent fashion. This was accomplished by simulating a martingale process for each asset class. By doing so, the models fit time-zero forward curves by construction, so that calibration involves only choosing the volatility structure for the martingale pertaining to each asset class. In this chapter we focus on specific choices of models for different asset classes, discussing how they can be implemented and calibrated within our framework.

3.1 Interest-Rate Models For relatively simple interest-rate products, arbitrage-free models with deterministic volatility have dynamics rich enough to reproduce simulated price distributions with the correct properties. In this section we describe in detail, within the framework defined in Chap. 2, the model with separable volatility structure introduced in (2.23), and we show in particular how the familiar Hull-White model is a special case of such a separable specification. For ease of exposition, we will mostly refer to a separable model driven by a single Brownian Motion. Products such as steepeners, however, which depend on different points of the yield curve, may require models with more than one stochastic driver or a richer volatility structure.1

1 There is a vast literature on interest-rate models. The reader can refer to the following books, Brigo

& Mercurio [18], Cairns [21], Filipovic [44], Hunt & Kennedy [64], Pelsser [85], or Rebonato [89] for more details. G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0_3, © Springer-Verlag Berlin Heidelberg 2009

45

46

3 Simulation Models

3.1.1 Separable Volatility In Sect. 2.3 we expressed all bond prices and the numeraire in terms of a family of N-martingales M.,T . In turn, for deterministic volatilities, (2.15) gives the integral form of Mt,T as t 1 2 σu,T dWu,T − Σt,T t , (3.1) Mt,T = M0,T exp 2 0 in terms of N-Brownian Motions WT ≡ (Wt,T )0≤t≤T and volatility functions σ ≡ (σt,T ). What we study here is the special case, already alluded to in (2.23), where the dependence of the volatility function on t and T can be separated into two terms. That is, we look at cases where the SDE for M can be written as (see also Sect. 2.3.1) dMt,T = Mt,T gT · (ft RdWt ),

(3.2)

with W a Brownian Motion in Rn , gT a deterministic vector in Rn , and with RT R being a positive semi-definite matrix such that the i’th component of RW is a standard Brownian Motion, which then gets time-changed by the (i, i)’th entry in the diagonal (n × n) matrix ft . Thus, by defining a new, time-changed process (also in Rn ) t Xt = fu RdWu , (3.3) 0

the expression for Mt,T can be written as 1 2 Mt,T = M0,t exp gT · Xt − Σt,T t . 2

(3.4)

2 t is the variance of g · X , that is, The term Σt,T T t 2 := t −1 Var (gT · Xt ) = gT · Σt,T

=: gT · F2t gT ,

t

(fu R)T (fu R)du gT

0

(3.5)

serving to define the matrix F. While we have written the model in general for n factors, to simplify the notation we will mostly restrict ourselves to the one-factor case in what follows; the vector gT and the matrix ft are then real-valued functions of T and t , respectively. In general Σt,T will be characterised by a number of parameters, and in a simulation framework, it is necessary to first calibrate these to instruments whose values are observable in the market. Thus, for instance, by writing the time-t price of a T -bond in terms of the martingale M, we get that Dt,T =

D0,T Mt,T D0,t Mt,t

3.1 Interest-Rate Models

47

=

1 2 D0,T exp (gT Xt − 2 Σt,T t) , 2 t) D0,t exp (gt Xt − 12 Σt,t

(3.6)

where in this one-factor case the variance is simply 2 tΣt,T = tFt2 gT2 .

(3.7)

The functional dependence of F and g on T and t may now be chosen to have the desired behaviour for the volatility of the T -bond. For instance, taking ft ≡ 1 and (3.8) gT = a 1 − e−κT results in bond volatilities (gT − gt )Ft = ae−κt 1 − e−κτ

(τ ≡ T − t),

(3.9)

which decrease with with time t but increase with tenor τ . In other words, bond options (or, equivalently, caplets; see below) priced with this model would exhibit implied volatilities that decrease with expiry but increase with tenor. One will need to choose the number of model parameters so as to strike the right balance between having a model that is parsimonious enough and having a fit to market prices that is good enough. It is also important to note that calibration can influence materially the counterparty risk profile. In the next sections we will show how to calibrate the model, using caps, floors, and swaptions.

3.1.1.1 Calibration to Caps A cap is a market instrument that pays, at pre-specified points in time, T1 < · · · < Tn , the amount by which the observed Libor rate exceeds a given level, K, referred to as the cap strike. Thus, the time-zero value, denoted C0,Tk (K), of the k’th payment made by the cap is C0,Tk (K) = D0,Tk EN

Tk

+

αk LTk−1 ,[Tk−1 ,Tk ] − K ;

(3.10)

the payoff above is referred to as a caplet. Tk indicates the maturity of the caplet, and NTk the Tk -forward measure. On the right, the payoff is a call option on the Libor rate, fixed at Tk−1 , for the period [Tk−1 , Tk ] of length αk = Tk − Tk−1 . Changing measure from NTk to N in (3.10) using (2.7), we get that + C0,Tk (K) = αk D0,Tk E MTk−1 ,Tk LTk−1 ,[Tk−1 ,Tk ] − K = αk D0,Tk E

¯ Tk−1 ,Tk L¯ k MTk−1 ,Tk−1 − KM

+

(from (2.19))

48

3 Simulation Models

MTk−1 ,Tk L¯ k ≤ MTk−1 ,Tk−1 K¯

K¯ Tk MTk−1 ,Tk−1 ¯ − αk D0,Tk KN ≥ , MTk−1 ,Tk L¯ k

= αk D0,Tk L¯ k NTk−1

(3.11)

where L¯ k = L0,[Tk−1 ,Tk ] + αk−1 and K¯ = K + αk−1 . Define M¯ tk =

Mt,Tk−1 . Mt,Tk

(3.12)

Since M¯ k and (M¯ k )−1 are respectively martingales in NTk and NTk−1 , evaluating the probabilities in (3.11) involves knowing the volatility structure of M¯ k . In particular, in the case of a separable volatility structure, we can write these probabilities as NTk−1

M¯ Tkk−1

N

Tk

−1

M¯ Tkk−1

where dk :=

L¯ k = Φ (dk ) , and K¯ K¯ = Φ dk − sTk−1 ,Tk Tk−1 , ≥ L¯ k ≤

√ ln L¯ k /K¯ sTk−1 ,Tk Tk−1 . + √ 2 sTk−1 ,Tk Tk−1

(3.13)

(3.14)

The variance of M¯ Tkk−1 is sT2k−1 ,Tk = gTk−1 − gTk · F2Tk−1 gTk−1 − gTk ,

(3.15)

where FTk−1 is the diagonal matrix defined in (3.5).

3.1.1.2 Calibration to Swaptions An interest-rate swaption is an instrument that gives the holder the right to enter into a fixed-for-floating interest-rate swap. Such instruments can be valued in closedform in the case of separable volatility one-factor models, and can therefore be used for calibration to observed market prices. Consider for concreteness a swaption which can be exercised at time T to enter into a swap where the holder pays a fixed annualised coupon of K and receives the Libor rate of interest. Write, again, T1 < · · · < Tn for the times when the swap payments are exchanged, and define (Tk − Tk−1 ) = αk for all k = 1, 2, . . . , n. Assume that T = T0 = T1 − α1 , so that the swaption exercise time coincides with the time of the first fixing on the floating rate component of the underlying swap. Then the

3.1 Interest-Rate Models

49

time-zero value, S0,T ,Tn (K), of the swaption is given by + n NT αk DT ,Tk S0,T ,Tn (K) = D0,T E 1 − DT ,Tn − K k=1

=E

NT

D0,T −

n k=1

where

ak =

Kαk , 1 + Kαk ,

MT ,Tk ak D0,Tk MT ,T

+ ,

k = 1, 2, . . . , n − 1, k = n.

(3.16)

(3.17)

In the special case of a one-factor model, (3.16) can be expressed as a sum of put M is a deterministic, monotonic options of different strikes. Indeed, if for any T , Mt,T t,t function of a single stochastic process Xt , then we can define hi (Xt ) := Mt,Ti /Mt,T h(Xt ) := D0,T −

n i=1

Mt,Ti ai D0,Ti = D0,T − ai D0,Ti hi (Xt ) Mt,T n

(3.18)

i=1

with h and the hi ’s being bijective. Therefore, there exists a family of strikes Ki∗ , i = 1, 2, . . . , n, such that D0,T −

n i=1

MT ,Ti ai D0,Ti MT ,T

+ = MT−1 ,T

n

ai D0,Ti (Ki∗ MT ,T − MT ,Ti )+ . (3.19)

i=1

In fact, the above is satisfied by having Ki∗ = hi h−1 (0) .

(3.20)

Once the strikes Ki∗ have been found using a root-searching algorithm, pricing the swaption is reduced to pricing put options of the form + M T T ,T i Ki∗ − EN MT ,T √ (3.21) = Ki∗ Φ (d) − Φ d − sT ,Ti T , where

√ ln Ki∗ sT ,Ti T d := , √ + 2 sT ,Ti T 2 sT2 ,Ti = FT2 gT − gTi .

(3.22) (3.23)

50

3 Simulation Models

3.1.2 Example: Hull-White (Extended Vasicek) We analysed in the previous section how to calibrate with caps and swaptions a generic model specified in terms of separable volatility structure. We consider here a specific example of separable volatility model, the Hull-White model. The Hull-White model introduced in [63] is a short rate model with deterministic volatility, where the short rate r satisfies the SDE drt = (θt − art )dt + σ dWt ,

(3.24)

with speed of mean reversion a and volatility σ (see also Baxter & Rennie [10], Brigo & Mercurio [18], or Hull [62] for further details). Knowing the instantaneous forward rate f (0, t) at time zero for different values of t (that is, knowing the current yield curve) allows determination of the unknown θt in (3.24). Note also that (3.24) prescribes that all bonds in the economy will depend on the same source of randomness W . We can write for each t ≥ 0, rt = f (0, t) +

2 σ2 1 − e−at + σ 2 2a

t

e−a(t−u) dWu ,

(3.25)

0

in terms of f (0, t). From this it is immediate that for each t , the short rate rt is a normal random variable with variance σ2

1 − e−2at . 2a

(3.26)

Moreover, note that from (3.25), the Hull-White numeraire process satisfies Nt−1

t = exp − f (0, u)du −

2 σ2 1 − e−au du 2 0 0 2a t u −a(u−s) − σ e dWs du 0

t

0

= D0,t exp −

t

0

= D0,t exp −

0

t

2 σ2 1 − e−au du − 2a 2 2 σ2 1 − e−au du − 2 2a

t

u

σ 0

e 0

t

σ 0

−a(u−s)

t

e

−a(u−s)

dWs du

dudWs , (3.27)

s

where the boundedness of the integrand allows us to use the Fubini theorem2 in the last equality. We now attempt to write our martingale M so that bond-prices in our framework have the same volatility structure as in the above Hull-White model. First, in the 2 For

the generalisation of this result to stochastic integrals, see, for example, Protter [86].

3.1 Interest-Rate Models

51

framework of Sect. 2.3 we have that Dt,T = and

D0,T Mt,T , D0,t Mt,t

t 1 2 Nt−1 = D0,t M0,t exp σu,t dWu,t − Σt,t t , 2 0

where 2 Σt,t :=

1 t

0

t

2 σu,t du.

(3.28)

(3.29)

(3.30)

Comparing (3.27) and (3.29) we can then set 2 σt,t =

2 σ2 1 − e−at . 2 a

(3.31)

To identify the form of Σt,T for T = t in terms of the Hull-White parameters, we write Dt,T in terms of the martingale Mt,T : D0,T M0,T + log D0,t M0,t t t 1 2 1 2 + σu,T dWt − Σt,T t − σu,t dWt − Σt,t t , 2 2 0 0

log Dt,T = log

which means that

(3.32)

. d log Dt,T = σt,T − σt,t dWt ,

(3.33) . where = signifies that the two sides differ by a term in dt only. On the other hand, in the Hull-White model, we can write

. d log Dt,T = −σ Bt,T dWt , (3.34) where Bt,T :=

1 1 − e−a(T −t) . a

We therefore have σt,T − σt,t = −σ Bt,T = −

σ 1 − e−a(T −t) a

(3.35)

(3.36)

with σt,t as in (3.31). Putting everything together, our framework can therefore be specialized to HullWhite volatility dynamics by choosing σ σ σt,T = eat e−aT + − e−at . (3.37) a a

52

3 Simulation Models

Referring to the separable specification of volatility discussed in Sect. 3.1.1, the Hull-White model is achieved by having n = 2 and ft = Diag eat , e−at ; σ σ T ; e−aT , − gT = (3.38) a a 1 1 1 . R= √ 2 1 1 In effect, what happens here is that our general volatility function σt,T is a linear combination of two separable forms. The degeneracy of the matrix R means that the components of the √Brownian Motion RW are both equal to the single Brownian Motion (W1 + W2 )/ 2, consistent with there being only one source of noise as in the original Hull-White model.

3.2 Equity and FX Models The vast literature on option-pricing models, stemming from the seminal BlackScholes paper [15], consists of various attempts at dealing with the fact that stock prices and foreign exchange rates do not follow the simple dynamics of the BlackScholes model.3 In particular, financial asset price returns are not normally distributed (exhibiting skewness and fat tails), and volatilities vary with time and with the price level itself. In fact, it is market practice to quote option prices in terms of implied volatilities, that is, the value at which to evaluate the Black option-pricing formula that recovers the market option price. At a general level, the devices for modelling these departures from asset price log-normality are, among others, (i) Building a market model whereby stock and FX forward prices are modelled directly, employing Black-Scholes volatilities implied from observed option prices; (ii) Making the volatility function depend on the level of the underlying, a class of models commonly referred to as local volatility models; (iii) Allowing the volatility of the underlying process to be itself stochastic, thus introducing an additional source of noise. This class of models is referred to as stochastic volatility models; (iv) Having a stock price process with a discontinuous component such as a Lévy process. Our goal in this section is to specify equity and FX models within our modelling framework, and to show how they can be calibrated to market instruments. Recall 3 For

a survey of stylised facts about asset price returns, see, for example, Cont [29].

3.2 Equity and FX Models

53

from Sect. 2.4 the expression Ft,T = F0,T F¯t,T = F0,T Yt

M˜ t,T Mt,T

(3.39)

for the evolution of the FX forward price in terms of the martingales M and M˜ driving the reference and foreign rates of interest. Typically, models of the types enumerated above, designed to reproduce some desired feature of stock prices or foreign exchange rates, do away with complexity arising from discounting by assuming interest rates to be deterministic or constant. In terms of our framework this amounts to having M˜ ≡ M ≡ 1 (whence Y ≡ F¯ ), and therefore in what follows we will sketch the details of how the Black, local volatility and stochastic volatility models can be applied to the process Y . To see how this relates to the more familiar situation where the spot price is modelled directly, note that Y = F¯ means Yt = Ft,T /F0,T . In particular, for T = t we then have St ≡ Ft,t = F0,t Yt , so that F˙0,t dYt dSt = dt + ; St F0,t Yt

(3.40)

that is, imposing a volatility on Y amounts to doing the same on the spot price S. Because we need to deal with all asset types (including interest-rate products) simultaneously, we also outline in Sect. 3.2.5 the form these models take when rates are stochastic, in which case the martingale terms in (3.39) contribute additional volatility to F¯ . The key idea will be to reduce the general case to the case of nonstochastic rates by taking an appropriate conditional expectation. We also look at the simpler approach, mentioned in (2.53), where we model forward rates (for equity and foreign exchange assets) that are independent of interest rates.

3.2.1 Black Model The Black model extends the original Black-Scholes model, by writing localmartingale dynamics for asset forward prices, of the form dYt = Yt g(t)dWt ,

(3.41)

with W an N-Brownian Motion and g(t) a deterministic function in t . In order that this specification of forward prices match observed option prices on the underlying asset, one simply compares the variance of Y in (3.41) to the observed Black-Scholes implied volatility for options of different maturities. That is, if maturities T1 , . . . , Tn are available, then g has to be consistent with

1 Ti

0

Ti

12

gu2 du

= Σ BS (F0,Ti , Ti ),

i = 1, 2, . . . n,

(3.42)

54

3 Simulation Models

where Σ BS (K, T ) is the Black-Scholes implied volatility for an option of expiry T and strike K. An alternative approach would be to calibrate to variance swap strikes, if a market for these is available. The constraint on g in this case is 1 Ti

Ti

0

gu2 du = E[VTi ],

(3.43)

where for each t > 0, the time-t realised variance Vt is defined to be 1 t [ln Fu,t ]u du, Vt := t 0

(3.44)

in terms of the quadratic variation process,4 [ln F ], of ln F . Now, the realised variance can be expressed as an integral over call and put prices. Indeed5 we find +∞ F0,T P (K, T ) C0 (K, T ) 0 −1 2 E[VT ] = D0,T dK + dK , (3.45) T K2 K2 0 F0,T in terms of the time-zero prices of calls (resp. puts) C0 (K, T ) (resp. P0,T (K, T )). In practice, there are subtleties in how to compute the fair variance swap strike, which we will not discuss here.

3.2.2 Local Volatility The Black model of the previous section is calibrated to a different volatility for each at-the-money option maturity, but still makes the implicit assumption that options of different strikes exhibit the same implied volatility. In practice, implied volatilities observed in the market vary not just with option expiry but also with the strike of the option, an effect usually called a smile or skew. That is to say, market prices 4 If

M is a continuous local martingale, then there exists a unique increasing continuous process, [M], called the quadratic variation process of M, such that M 2 − [M] is a continuous local martingale. See Rogers & Williams [94] for the full story. 5 Taylor’s

expansion series with integral remainder gives for any C 2 (R) function that b f (b) = f (a) + f (a)(b − a) + f (x)(b − x)dx a

= f (a) + f (a)(b − a) +

a

b

f (x)(b − x)+ dx −

b

f (x)(x − b)+ dx.

a

We apply this to f (x) = ln x with a = F0,T and b = FT ,T . Taking N-expectations and noting that E[FT ,T ] = F0,T yields the result.

3.2 Equity and FX Models

55

for options are inconsistent with assuming that the volatility of the underlying price process does not change with the price level. One way of building this effect into the model is to modify the Black SDE (3.41) to dYt = g(t, Yt )Yt dWt ,

(3.46)

where the volatility function now depends also on Y . Models of the form (3.46) are referred to as local-volatility models. The departure of (3.46) from the Black model can be gauged by analysing the special case where the volatility term takes the form g(t, x) = g(t)f (x). Expanding the function f around the value 1 and comparing to the implied volatility, we have (to second-order) m m 1 f (x)dx ≈ α + β(x − 1) + γ (x − 1)2 dx 2 1 1 ≈ (m − 1)

Σ BS (mF0,T , T ) , Σ BS (F0,T , T )

(3.47)

where α ≡ f (1), β ≡ f (1) and γ ≡ f (1), and where for each T , g relates to the at-the-money implied volatilities,

T

g 2 (u)du = (Σ BS (F0,T , T ))2 T ,

0

as in the Black model. Doing the integration, we see that we need to have 1 1 Σ BS (mF0,T , T ) 2 ≈ 1 + β(m − 1) + γ (m − 1) , Σ BS (F0,T , T ) 2 6

(3.48)

which can be used in the Black formula to calculate approximate prices for options in this model, an observation that is important for calibration. From (3.48), we interpret the coefficients β and γ in terms of the at-the-money skew and convexity, that is 1 ∂Σ BS (K, T ) = βΣ BS (F0,T , T ) (3.49) ATM Skew = K ∂K 2 K=F0,T 2 BS (K, T ) 1 2∂ Σ = γ Σ BS (F0,T , T ) (3.50) ATM Convexity = K 2 3 ∂K K=F0,T showing that if it is deemed suitable to have f quadratic, then (3.48) characterises f in terms of observed implied volatilities. We also read off Derman’s approximate rule of thumb (see Derman [36]) that near m = 1, the local volatility f changes with m twice as fast (at a rate of β) as the implied volatility (which changes at a rate of 1 2 β). The most well known example of a local volatility model is the Constant Elasticity of Variance (CEV) model, first studied by Cox & Ross [32] (see also

56

3 Simulation Models

Schroder [97]). In our notation, the CEV model is obtained by choosing g(t, x) = σ x β ,

(3.51)

where σ > 0 is a positive real and β ≡ f (1) is a constant skewness parameter6 as seen in (3.48). For β = 0, the CEV model is obviously the Black Model in (3.41). For β < 0 (resp. β > 0), the volatility decreases (resp. increases) with x. This results in the distribution of Yt being skewed to the left (resp. to the right), as shown in Fig. 3.1. Fig. 3.1 Distribution of FT ,T = ST with F0,T = 100, σ = 20% and T = 1

Taken at face value, local volatility models impose on asset prices a volatility term that is a deterministic function of the asset price level, an assumption that might be questionable in practice. However, Dupire [41] showed the existence of a diffusion process consistent with the observed local volatility surface, and the diffusion coefficient for this process is a local volatility function. We review the derivation of this result, following closely the analysis of Dupire’s idea in Gatheral [48]. Consider the time-zero price, C(F0,T , mF0,T , T ) say, of an option of expiry T and strike mF0,T , where F0,T is the time-zero price of the T -forward and m > 0 is the strike moneyness. If we write pt (·, ·) for the risk-neutral transition density of Y , the call price has the representation C(F0,T , mF0,T , T ) =

∞

D0,T (yF0,T − mF0,T )pT (1, y)dy.

(3.52)

m

The Kolmogorov equations for the transition density pT (·, ·) read p˙ t (x, y) = Gx pt (x, y) = Gy∗ pt (x, y), 6 The

(3.53)

parameter β is also related to the elasticity of the variance in this model, which is x(f 2 ) (x)/f 2 (x) = 2β.

3.2 Equity and FX Models

57

where G , G ∗ are, respectively, the infinitesimal generator7 of Y and its adjoint, and where p˙ t represents differentiation in t . Differentiating (3.52) in T and using the first part of (3.53), we obtain the Black-Scholes differential equation. Because we are interested in a differential equation involving derivatives in the strike and not in the initial forward price F0,T , we use the second part of (3.53) to get ˙ C(y, t) =

˙ F˙0,t F˙0,t D0,t + yC (y, t) C(y, t) − D0,t F0,t F0,t 1 + g 2 (t, y)y 2 C (y, t), 2

(3.54)

where C(y, t) is an abbreviation for C(F0,T , yF0,T , t), C and C are derivatives of C in its argument y, and C˙ is the derivative in t . Inverting (3.54) gives us an expression for the local volatility function g in terms of option moneyness and maturity, that is g 2 (t, y) =

˙ C(y, t) −

D˙ 0,t D0,t

+

F˙0,t F˙0,t F0,t C(y, t) + F0,t yC (y, t)

1 2 2 y C (y, t)

.

(3.55)

Although C and the derivatives C and C are related to market call prices, the final goal will be reached only when we express g in terms of what is directly observed in the market, namely the implied volatility surface. Gatheral [48] shows the required expression to be ϕ ∂w 1 1 1 ϕ2 ∂w 2 1 ∂ 2 w g (t, y) = w˙ 1− + + − − + 2 , w ∂ϕ 4 4 w w ∂ϕ 2 ∂ϕ 2 2

(3.56)

where w(F0,T , y, T ) = T Σ BS (F0,T , yF0,T , T ),

(3.57)

is the Black-Scholes implied total variance, w˙ ≡ ∂w/∂T ,

(3.58)

ϕ ≡ ln y

(3.59)

and where

is the log-moneyness of the option strike. In practice, to get around difficulties arising from the volatility surface not being smooth or granular enough, one would parametrize the total variance surface w in t X is a diffusion, then for smooth f the process Yt = f (Xt ) − 0 (G f )(Xs )ds is a martingale. ∗ is then defined by having g(x)(G f )(x)dx = (G g)(x)f (x)dx for smooth f and g. From this characterisation, we see that Y is a martingale, supermartingale or submartingale according as f is harmonic (G f = 0), superharmonic (G f ≤ 0), or subharmonic (G f ≥ 0) for G .

7 If

G∗

58

3 Simulation Models

terms of powers of moneyness K/F0,T , and use derivatives of this parametrized form in computing (3.56). Figure 3.2 shows the implied volatility as a function of strike for chosen values of skew and convexity. Similarly, Fig. 3.3 shows the effect of skew on the resulting distribution for the log-price of the stock.

3.2.3 Stochastic Volatility The class of stochastic volatility models allows volatility of an asset price and the asset price itself to be altogether different processes, by writing dynamics of the form √ dYt = Yt vt dWt

(3.60)

dvt = α(Yt , vt , t)dt + β(Yt , vt , t)dZt

(3.61)

with W and Z being N-Brownian Motions having instantaneous correlation ρ. Now from Dupire’s result we know that prices of options (equivalently, the implied volatility surface) of some given expiry T are consistent with a distribution of asset prices at T arising from some local-volatility model. In fact, it is a consequence of a more general result of Gyöngi [54] that if the SDE for Y in (3.60) admits a unique solution, then the SDE dXt = Xt b(t, Xt )dWt ,

X0 = Y0 ,

(3.62)

has a weak solution8 having the same law as Y . Moreover, the diffusion term b has the representation b2 (t, y) = E[vt | Yt = y],

(3.63)

thus exposing the link between the stochastic volatility process v and the Dupire local volatility function b. For more details on diffusion and stochastic volatility models we refer the reader to Andersen & Piterbarg [3], Cox [31], Dupire [41], Derman & Kani [37], and Hagan et al. [55]. A well-known example of a stochastic volatility model is the Heston model (see Heston [61]), in which the asset price variance is a diffusion of CIR type, that is √ dvt = κ(θ − vt )dt + σ vt dZt .

(3.64)

This diffusion is mean-reverting to a level θ > 0, with κ > 0 controlling the speed of reversion. The real parameter σ > 0 is commonly known as the volatility of volatility. Correlation between the Brownian Motions Z (driving volatility) and W (driving the asset price Y ) controls the implied volatility skew, with negative skew resulting 8 For the concept of a weak solution to an SDE, see any standard text on the subject, such as Rogers & Williams [94].

3.2 Equity and FX Models

59

from negative correlation (much like negative β in the CEV model). Euler and Milstein schemes are not the best choice for discretizing and simulating the pair of SDEs (3.60)–(3.61); Andersen [1] presents an efficient method that makes use of moment-matching techniques. For the Heston model (part of a bigger class of generally tractable models—see Duffie et al. [40]), the characteristic function of the asset price distributions is known in closed form. By this, and as shown by Carr & Madan [24], the characteristic function of the option price can itself be written down in closed form, allowing option prices to be recovered by numerical inversion. In general, if st = ln (Yt ) and φT (u) = E[exp (iusT )]

(i 2 ≡ −1),

(3.65)

is the characteristic function of sT , then the modified call prices −1 ζ m c(m, T ; ζ ) ≡ F0,T e C(F0,T , em F0,T , T )

(3.66)

with strike K, log strike moneyness m = ln (K/F0,T ), maturity T and real ζ > 0, have Fourier transform ∞ D0,T φT (v − (ζ + 1)i) , (3.67) eivx c(x, T ; ζ )dx = 2 ψT (v) ≡ ζ + ζ − v 2 + i(2ζ + 1)v −∞ which shows why positivity of ζ is required to avoid having ψT singular at the origin. Fourier techniques such as FFT can be used to calibrate the model using observed call prices, by numerically inverting the option price transform (3.67). In the particular case of the Heston model9 the transform (3.65) is log-affine in the starting point V0 of the variance process V , namely φT (u) = exp {C(T , u) + D(T , u)V0 },

(3.68)

where

κθ c(u)ed(u)T − 1 C(T , u) = 2 (κ − ρσ ui + d(u)) T − 2 ln , c(u) − 1 σ κ − ρσ ui + d(u) ed(u)T − 1 , D(T , u) = σ2 c(u)ed(u)T − 1

with c(u) =

κ − ρσ ui + d(u) , κ − ρσ ui − d(u)

d(u) =

(ρσ ui − κ)2 + (iu + u2 )σ 2 .

(3.69)

(3.70)

Note that in (3.68), we have φT (−i) = E[YT ] = 1, because C(T , −i) = D(T , −i) = 0; this is of course as expected from the martingale property of Y . 9 See,

for example, Kahl & Jaeckel [67].

60

3 Simulation Models

3.2.4 Jump Models Processes with jumps are an attractive tool for modelling asset prices, with Merton [81] being the first to explore option pricing in models where the stock price has a discontinuous component. Geman et al. [49] and Carr et al. [26] present a case for using pure jump processes (that is, with no diffusion component) to model asset price returns. Linked to this idea is the use of non-decreasing jump processes10 to time-change continuous diffusions, thus providing a powerful device for matching the implied volatility skew (see also Carr & Wu [25], Eberlein et al. [42], Kou [71], and Mendoza et al. [79]). The basic class of jump processes with which anything tractable can be done is Lévy processes, because the infinite divisibility property gives the characteristic function a form that enables calculation of various Laplace transforms and therefore opens the way to inversion of option prices as for the Heston model. For more information on Lévy processes, we refer the reader to Barndorf-Nielsen et al. [9], Bertoin [13], Boyarchenko & Levendorskii [16], and Rogers & Williams [94]. For work on models combining stochastic volatility with Lévy processes see, for example, Carr et al. [27]. There is a vast literature on the use of jump processes in financial modelling, of which Cont & Tankov [30] is a good survey. Schoutens & Symens [96] studies the pricing of options by simulation in jump models.

3.2.5 Extension to Stochastic Interest Rates The previous sections have reviewed standard specifications for the Black, local volatility and stochastic volatility models. Because the primary goal of these models is to explain or fit observed volatilities implied from option prices, the complexities brought about by stochastic interest rates are avoided by assuming rates and bond prices to be deterministic. The assumption of deterministic rates is a severe constraint if models are to be used for credit exposure computation, but what we have done above has not been in vain. In fact this section will describe how, by introducing an extra conditioning step, the models for deterministic interest rates can be re-used to take into account stochastic interest rates. In our framework, where we have (see (3.39)) Ft,T = F0,T F¯t,T = F0,T Yt

M˜ t,T , Mt,T

(3.71)

deterministic rates are equivalent to the martingales M and M˜ being identically equal to one, and the asset pricing models as we have described them then apply to the process Yt = Ft,T /F0,T , where YT then has the interpretation as the moneyness of the time-T spot price relative to the time-zero forward price. 10 Real-valued

increasing Lévy processes are termed subordinators.

3.2 Equity and FX Models

61

We now review how such models can be extended to the case when the rates martingales are not deterministic, similar to what is done by Andreasen in [5], and assuming throughout that M and M˜ follow log-normal dynamics. The key idea is to write the process Y as the product Y Y ⊥ of two independent martingales. Conditioning on the value of Y , an option on Y ⊥ can be priced in the stochastic or local volatility models, with the final option price then being an integral over the law of Y of the conditional option prices. To see this, in (3.71) we start by writing ˜ t,T ˜ t,T M M =: Yt Yt⊥ , (3.72) Yt⊥ ≡ F¯t,T F¯t,T = Yt Mt,T Mt,T ˜ 11 ). Intuwith Yt independent of Yt⊥ (but not, of course, independent of (M, M) ⊥ itively, as the notation is intended to help convey, Y relates to the stochastic component in asset forward prices that is ‘orthogonal’ to rates. Of course, for deterministic rates, Y ⊥ coincides with Y and Y is identically equal to one. The advantage of this decomposition is that it allows to calibrate the model in two distinct steps, separating the asset price and interest rate components, as we see now. Consider a call option of maturity T and strike K = mF0,T , which has price + C(F0,T , mF0,T , T ) = E NT−1 FT ,T − mF0,T . (3.73) Conveniently switching to the T -forward measure, we have + T F¯T ,T F0,T − mF0,T C(F0,T , mF0,T , T ) = D0,T EN + T F¯T ,T − m = D0,T F0,T EN = D0,T F0,T EN

T

+ F¯T,T YT⊥ − m/F¯T,T .

(3.74)

The crucial next step is now to invoke the independence of Y and Y ⊥ to develop the call price as C(F0,T , mF0,T , T ) + T T YT⊥ − m/y y = F¯T,T = F0,T EN F¯T,T D0,T EN = F0,T EN

T

F¯T,T C ⊥ 1, m/F¯T,T , T ,

(3.75)

where C ⊥ (y ⊥ , k, T ) is the price of an option on Y ⊥ (whose initial value is y ⊥ ) struck at k, of expiry T . 11 We emphasise again that, for tractability reasons, we will assume Y follows a log-normal prot cess.

62

3 Simulation Models

The usefulness of decomposing Y is now clear: to compute the price of an option on F , we first compute prices of an option on Y ⊥ , using some preferred asset-pricing model and unencumbered by rates stochasticity, and then incorporate the effect of rates by integrating over the law of F¯ . ˜ then F¯ will be If we take log-normal Black models for each of Y , M and M, T also log-normal (and a N -martingale). We then have the equality in law √ 1 2 ¯ FT ,T ∼ exp Z T ΣT − (ΣT ) T ≡ E (Z, T , ΣT ) (Z ∼ N (0, 1)), (3.76) 2 where the total variance T (ΣT )2 is a function of the volatilities and covariance ˜ Thus, if gt = (gt , g˜ t , gt ) is the vector of Black volatilites matrix of Y , M and M. ˜ M), then we have of (Yt , M, T (ΣT )2

=

T

gu · Rgu du,

(3.77)

0

where, with obvious notation, the matrix ⎛ 1 −ρY,M˜ ⎝ 1 R = −ρY,M˜ ρY,M ρM,M˜

⎞ ρY,M ρM,M˜ ⎠ 1

(3.78)

has as entries the pair-wise correlations between Brownian Motions driving Y , M ˜ and M. With this setup, we can now write the call price fairly explicitly as a Gaussian integral, for we have from (3.75) that C(F0,T , mF0,T , T ) ∞ E (z, T , ΣT )C ⊥ 1, m/E (z, T , ΣT ), T φ(z)dz = D0,T F0,T −∞ ∞

= D0,T F0,T

−∞

√ C ⊥ 1, m/E (z + T Σ , T , ΣT ), T φ(z)dz,

(3.79)

where the last equality follows immediately once we interpret the ‘Doléans’ exponential12 E defined in (3.76) as a change of measure. The integral in (3.79) readily lends itself to numerical methods (such as gaussian quadrature) as long as the call price is available. In the case of stochastic volatility models such as the Heston model, call prices are known only up to a Fourier transform, so FFT methods will be needed to compute the call prices by inversion, as discussed in Sect. 3.2.3. a martingale M with M0 = 0, the Doléans exponential of M is the exponential martingale process Et (M) = exp (Mt − 12 [M]t ), where [M] is the quadratic variation process of M. Our use of the name is because the law of (3.76) is the law of ET (ΣT B) for some Brownian motion B. 12 For

3.2 Equity and FX Models

63

There is nothing to stop us from decomposing the process Y ⊥ itself as the product of independent processes, as long as the characteristic functions of the time-t laws of those processes (and hence the characteristic functions of call option prices) are known in closed form. This allows the pricing of options in models where not only are rates stochastic, but also where different features of the volatility surface can be accommodated by employing independent local volatility, stochastic volatility and even jump components, in the spirit of Andreasen’s [4, 5].

3.2.6 A Simpler Approach: Independent Interest Rates It is worth contrasting the decomposition of Y in (3.72), whereby Yt⊥

˜

Mt,T Yt Mt,T

= F¯t,T

−1 ,

Y , Y ⊥ independent,

(3.80)

to the approximative one of (2.53), where we had simply Yt ≈ Fˆt,T

M˜ t,T Mt,T

−1 ,

˜ M) independent, Fˆ , (M,

(3.81)

with Fˆ and F¯ having the same marginal laws. Expression (2.54) showed how the inaccuracy brought about by (3.81) can be mitigated somewhat, and in Chap. 6 we will analyse by means of concrete examples the impact this inaccuracy has on pricing. Benhamou [11] analyses the bias introduced by neglecting the stochasticity of interest rates when deriving the Dupire formula.

3.2.7 Different Models for Different Markets Certain models may be better suited to equity markets than to FX markets. For example, equity markets usually have negative skews, whereas FX markets tend to have smiles. In other words, while the market-implied volatilities in equity markets are impacted by skew, those in FX markets are impacted by kurtosis. One possible economic reason for this is that traditional investors in equities can only be long, whereas one can always go short in foreign exchange markets by switching one’s holdings from one currency to another. Figure 3.2 shows typical implied volatilities for equities and foreign exchange markets. Figure 3.3 shows how the shape of the implied volatility surface impacts the distribution of log-FX rates. As described in Chap. 2, inflation markets can be seen as an extension of foreign exchange markets, with the exception that inflation markets are less complete. Indeed, while options on FX rates are commonplace, options on inflation indices

64

3 Simulation Models

Fig. 3.2 Typical implied volatilities for Equity Markets (at-the-money skew = −10%, at-the-money convexity = 0%) and Foreign Exchange Markets (at-the-money skew = 0%, at-the-money convexity = 30%, see (3.49))

Fig. 3.3 Implied distributions for Foreign Exchange Markets (above: at-the-money skew = 0%, at-the-money convexity = 30%) and Equity Markets (below: at-the-money skew = −10%, at-the-money convexity = 0%) compared to the standard normal distribution, see (3.49)

are not. Therefore using complex dynamics for inflation indices and real rates may be unnecessary, or even counterproductive since there are not enough liquid instruments available to calibrate the model.

3.3 Credit Models

65

3.3 Credit Models Credit Derivatives are products whose payoff is related to credit quantities such as credit spreads, credit default losses, or rating migrations. For credit derivatives relating to a portfolio of more than one entity, an essential modelling element is the inter-dependence between the spread and default times of the individual entities. Indeed, computing prices and simulating price distributions for credit products is challenging as (i) the choice is not clear as to what the best model is for simulating credit spreads and default times, and (ii) there are several ways of introducing default dependence between different credit entities (see for example Duffie & Singleton [39], Lando [72], or Schönbucher [95]). This section describes a possible model for computing the credit exposure posed by credit-related products. The starting point, as described in Sect. 2.7, is to use CDS term structure to compute values at time zero of the default probabilities for each single reference name. Once this is done, stochastic default probabilities for each reference name can be simulated following some chosen dynamics for the martingales M¯ appearing in Sect. 2.7.2; this is what we do in Sect. 3.3.1 below. We also describe how the volatility parameters in the model can be calibrated to market quotes for options on CDSs. For the dependence between different reference entities, we have chosen to work with a Gaussian dependence structure, as hinted at in (2.72). We detail in this section how we propose to calibrate this dependence structure to quotes on CDO tranches. At this point it is worth highlighting the difficulties one encounters in simulating price distributions of credit portfolios. When faced with the task of pricing correlation-dependent products such as, say, CDO tranches, it is standard practice to match market prices for different tranches by tuning model parameters (usually the correlation input) individually for each tranche.13 Such an approach is useless, however, in a simulation model attempting to produce correct default loss distributions for different tranches simultaneously. Consequently, the correlation structure in our model has no meaning beyond that given to it in the modelling expressions (2.72). In what follows, we consider a portfolio of n defaultable entities, and let Ai , Ri and τ (i) respectively define the nominal amount, the recovery fraction upon default, and the time of default of name i. Recall from (2.74) the portfolio loss process L ≡ (Lt )t≥0 , n n (1 − Ri )Ai 1τ (i) ≤t Ai . (3.82) Lt = i=1

i=1

The quantity (i) qs,t ≡ (1 − p (i) (s, t)) = N τ (i) ≥ t | Fs , τ (i) > s , 13 Often

this parameter is called base correlation by practitioners.

(3.83)

66

3 Simulation Models

will denote the probability, based on time-s information, that name i survives beyond time t , having survived until time s < t.

3.3.1 Simulation of Single-Name Default Probabilities and Default Times Here we specify concrete dynamics for the martingale process M¯ in (2.69). We choose a volatility term in the SDE for M¯ that has a separable dependence on time and maturity, and which leads to normally-distributed survival probabilities.14 More specifically, we set M¯ 0,T = q0,T ,

d M¯ t,T = ft gT dWt ,

(3.84)

or, in integral form, M¯ t,T = q0,T + gT Xt ∼ N (q0,T , gT2 Ft2 t), with

Xt =

(3.85)

t

fu dWu ,

(3.86)

0

and with Ft2

=t

−1

t 0

fu2 du

(3.87)

being the normalised variance of the time-changed process X. Note that the separability of the volatility coefficient of M¯ t,T is essential to allow fast access to any martingale value and therefore to any needed quantity (for instance, a par CDS spread). We now look at how to parametrize the functions f and g and calibrate them to prices of options on CDSs. Consider an option, expiring at time T ≡ T0 , to enter into a CDS contract that pays protection on a chosen reference name in return for a fixed strike coupon K at coupon payment dates T1 , T2 , . . . , Tn . Assuming a fixed ¯ the no-arbitrage time-zero price of such an option CDS recovery rate of R ≡ 1 − R, is + n ! " D T ,T i CDSS(0, K, T , Tn ) = E R¯ qT ,Ti−1 − qT ,Ti − αi KqT ,Ti , M¯ T ,T NT i=1

(3.88)

14 We

will see later in this chapter how to put bounds on the proportion of simulated probabilities not in [0, 1].

3.3 Credit Models

67

with αi ≡ (Ti − Ti−1 ). Expressing now the bond DT ,Ti in terms of the martingale MT ,Ti and the survival probability qT ,Ti−1 in terms of M¯ T ,Ti−1 , we get CDSS(0, K, T , Tn ) = E

n DT ,T

i

i=1

NT

+ {R¯ M¯ T ,Ti−1 − (R¯ + αi K)M¯ T ,Ti }

. (3.89)

This formulation allows one to price semi-analytically any CDS option, as long as the expectation in (3.89) can be computed in closed form. This is the case for the normal model in (3.84). Indeed, in (3.89), because M¯ is normally distributed, so is the difference (M¯ T ,Ti−1 − M¯ T ,Ti ). Thus (neglecting rates stochasticity), CDSS(0, K, T , Tn ) = E

n

D0,Ti

√ γi (qi−1 , qi ) + γi (gi−1 , gi )FT T Z

+ ,

i=1

(3.90) where we have abbreviated gi ≡ gTi , qi ≡ q0,Ti , Z ∼ N (0, 1), and where ¯ − (R¯ + αi K)y. γi (x, y) ≡ Rx

(3.91)

The sum appearing in (3.90) is just a normal random variable.15 We can write (3.90) as CDSS(0, K, T , Tn ) = μΦ(μ/η) + ηφ(μ/η),

(3.92)

where μ and η are the mean and standard deviation, respectively, of the sum in (3.90), i.e. μ≡

n

D0,Ti γi (qi−1 , qi )

i=1

η≡

n

D0,Ti |γi (gi−1 , gi )|FT

√

(3.93) T.

i=1

In what we have above, M¯ t,T is a normal variate centered around q0,T . It is desirable to have the martingale M¯ stay within the interval [0, 1] with high probability. This condition puts constraints on the forms one can choose for the volatility functions f and g. In more detail, if we want 0 ≤ M¯ t,T ≤ 1 15 Note

that given a random variable Z ∼ N(0, 1), a ∈ R, and b > 0, we can write E[a ± bZ]+ = aΦ(a/b) + bφ(a/b),

where φ is standard normal density and Φ is its primitive.

(3.94)

68

3 Simulation Models

Fig. 3.4 Quantiles of 5Y CDS spreads resulting from simulating survival probabilities according to the separable model in (3.84). Model parameters for the volatility functions f and g were (κ, κg , σ ) = (0.4, 0, 1)

to be true with probability p, then f and g will need to satisfy √ q0,T + qgT Ft t ≤ 1 √ 0 ≤ q0,T − qgT Ft t,

(3.95)

where q = Φ −1 ( 12 (p + 1)). Both these conditions can be summarised as √ min(q0,T , q¯0,T ) , gT Ft t ≤ q

(3.96)

q¯0,T = 1 − q0,T . Since the right hand side above involves only T , it is preferable to work on the assumption that F is known and then to ensure that gT ≤

min(q0,T , q¯0,T ) √ ; q maxt>0 (Ft t)

(3.97)

one satisfactory choice is gT = σ exp(−κg T )

min(q0,T , q¯0,T ) √ q maxt>0 (Ft t)

(3.98)

with κg ≥ 0 and σ ≤ 1. From (3.84), choosing, then, say, ft = exp(−κt),

κ ≥ 0,

(3.99)

whence tFt2 = (1 − e−2κt )/(2κ)

(≤ (2κ)−1 ),

(3.100)

3.3 Credit Models

we have finally the specific form of g as √ gT = q −1 σ 2κ exp (−κg T )(min(q0,T , q¯0,T )).

69

(3.101)

We can now use this parametrization to compute options on CDS (as described above) and we can derive Black implied volatilities. In this model Black-implied volatilities of CDS option prices can be seen to (i) decrease with option expiry; (ii) increase as a function of maturity of the underlying CDS, then decrease again; (iii) increase with the level of par CDS spread. Figure (3.5) displays implied volatilities for options of different expiries, exercising into CDSs of varying maturities.

Fig. 3.5 Black-implied volatilities for options of various expiries giving the holder the right to enter into CDSs of different maturities. Volatility generally decreases with option expiry (x-axis). It increases to a peak (for the CDS of 5Y maturity) and then decreases again with maturity of the underlying CDS. Model parameters used were (κ, κg , σ ) = (0.4, 0, 1)

3.3.2 Inter-Name Default Dependence In the spirit of the Gaussian dependence model prescribed in (2.72), and in the con¯ we now write down expressions for the joint law text of the normal model for M, of default times of different entities; these are of course a function of the correlation parameters ρ and η in (2.72). In turn, these joint laws will be used to compute semi-analytically the prices of the CDO tranches that are chosen as calibration instruments, allowing calibration of the correlation structure to market information.

70

3 Simulation Models

Consider the martingale process M¯ (i) driving the stochastic default probabilities for reference name i, namely (i) (i) (i) (i) d M¯ t,T = ft gT dWt ,

(i) (i) M¯ 0,T = q0,T .

(3.102)

We will force the M (i) to depend on each other by decomposing the Brownian Motion W (i) as dWt(i) = η(i) · dZt + η¯ (i) dZt(i) ,

(3.103) (i)

as in (2.72). Zt is a d-dimensionalBrownian Motion and Zt is a univariate independent Brownian Motion; η¯ (i) = 1 − η(i) · η(i) . This results in t (i) Xt = fu dWu(i)

0

t

= 0

(i) fu η(i) · dZt + η¯ (i) dZt t η(i) · Z + η¯ (i) Z (i) ,

(i) √

∼ Ft

(3.104)

where tFt2 is the variance of X and where Z, Z (i) now denote random variables √ (i) √ with same laws as Zt / t , Zt / t , respectively. Similarly, we force the uniform variables16 U (i) to depend on each other by setting (3.105) U (i) = 1 − Φ ρ (i) · M + ρ¯ (i) M (i) , again as in (2.72). With this setup, we can now write down the joint law of default times of different entities. Indeed, the cumulative distribution function of the default time of entity i is (i) N τ (i) < t = N M¯ t,t < U (i) (i) (i) (i) = N q0,t + gt Xt ≤ U (i) (i) (i) (i) = N 1 − U (i) ≤ (1 − q0,t ) − gt Xt (i) (i) (i) (i) (i) (p¯ 0,t ≡ 1 − q0,t ) = N Φ ρ (i) · M + ρ¯ (i) M (i) ≤ p¯ 0,t − gt Xt = N Φ ρ (i) · M + ρ¯ (i) M (i) (i) (i) √ (i) tFt η(i) · Z + η¯ (i) Z (i) ≤ p¯ 0,t − gt 16 . . . from

which we insist that default of i happen as soon as M¯ t,t < U (i) is true. . . . (i)

3.3 Credit Models

71

= N ρ (i) · M + ρ¯ (i) M (i) (i) (i) √ (i) η(i) · Z + η¯ (i) Z (i) , ≤ Ψ p¯ 0,t − gt tFt

(3.106)

where Ψ ≡ Φ −1 and, we recall, ρ¯ (i) ≡ 1 − ρ (i) · ρ (i) , η¯ (i) ≡ 1 − η(i) · η(i) . Note that the special case η ≡ 0 (or, equivalently, gt ≡ 0) corresponds to survival probabilities that are not stochastic, so that for each T , (i) (i) (i) M¯ t,T = q0,T = qt,T ,

t ≤ T.

(3.107)

The model we have presented collapses in this case to the familiar static one-factor Gaussian copula model. The reason for decomposing the uniform U (i) and the time-changed process Xt in terms of sums of independent terms is that by conditioning on the values of M and Z, which are common to all reference names i, we can obtain the conditional joint law of default times of several reference names. Indeed, from (3.106), expanding (i) Ψ ≡ Φ −1 to first order around p¯ 0,t , we end up approximating the conditional default probability for name i as N τ (i) < t M, Z (i) (i) ≈ N ρ¯ (i) M (i) + Ψ σ η¯ (i) Z (i) ≤ Ψ (i) − Ψ σ η(i) · Z − ρ (i) · M Ψ (i) − Ψ (i) σ η(i) · Z − ρ (i) · M , (3.108) =Φ 1 [(ρ¯ (i) )2 + σ 2 (η¯ (i) )2 (Ψ (i) )2 ] 2 (i)

where Ψ (i) ≡ Ψ (p¯ 0,t ) indicates the inverse cumulative normal distribution function,

Ψ (i) ≡ Ψ (p¯ 0,t ) is its derivative, easily written in terms of the Gaussian density, and √ where we have abbreviated σ ≡ gt Ft t. Because, by construction, the default times τ (i) are conditionally independent given M and Z, the conditional loss distribution at any time t of a portfolio of names is the distribution of a sum of independent single-name loss distributions. Thus, if L(i) is the cumulative loss process for name i, then n (i) (3.109) N(Lt ∈ (x, x + dx) M, Z) = N Lt ∈ (x, x + dx) M, Z (i)

i=1

is the law of a sum of independent random variates. This law can be computed numerically either using Fourier inversion or, as we describe in Sect. 3.3.3 below, by recursive methods. Having obtained the conditional loss distribution, the full unconditional loss distribution is only an integration step away, since the integral N (Lt ∈ (x, x + dx)) = N (Lt ∈ (x, x + dx) | M = m, Z = z) φ(dm, dz) (3.110)

72

3 Simulation Models

with respect to the gaussian densities of M and Z can be accomplished efficiently using a quadrature method. Knowing the distribution of the portfolio loss, given in (3.110), allows us to compute prices of derivatives on the portfolio loss, in particular CDO tranches, by computing the required expectations numerically.

3.3.3 Technical Note: Recursion The law of a sum of independent discrete random variables can be computed by a simple recursive procedure. Suppose we are given n independent discrete random variables Y1 , Y2 , . . . , Yn , assume that the support of Yi is the set {0, 1, 2, . . . , yi }, i = 1, 2, . . . , n, and let pi (k) = N(Yi = k). Consider the random variables Sj =

j

Yi ,

j = 1, . . . , n.

(3.111)

i=1

#j This has support in {0, 1, . . . , sj := i=1 yi }, and distribution {p(j, k) = N(Sj = k)}. The probabilities p(j, k) can be found by a recursive procedure, as follows: (i) Start with p(0, 0) = 1, p(0, k) = 0, k = 1, 2, . . . , sn . (ii) For each j = 1, . . . , n, compute p(j, k) for each k from 0 to sj : p(j, k) =

k

p(j − 1, i)pj (k − i).

(3.112)

i=0

Note that in the case where the Yi are Bernoulli variables with two possible values, as in the case of reference names that either default or not, all terms but two vanish in the sum (3.112). Now, as we have seen above, conditional on the market variables M and Z, the portfolio loss distribution Lt is a sum of independent distributions, so recursion can be applied once the loss distribution is suitably discretized. To do this, consider the process for the i’th reference name, Lt := A−1 R¯ i Ai 1τ (i) ≤t , (i)

A :=

n

Ai ,

(3.113)

i=1

with Ri = 1 − R¯ i being the fractional recovery for i and Ai the corresponding notional at risk for name i. In terms of this the fractional portfolio loss (3.82) is Lt =

n i=1

(i)

Lt .

(3.114)

3.3 Credit Models

73

We discretize the support set of Lt , namely −1 ¯ Ai Ri for some I ⊆ {1, 2, . . . , n} , A =0∪ z:z=A

(3.115)

i∈I

by choosing a real number and integers ψi and writing A−1 Ai R¯ = ψi + ri ,

i = 1, 2, . . . , n,

(3.116)

with each remainder term ri satisfying ri < . In this way, the random variable Lt , with support A , may be approximated by a discrete random variable of support {0, , 2, . . . , K} for some sufficiently large integer K. In other words, the loss suffered by each reference name upon default, expressed as a fraction of the total portfolio notional A, is an integer multiple of a basic loss quantum, . In particular, the loss suffered by reference name i takes values in {0, ψi } and the number of loss quanta has distribution ⎧ (i) ⎪ ⎨1 − q0,t , k = 0 (i) (3.117) pi (k) = q0,t , k = ψi ⎪ ⎩ 0, otherwise, with the quantities on the right being obtainable from the observed credit spreads for reference name i. Having discretized the support set A , the conditional portfolio loss process n (i) Lt (M, Z) = Lt (M, Z)

(3.118)

i=1

is a sum of discrete independent random variates, whose law can now be obtained using recursion. Clearly, the smaller the value one chooses for , the better will Lt be approximated by the corresponding discretized distribution, but this will necessitate a larger value of the integer K and result in longer computational time.

3.3.4 Properties of the Loss Distribution: Large Homogeneous Portfolio The portfolio loss distribution, and the prices of CDO tranches, can be written in closed form in the special case of a one-factor Gaussian copula model, under the assumptions that the number of names in the portfolio is arbitrarily large. To derive the closed form limiting distribution, we assume that the linear dependence parameter ρ i in (2.72), the recovery fraction Ri , and the default probabilities

74

3 Simulation Models (i)

1 − q0,t are identical for all i. That is, the portfolio is homogeneous, and contains an arbitrarily large number of identical reference names. Denoting by Lˆ the process whose value is the fraction of names that default in the portfolios, Vasicek [105] showed that the law of the time-t loss can be written as ρΦ ¯ −1 (h) − Φ −1 (1 − q0,t ) ˆ , h ∈ [0, 1], 0 ≤ ρ ≤ 1, (3.119) N Lt ≤ h = Φ ρ where we have dropped the now-irrelevant superscripts on the survival probability (i) q0,t and the correlation ρ (i) . Note that for ρ = 0, when defaults happen independently, the above says what we expect from the law of large numbers, namely that the proportion of losses will coincide with the probability (= 1 − q0,t ) that a single name defaults. In the limiting case ρ → 1, Lˆ is a Bernoulli distribution. More generally, for intermediate values of ρ, increasing ρ serves to skew the loss distribution to the right, with the consequence that larger mass is assigned to a larger number of defaults. Figure 3.6 shows this effect. What we infer from this is that the protection value of CDO tranches of the form [0, kd ], the so called equity- or basetranches, will decrease as ρ increases. Conversely, protecting senior tranches of the form [ka , 1] will cost more as ρ increases. For mezzanine tranches with ka and kd strictly different from 0 or 1, the behaviour of the protection price as a function of correlation will depend on the values of ka , kd and ρ. Fig. 3.6 Inverse CDF of Lˆ for a probability of default of 5%, and for various values of correlation ρ ∈ [0, 1]

It is tedious but not to difficult to also write down the price of protection for a CDO tranche in this limiting model. Recall that a CDO tranche is the difference of two options on the portfolio loss, so we need to be able to compute expectations of the form + , (3.120) E Lˆ t − Kˆ where Lˆ t , a random variable with support [0, 1], is the proportion of losses suffered by the portfolio by time t , and where Kˆ ∈ [0, 1] is the call strike. The expecta-

3.4 Choice of Model

tion (3.120) can be shown to equal + ˆ γ ; −ρ¯ , E Lˆ t − Kˆ = Φ2 −Φ −1 (K),

75

(3.121)

with Φ2 (·, ·; η) being the bivariate normal cumulative distribution with correlation ˆ owing to the η. Noticing that the real portfolio loss at time t will be Lt = (1 − R)L, recovery fraction R, we have the price of a call option on the portfolio loss, struck ˆ as at K = (1 − R)K,

+ E (Lt − K)+ = (1 − R)E Lˆ t − Kˆ , (3.122) whence

ˆ γ ; −ρ¯ . E (Lt − K)+ = (1 − R)Φ2 −Φ −1 (K),

(3.123)

3.3.5 Calibration of Correlation We have seen that within the context of a Gaussian dependence model, the expression (3.110) for the portfolio loss distribution can be computed semi-analytically after conditioning on M and Z. Clearly, the loss distribution obtained will depend on the dimensionality of the factors M and Z and on the dependence parameters, ρ (i) and η(i) chosen for each reference entity i. This points to a way for calibrating the chosen dependence model, namely by choosing dependence parameters in such a way that model prices for chosen tranches are sufficiently close to market-observed prices. The elements of M and Z can be thought of as market factors explaining the co-dependence between reference names; for example, geographical region and credit spread level (rating).

3.4 Choice of Model This chapter has described various models for asset and derivatives pricing, placing them in the context of the framework of Chap. 2. What we have built is a hybrid model tailored to price hybrid products whose underlying elements are the transactions in the counterparty portfolio. The particular choice of model for each asset class is driven mainly by a balance between accuracy and simplicity, and to a certain extent by the type of products present in the portfolio. In any case, however, we need to take into account that (i) The goal is the pricing and hedging of counterparty exposure, and accuracy is therefore key. (ii) Scenario consistency, and therefore the simulation of all processes simultaneously, is essential.

76

3 Simulation Models

3.4 Choice of Model

77

(iii) When portfolios are large, a compromise between accuracy and speed of computation needs to be found. (iv) The interest-rate model has to be common across all asset classes. Given the constraint of simultaneous simulation of scenarios for a large number of processes, and the valuation of portfolios of thousands of (exotic and plain vanilla) transactions, we found that one factor models with separable volatility structures (as described in Chap. 2) worked well for our purposes. In our experience, the sophistication of models used under such constraints is less important than having a setup that allows a consistent framework that can be extended in a modular way as new products require.

Chapter 4

Valuation and Sensitivities

Conceptually there are two steps in computing credit exposure: simulation followed by pricing. First, one needs to simulate scenarios from the distribution of the underlying processes that drive the price of the product concerned. Secondly, the price of this product needs to be evaluated at each time in the simulation schedule for each of the simulated scenarios. In the previous chapters we have considered a general simulation framework and we have specified simple models used in practice. The aim of this chapter is about the second step, pricing. In simple cases pricing can be performed in closed form or semi-analytically. If a closed-form valuation, which maps scenarios to price, is not available, then the pricing step needs also to be carried out by simulation. This implies that for products with no closed-form valuation, the problem of computing price distributions entails performing simulations (for pricing) within simulations (of scenarios for the underlying processes), an approach that quickly becomes unfeasible for any reasonable simulation size. American Monte Carlo (AMC) is a simulation technique that has been applied to the problem of pricing financial products with features of early exercise. As we describe later, when used in this way, the AMC method yields not just (an estimate of) the price of the product but also the price distribution at each point in a grid of discretized time-points. This price distribution is exactly what can be exploited for estimating the quantiles that define the level of credit exposure, so that the AMC technique can be applied to problems not just of pricing but also of credit exposure computation. In this chapter we focus on computing exposure using AMC. We briefly describe the theoretical basis of AMC, show different algorithms used in practice, and indicate our choice of algorithm for the computation of credit exposure. Finally, we describe different techniques to compute price sensitivities, which will be key later in Chap. 14, when discussing pricing and hedging counterparty risk. G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0_4, © Springer-Verlag Berlin Heidelberg 2009

79

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4 Valuation and Sensitivities

4.1 American Monte Carlo: Mathematical Notation and Description When it comes to pricing financial products with very intricate payoff structures, or which depend on several underlying factors with complex dynamics, (Monte Carlo) simulation has become the tool of choice. As long as one has a means of sampling from the random distribution of the underlying drivers, there is virtually no limit to the payoff structures that can be priced. However, applying a simulation method to pricing of products with early-exercise features is not a straightforward task, the reason being that at any point in the life of such a product, the value depends on exercise decisions made at times in the future. In turn, the decision to exercise or not would depend on the perceived value of not exercising as compared to the intrinsic value upon exercise. In principle, this causes the pricing problem for an early-exercise product to mushroom into similar pricing problems on each simulated path, at each time-point considered. The number of simulations required quickly grows enough to thwart any attempt at pricing by straightforward simulation. The technique that has now become known as American Monte Carlo attempts to get around this problem by performing one set of simulations and then estimating (rather than pricing through new simulations) at each point in time the value of not exercising (once this value is known, the task of comparing it with the intrinsic value of exercising is relatively easy). One by-product of pricing by American Monte Carlo1 is that apart from the desired price, the method produces also samples from the price distribution at times between the pricing time and the expiry of the product. This feature makes it well-suited (with modifications) to estimating the counterparty or market risk posed by a particular product, such exposure being based merely on the quantiles of the product price distribution at different times.

4.1.1 Mathematical Formulation We presented in Chap. 2 a definition of credit exposure for a generic product with early-exercise features which we have denoted by P . At the outset, the holder of P is entitled to a cashflow X ≡ (Xt ). We denote by TX the maturity of X, so that Xt = 0 if t > TX . Apart from the cashflows X, P also gives the holder the option to replace, at specific points in time, their entitlement to X with an alternative product, which we call the post-exercise portfolio, denoted by Q, and which has maturity TY so that Q has value zero at times after TY . We write T = {τ1 , τ2 , . . . , τnE } ∪ {∞}

(4.1)

for the set of nE times at which the option may be exercised to give up X in exchange for Q. If exercise happens at τE ∈ T , then the value provided by P until the exercise 1 . . . for

products both with early-exercise and without it. . . .

4.1 American Monte Carlo: Mathematical Notation and Description

81

time is embodied in (2.3), Πtno

= Nt E

τE ∧TX

t

Xu du Ft , Nu

(4.2)

where the superscript on the left indicates that this is the value of the no-exercise flows X. The optimality criterion by which the holder chooses the optimal time, τE∗ , at which to exercise the option, will be defined shortly below. There are several possibilities for the form that the alternative holdings represented by Q may take. (i) Physical Settlement. In this case, the cashflows (Xt ) provided by the noexercise portfolio Π no are replaced by cashflows (Yt )0≤t≤TY changing also the maturity of the transaction from T = TX to T = TY . The price distribution of the trade will take values for all t in [0, TX ∨ TY ]. An example of this type of product is a physically settled swaption. (ii) Cash Settlement is different from physical settlement in that the net present value at time of exercise of all the flows (Yt ) is exchanged at exercise time τE , and the transaction then terminates. The price distribution will take values for all t in [0, τnE ]. An example is a cash settled swaption. (iii) Intrinsic Exercise. Here, the option holder receives the time-τE flow, YτE , and no further cashflows. The price distribution will take values for at most all t in [0, τnE ]. An example is a Bermudan option, or a cancellable swap. (iv) No exercise at all. For this case, we simply set T = {∞} =: T ∞ ,

(4.3)

expressing the fact that exercise will never happen, and therefore that the holder of the no-exercise portfolio Π no will receive flows (Xt ) until expiry time TX . As highlighted in Chap. 2 we can then write the price distribution of product P as,

Vt =

VtP , Q Vt ,

t < τE∗ t ≥ τE∗ .

The first element VtP is given by, Q τE ∧TX X VτE u VtP = Nt sup E du Ft + E Nu NτE t τE ∈Tt

(4.4)

Ft

(t < τE∗ ),

(4.5)

where Tt = {τ ∈ T | τ ≥ t}. Q Vt ,

(4.6)

The second element, can have different formulations depending on the type of callability. In practice, the flows X and Y ensuing from P and Q are not continuous but occur at discrete time points. For simplicity, we will nevertheless consider X

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4 Valuation and Sensitivities

and Y to be defined for any t ≥ 0 and set Xt = 0 (resp. Yt = 0) if X (resp. Y ) provides no cashflow at t . Note that physical and cash settlement provide the same value to the option holder. Exercising intrinsically into the cashflow (Yt ), however, provides less value, since the option holder is then not entitled to flows Yt for t > τE . Q Symbolically, if Vt is the time-t value to the option holder who has exercised at τE < t, we will have, in terms of notation introduced in Chap. 2,

T Πtex = Nt E[ t Y NYuu du | Ft ], for non-intrinsic exercise Q (4.7) Vt = for intrinsic exercise. πtex = Yt 1t=τE , The value of non-intrinsic exercise is unaffected by whether settlement is physical or in cash form, save for the fact that the holder receives the flows (Yu ) in the former case, and the one-off payment Πtex at τE in the latter. At each time t , the holder of the product P attains the value Vt by choosing his exercise time τE∗ ∈ T so as to maximise the net present value of his cashflows. The problem we want to consider is how to evaluate Vt . At each time τi ∈ T where the option holder may potentially exercise his option, the decision whether to exercise or to continue will be based on the information observed in the economy. Formally, then, we suppose that at any time t, the information set for the model consists of a σ -algebra Ft , part of a filtration (Ft )0≤t≤T generated by J underlying stochastic processes, say Ξ ≡ (ξ1 , . . . , ξJ ),

(4.8)

that drive the economy. For our model, these stochastic drivers will be, for instance, the collection of Brownian Motions, which the martingales (Mt,T ) depend on. We also suppose, not unreasonably, that the holder of the option knows at t whether exercise has taken place yet, that is, Ft ⊃ {τE ≤ t}. The price processes VtP and Q Vt are assumed to be adapted to (Ft ). Further, we suppose that there is a vector process Θ ≡ (θ1 , . . . , θnobs ),

(4.9)

which generates a filtration σ (Θ) with σ (Θt ) ⊆ Ft , for each t ∈ [0, T ]. Thus, for instance, θj could be the value process of a market instrument whose value depends on the same underlying factors, Ξ , in the economy as does the price process V P . The instruments whose price processes are the θj are referred to as observables. Their importance will become clearer when we will discuss the optimal decision algorithm within the American Monte Carlo framework. The key is that at any particular time t , we will assume that the decision whether or not to exercise will be driven by time-t values of the observables.2 2 Taken

at face value, this assumption would seem to exclude path-dependent products; in fact it does not. It is perfectly legitimate to take as observable the price process of a path-dependent instrument, and then allow its time-t price to drive time-t exercise decisions.

4.1 American Monte Carlo: Mathematical Notation and Description

83

Pricing of P , and estimation of the optimal exercise rule τE∗ , is via Monte Carlo simulation; to this end we assume we have simulated realisations {ξˆj,k }, (ν)

1 ≤ j ≤ J, 0 ≤ k ≤ K, 1 ≤ ν ≤ n

(4.10)

of the driving factors Ξ in the economy; here, j indexes the j ’th driving factor, k indexes the k’th time-point tk in a partition, say P := {0 = t0 , t1 , . . . , tK = T }

(4.11)

of the time-interval [0, T ], and ν indexes the ν’th simulation out of a total of n. The ˆ serves to indicate a sampled value. Thus, for each tk , we have samples of size n drawn from the distributions of each of the driving factors ξj . Similarly, we assume we also have simulated realisations {θˆm,k }, (ν)

1 ≤ m ≤ nobs , 0 ≤ k ≤ K, 1 ≤ ν ≤ n,

(4.12)

from the laws of the nobs observables θ1 , . . . , θnobs .

4.1.2 Practical Examples Let’s consider some examples that illustrate how these equations should be interpreted in practice.

4.1.2.1 Non Exercisable Trades For a transaction that does not allow early exercise, it is necessary to set, T ≡ T ∞ = {∞}.

(4.13) Q

In principle, we could allow T to be unrestricted and set Vt = −∞ for each t ≥ 0.3 Algorithmically, the first approach is neater, since then one does not need to even consider exercising at any t.

4.1.2.2 Simple Examples with Exercise We now turn to look at how simple trades, a Bermudan put option, a cancellable swap and a European swaption, are represented in this framework. Bermudan Put Option: Consider a 5 year contract on a stock S, which, at given dates (e.g. every year), gives the right to the holder of the option, to sell the stock at a predefined strike K. 3 This

Q

is the case of long callability. In the case of short callability we write Vt = +∞.

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4 Valuation and Sensitivities

The set of exercise dates T is defined as, T = {1, 2, 3, 4, 5} ∪ {∞}.

(4.14)

The no-exercise portfolio is defined as Xt = 0

∀t,

(4.15)

and the exercise portfolio as, K − S, Yt = 0,

if t ∈ {1, 2, . . . , 5} otherwise.

(4.16)

Note that we have written K − S and not (K − S)+ as the optimal exercise strategy is performed by the decision algorithm. Cancellable Swap: Consider a contract in which we start off with a 10-year swap with unit notional where we pay yearly coupons of 5% per annum and receive the 12-month libor fixed a year in advance, and in which we have the option to cancel the swap after 5 years, each year, for a fixed fee of 1%. In this case, the non-exercise payoffs (Xt ) would be defined as: Xt =

Lt−1 [t − 1, t] − 0.05, if t ∈ {1, 2, . . . , 10} 0, otherwise.

(4.17)

The set of exercise dates T is defined as: T = {5, 6, 7, 8, 9, 10} ∪ {∞}. As for the exercise portfolio, the cash flows (Yt ) are defined as −0.01, if t ∈ {5, 6, . . . , 10} Yt = 0, otherwise,

(4.18)

(4.19)

to reflect the penalty due at time of exercise. Given that there is a fixed one-off penalty upon exercise, the transaction has an intrinsic optionality feature, with Q

Vt = πtex = Yt 1t=τE ,

t ∈T .

(4.20)

Physically Settled European Swaption: Consider a contract where we have the right but not the obligation to enter in 5 years’ time, into a 5-year swap of unit notional, in which we would pay yearly coupons of 5% per annum and receive the 12-month libor rate. If, in 5 years’ time, we decide to exercise, then we are subject to market and counterparty risk for the remaining 5 years of existence of the swap. Otherwise, the trade terminates with no further exchange of cashflows.

4.1 American Monte Carlo: Mathematical Notation and Description

85

In this example, the set of exercise dates is: T = {5} ∪ {∞}.

(4.21)

Xt = 0,

(4.22)

The non-exercise payoffs X are t ≥ 0.

For the exercise payoffs, we have Lt−1 [t − 1, t] − 0.05, if t ∈ {6, 7, . . . , 10} Yt = 0, otherwise.

(4.23)

Choosing to exercise entails entering into the swap, so that we have non-intrinsic optionality with 10 Yt Q i Ft , t ≥ 0. Vt = Nt E (4.24) N t ≥t ti i

In particular, we have Q

V5 = A5 (S5 − 0.05),

(4.25)

where At is the annuity at t of a 5-year swap with yearly coupons, and St is the par rate of such a swap. Hence the optimisation program (4.5) reduces to V5 = max (0, A5 (S5 − 0.05)) , and

A5 (S5 − 0.05)+ Vt = Nt E N5

Ft ,

t ∈ [0, 5].

(4.26)

(4.27)

Many more types of transactions will be considered in detail at a later stage of this book.

4.1.3 Backward Induction Algorithm There are several approaches that may be employed to compute the optimal exercise decision rule. In general a recursive procedure is used. This involves estimating at each time step tk the expected value of not exercising, conditional (on not having exercised prior to time tk and) on the time-tk value of the observables θj . The base case for the induction is the point in time where the prices of both products P and Q are trivial. This happens at time T ≡ TX ∨ TY , after which X and

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4 Valuation and Sensitivities

Y are both identically zero by definition. At this time, we have (ν) Q (Base case) VˆT = max (VˆTP )(ν) , (VT )(ν) 1T ∈T + (VˆTP )(ν) 1T ∈T / (ν) (ν) 1T ∈T ≡ max Xˆ T Ξˆ T , YˆT Ξˆ T (ν) + Xˆ T Ξˆ T 1T ∈T (4.28) / , 1 ≤ ν ≤ n, where the ˆ indicates sampled/estimated values and where we have made explicit that the payoffs XT and YT depend on the samples of the underlying driving factor Ξˆ T . For valuation times tk < T , we proceed inductively. Suppose that exercise has not happened prior to tk , and write Ftk for the expected value that would be gained by an agent who does not exercise at time tk , but who follows the optimal strategy at times after tk . By our assumptions, the expected value Ftk , (the continuation value), is a function of the time-tk observables Θtk : Ftk ≡ Ftk (Θtk ).

(4.29)

In practice, what we have is a finite sample of size n from the distribution of the time-tk observables Θtk , and from this we can hope to get a sample of size n from the law of the conditional-expected non-exercise value, Ftk (Θtk ): Fˆtk Fˆt ≡ k (Θˆ tk ) N tk N tk tk+1 Xu du Θˆ tk := E Nu tk τE ∧T N Ntk+1 Q 1 tk+1 + sup Xu du + VτE 1τE ≤T Θˆ tk E . Ntk+1 tk+1 Nu NτE τE ∈Tt k+1

(4.30) Again, the ˆ indicates sampled or estimated values.4 The expectation in the first line in the above expression is merely the value accumulated from hesitating (at time tk ) to exercise for one more time step (until tk+1 ). The remaining terms constitute the value to be gained from following the optimal exercise rule from time tk+1 onwards. To see this, note that the term in {} becomes

τE ∧T N Ntk+1 Q 1 tk+1 ˆ ˆ sup E Xu du + V 1τ ≤T Θt E Θt Ntk+1 Nu NτE τE E k+1 k tk+1 τE ∈Tt k+1

4 For

clarity, we have suppressed the explicit ν superscript indexing simulations, but (4.30) consists of n equations. In particular, the value Fˆtk is of course not the exact solution to the decision problem, as the supremum and expectation are obtained numerically from sampled values.

4.1 American Monte Carlo: Mathematical Notation and Description

=E

1 Ntk+1

E sup

τE ∈Ttk+1

τE ∧T tk+1

Nt Ntk+1 Xu du + k+1 VτQE 1τE ≤T Nu NE

87

ˆ ˆ Θtk . Θtk+1

(4.31) Notice that we have used here the so-called ‘tower-law’ of conditional expectations. Taking the Θtk -conditional expectation outside the sup operator is legitimate, because (and only because) Θtk is irrelevant to the maximisation conditional on Θtk+1 -information. Finally, invoking the definition (4.5), we translate (4.31) to E

1 Ntk+1

ˆ Vtk+1 (Θˆ tk+1 ) Θˆ tk .

(4.32)

Putting everything together, we can re-write (4.30) as ˆ tk+1 Vtk+1 Fˆtk Xu Fˆtk ˆ ˆ ˆ ˆ ≡ (Θtk ) = E du Θtk + E (Θtk+1 ) Θtk . N tk N tk Nu Ntk+1 tk

(4.33)

Recall that Fˆtk represents the ν’th estimate ‘drawn’ from the time-tk value distribution of P, conditional on exercise not having happened prior to tk and conditional also on it not happening at tk .5 In order to obtain the time-tk value Vˆtk it remains to decide whether exercising at tk results in value larger than Fˆtk , and to set (ν)

Q Vˆtk ≡ Vˆtk (Θˆ tk ) = max Vtk , Fˆtk ,

(4.34)

where Vtk (resp. Fˆtk ) denotes the estimated value of exercising (resp. not exercising) at time tk . This completes the inductive step to be made at time tk . In the case of short optionality, where the holder of the option is the payer and not the receiver of the cashflows X and Y pertaining to P and Q, (4.34) becomes Q

Q Vˆtk ≡ Vˆtk (Θˆ tk ) = min Vtk , Fˆtk .

(4.35)

This inductive step is then repeated until all time points Tk in the partition P are exhausted. One fine point to mention is the following. The value process V P is defined such that VtP is the value of P at time t, conditional on exercise not having happened prior to t. Now suppose that while performing the backward recursion, it is deemed optimal for the ν’th simulated path, to exercise at time tk . It may then happen that at some later step in the recursion (and so at an earlier time tj ), it is also deemed optimal to exercise at tj . By taking the exercise time τE to be the earliest time at which the recursion deems it optimal to exercise, which we assume to be the

5 In

(4.33) we have again suppressed the superscript indices (ν) for readability.

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4 Valuation and Sensitivities

case in the sequel, one ends up with a unique exercise strategy: Q τE ∧TX X VτE u ∗ du Ft + E τˆE = inf t ∈ T |Vˆt = sup E N N u τE t τE ∈Tt

Ft .

(4.36) In principle, computing the second conditional expectation appearing in (4.33) requires one to perform further simulations—effectively to repeat the pricing problem at each time point, and for each of the n sample values of the observables. As already mentioned, this quickly becomes an unfeasible task, and an alternative approach needs to be employed. Different approaches to Monte Carlo pricing hinge on different ways of estimating the second conditional expectation term in (4.33). There are many computational algorithms described in the literature. We move now to describing some of these approaches in details. We will then focus on our specific implementation for credit exposure computation.

4.2 AMC Estimation Algorithms As we concluded in the previous section, it is necessary to find clever ways to estimate the conditional-expectation function Vtk+1 Θtk → E (4.37) Θ t Ntk+1 k appearing in (4.33), where Θ are the so called observables. This is generally done using heuristics which have shown to work well in practice. We now describe some approaches described in the literature that have been used to accomplish this, namely the Tilley [103] and the Longstaff-Schwartz [76] algorithm. The approach we employ to compute counterparty credit exposure is a modification of the regression estimation of Longstaff and Schwartz in combination with Tilley’s bundling algorithm. In all these heuristics the idea is to approximate with simple functions the continuation value and to base the decision algorithm on these approximations. This can be achieved by interpolation methods (as in the Longstaff-Schwarz algorithm) or by splitting (bundling) the domain (as in Tilley algorithm). We will see that these two approaches can be used together to obtain an efficient heuristics which compute exposure for most of the products. In the next sections we will first analyse Tilley’s algorithm, as historically this was one of the first attempts to price American options.

4.2.1 Tilley’s Algorithm Tilley’s algorithm was initially designed for the particular example of American options written on a single underlying stock that pays no dividends. The algorithms

4.2 AMC Estimation Algorithms

89

take into account only one observable Θ ≡ θ , and starts by first sorting the values of the observables {θ (ν) } and then classifying the samples into a chosen number of equally sized bundles. The conditional expectation (4.37) is then estimated as the sample mean for each given bundle, that is, for each sample ν in some chosen bundle B, Tilley sets Fˆt(ν) k (ν)

N tk

=

(ν) 1 Vˆtk+1 , (ν) B ν∈B Ntk+1

(4.38)

to be the estimated value of not exercising at time tk , with B being the number of elements in the bundle B. Thus, Tilley’s algorithm uses the information from θ to partition the estimation set into different bundles. As we will see in the next section, an alternative approach is to regress the continuation value against the observables. Thus, Tilley’s method is akin to fitting a piecewise-linear function to a non-linear data set. We will see that the Longstaff-Schwartz approach fits a non-linear function to the entire data set. Notice also that there is more than one way of choosing the first path ν ∗ for which exercise is optimal; in his original paper, Tilley proposes one particular rule to choose a unique ν ∗ .

4.2.2 Longstaff-Schwartz Regression Longstaff and Schwartz [76] put forward an algorithm that models the continuation (ν) value Fˆtk at each time tk , as a regression on the time-tk value Θtk of the chosen observable of the discounted values computed at time tk+1 . The idea is that, at time tk , the sample conditional expected value of not exercising (that is the continuation value Fˆtk (Θˆ tk ) in (4.31)) can be expressed as a linear combination of basis functions of the time-tk observables. For this reason this algorithm is also called regression algorithm. These basis functions are generally polynomials. Thus, Ftk (Θtk ) := aj Lj (Θtk ). (4.39) j

Here it is supposed that the conditional expectation function is in a space that is spanned by the basis functions {Lj }, j = 1, 2, . . . . Notice that in general, Θ is an nobs -vector so that each Lj maps Rnobs to R. The fit coefficients aj are estimated through regression, as described below. For the Longstaff-Schwartz algorithm, the choice that needs to be made is what number of basis functions, μ say, one should use. Then, at the k’th time step, the regression coefficients (aj ) are estimated by regressing the n sample discounted values (ν)

Ntk

(ν) Ntk+1

(ν) Vˆtk+1 ,

ν = 1, 2, . . . , n,

(4.40)

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4 Valuation and Sensitivities

on the first μ basis functions evaluated on the sampled observables, L1 (Θˆ tk ), . . . , Lμ (Θˆ tk ), (ν)

(ν)

ν = 1, 2, . . . , n.

(4.41)

The regression carried out by the authors used only those sample values ν which are in the money. In particular, for the American put example considered by Longstaff (ν) and Schwartz, the time-tk regression used only those sample values Stk < K of the stock price for which exercise would yield non-zero payoff.

4.2.3 Biases of Estimates The Tilley estimation algorithm just described yields, of course, only an approximate solution to problem of pricing a product P with Bermudan exercise6 allowed at any of the time points τk ∈ T . The end result obtained is influenced by both upward and downward biases that arise from the necessity to use a finite number of simulations (see also Hyer [65]). (i) Granularity bias is a bias that arises because bundles are ‘too big’. The cause of this bias is that the conditional-expected value of not exercising is identical for all paths in the same bundle. Because the same non-exercise value is used for each path in a given bundle, the estimated exercise rule for paths in that bundle will be sub-optimal, causing a downward bias in estimated price. Granularity bias would be eliminated if each path were itself a bundle. (ii) Small-sample bias is that arising because bundles are ‘too small’. The fact that each bundle contains only a small finite number of paths causes the algorithm to work out a sub-optimal exercise rule, again causing a downward bias in price. (iii) Look-back bias is that arising because the same set of N paths is used in estimating the optimal decision rule as is used to compute the value yielded by that rule. Tilley showed by example that this type of bias results in an upward bias in the estimated price; he did this by comparing the value estimated by his algorithm to the value one obtains when using the same set of paths, but employing the exact known optimal exercise strategy. The presence of an upward bias may at first seem contradictory, as by definition no estimated exercise strategy can dominate the optimal one. Consider, however, a situation in which each bundle consists of a single path, and pick some path (equivalently, bundle) η. Then, the backward induction algorithm would compute (η) (η) VˆT = YˆT 6 If the product P allows American exercise, the need to discretize forces us to model a corresponding product with Bermudan exercise features; we will not discuss inaccuracies arising due to this. What we refer to in this paragraph, rather, are the discrepancies between the true and computed values for the problem of pricing a product P with Bermudan exercise features.

4.2 AMC Estimation Algorithms

91

N tk (η) (η) Fˆtk = E[Vˆtk+1 |Θtk ], Ntk+1

k = K, K − 1, . . . , 0

(η) (η) Q Vˆtk = max{Vtk , Fˆtk }.

(4.42)

Thus, the exercise rule for the path η would depend on the simulated values of that path alone. The time-zero price would then be the expected value over all (η) paths of the terminal payoff on each path, VˆT , discounted back and replaced by the exercise value whenever the latter is larger. Mathematically, the look-back bias arises because of the convexity of the max operator and Jensen’s inequality—the price is estimated as the expectation of maxima of per-path future values, rather than (correctly) as the maximum of expected values over all possible choices of exercise strategy.

4.2.4 An AMC Algorithm to Compute Credit Exposure The drawback of Tilley’s approach is in its use of a single observable to carry out bundling. In practice, transactions will depend on several underlying variables, each of which should be considered in partitioning the sampled paths. On the other hand, the Longstaff and Schwartz method computes a regression to estimate the continuation value Fˆtk , but does not employ bundling. A possible extension is a combination of modified algorithms based on these two basic ideas (see also Hyer [66]). Furthermore, motivated by Sect. 4.2.3, we introduce a bias correction device to improve the quality of estimates Fˆtk .

4.2.4.1 Recursive Bundling Tilley defines bundles by classifying the number of simulation paths according to the level of a single observable. We generalise this notion by bundling recursively on all the nobs observables in the process Θ. To see how this can be accomplished, consider a particular time point tk . Choosing integers m1 , m2 , . . . , mnobs ,

(4.43)

we start by classifying paths into m1 bundles based on the level of the observable θ1,tk . Each of these is then subdivided into m2 bundles using the level of the observable θ2,tk . The procedure is then repeated for all observables, resulting in a total of m1 m2 . . . mnobs bundles. The reason for performing bundling is to classify the simulation paths into subsets such that for any two paths in a particular bundle, all observables have similar values. For observables with continuous distributions, this is accomplished well enough by the above recipe, which allocates paths equally across bundles. However,

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4 Valuation and Sensitivities

for a discontinuous observable such as, for instance, that corresponding to the default indicator of a reference credit, equal allocation can result in two paths with the same values for an observable being in different bundles. To prevent this, we perform an additional clustering check, and shift paths from one bundle to another, if by doing so the distance between the particular path and its closest neighbour in the bundle is reduced.

4.2.4.2 Regression We adapt the Longstaff Schwartz method by performing regression on each bundle. Thus, for a typical bundle, B, say, we find parameters a and b that fit the model Vˆtk = E

N tk ˆ Vt Ntk+1 k+1

J L j Ft = a θ + bl θl+1,tk + εtk , j 1,tk k j =0

(4.44)

l=0

where εtk ∼ N (0, σt2k ), and εtj is independent of any εtk for j = k. What we are doing here, then, is to estimate the conditional expectation as a polynomial of order J in the first observable and a linear function in the remaining L + 1 observables. Note that cashflows paid at time tk are not included in the regression step, as these are known quantities.

4.2.4.3 Bias Correction We pointed out, in Sect. 4.2.3, that several biases arise in the Tilley estimation algorithm. Fries [46] describes how the look-back bias (which he refers to as foresight bias) can be removed analytically. Consider our regression model (4.44). For each tk , this models the conditional expectation function Vtk+1 Θtk → E Θt (4.45) Ntk+1 k as Θtk → f (Θtk ).

(4.46)

Suppose that ε ∼ N (0, σ 2 ) is the Monte Carlo error in the estimator f , so that Vtk+1 Θ ) = E + ε. (4.47) f (Θtk t Ntk+1 k If K is the intrinsic value of exercising at time tk , then look-back bias arises from Jensen’s inequality, because the value E[max(K, f (Θtk ))]

(4.48)

4.3 Post-Processing of the Price Distribution

93

of the exercise strategy computed by the algorithm differs from the theoretical value Vtk+1 Θt . (4.49) max K, E Ntk+1 k Analytical removal of the bias is possible once we note that for any real a, b, and for ε ∼ N(0, σ 2 ), we have E max(a, b + ε) = σ φ(η) + ησ Φ(η) + a, (4.50) with η ≡ (b − a)/σ . Similarly, for short optionality, the bias correction can be removed using: E min(a, b + ε) = −E max(−a, −b − ε) . (4.51)

4.3 Post-Processing of the Price Distribution The inductive procedure outlined in Sect. 4.1.3 provides, at each valuation / decision time tk , an (estimate) of whether exercise at tk is optimal if it has not happened prior to tk . In order to obtain the correct estimate for the value of the problem, then, one needs to locate the earliest time τE∗ at which exercise has been estimated to be optimal to the alternative of continuing. Symbolically, for each path ν, we set (ν) (ν)∗ Q Q (4.52) τE = inf tk ∈ T Vˆtk = max Fˆtk , Vtk = Vtk . Following this, we then have, for each tk ≥ τE∗ , ⎧ Q ∗ for non-intrinsic cash settlement ⎪ ⎨Vtk 1tk =τE , Q Vˆtk = Vt , for non-intrinsic physical settlement ⎪ ⎩ k πtk = Ytk 1tk =τE∗ , for intrinsic exercise,

(4.53)

where we have, for ease of notation, suppressed the superscript (ν) indexing paths.

4.4 Practical Examples Revisited We revisit here our two illustrative examples from Sect. 4.1.2 in order to show how our algorithm would work in a concrete setting. Cancellable Swap: In this example, we considered a cancellable swap where the holder has the option, at each τk ∈ T , to exit the transaction for a fixed penalty. Clearly, then, at each τk , it would be rational to exercise the option if the estimated value of continuing to receive the swap payments is less than the reward Yτk = −0.01 suffered from cancellation.

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The observables Θ, on which the estimation of the conditional expectation is based, should be chosen while keeping in mind the underlying variable in the trade. In this case, quantities such as the Libor rate or the fair swap rate are valid observables. Using these, the backward induction would, at each time τk estimate the value of the cancellable swap assuming exercise has not previously taken place, and would decide to cancel the swap if its value is lower than the exercise penalty. Physically Settled European Swaption: Recall that for the swaption example, the only allowable exercise time was tk = 5. If we assume that the value of the swap annuity, At , and the fair swap rate, St , are known at each t, then the decision needs to be made only at tk = 5, whether entering into the swap (with value A5 (S5 − 0.05)), has value that exceeds the zero-value of the strategy of allowing the swaption to expire unexercised. At times t prior to tk = 5, the swaption has continuation value Fˆt which will be estimated using regression and the chosen observables. Again in this case, since the trade depends on the fair value swap rate S, it makes sense to choose this and / or related quantities as the observables Θ in the estimation.

4.5 Computing Price Sensitivities No valuation framework would be complete without the capability of computing the sensitivities of a trade to its underlying risk drivers. The usual definition of a price sensitivity is the partial derivative of the price with respect to a given risk driver (keeping all remaining risk drivers constant). Let (ξ (i) ) denote the stochastic processes driving the pricing of a product and for which we want to evaluate the sensitivities. Examples of risk drivers are the stock price, the FX rate, the swap rate, the zero rate, or the volatility level. If V0 = V (Ξ0 ) = V (ξ (i) (0), . . . , ξ (n) (0)) is the time-zero price of the transaction as an explicit function of the underlying risk drivers, then it is usual to define delta of the trade with respect to ξ (i) as ∂V , (4.54) Δ(i) := (i) ∂ξ Ξ =Ξ0 the partial derivative evaluated at the time-zero value of the risk drivers. Similarly, the gamma of the trade with respect to ξ (i) and the cross-gamma of the trade with respect to ξ (i) and ξ (j ) are ∂ 2 V (i) (4.55) Γ := ∂(ξ (i) )2 Ξ =Ξ0 and Γ

(i,j )

∂ 2V := (j ) (j ) . ∂ξ ∂ξ Ξ =Ξ0

(4.56)

4.5 Computing Price Sensitivities

95

4.5.1 The Classical Approach In the banking industry, the standard way of computing price sensitivities is via a finite-difference approximation,7 that is, modifying the set of market data to produce a small change in the value of the risk driver of interest and then re-valuing the trade.8 There are two obvious problems which can arise from this methodology. (i) Computational Speed. Computing finite differences requires a full revaluation of the price distribution for each change in the market data. Thus, the first and second derivative of a product with respect to only one risk driver require already tripling the computation effort. (ii) Flexibility. It may not be possible to perturb a chosen risk driver while keeping all other drivers constant. An example is the swap rate which is the combination of several stochastic quantities. This constrains the set of drivers for which it is possible to specify sensitivities.

4.5.2 Price Sensitivities through Regression Within our framework, we can estimate price sensitivities at almost no extra computational cost. The idea is to regress incremental change in values of the price distribution against corresponding incremental values of the risk driver of interest. In detail, choose ε > 0 to be small and write ΔV = Vε − V0 ≡ V (Ξ (ε)) − V (Ξ (0)).

(4.57)

Assuming that a sample of the price distribution at time ε is available, (which we can ensure at valuation stage), and choosing some ξ (i) to be the risk-driver of interest, we write ΔV as a polynomial in Δξ (i) = ξ (i) (ε) − ξ (i) (0), ΔV =

m

k ak Δξ (i) .

(4.58)

k=0

Regression gives us estimates of the weights ak , which, in turn, enable us to compute the k partial derivatives dkV = k!ak . d(ξ (i) )k

(4.59)

It is important to note that this is the total derivative of the transaction value with respect to the chosen ξ (i) , and that ξ (i) may itself be correlated to other risk factors 7 There are several ways to compute sensitivities. For a survey see for example the book by Glassermann [50]. 8 Practitioners

often refer to this technique as bumping.

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that affect V . Obtaining the true partial derivative of V with respect to a chosen factor, eliminating the effect of correlation between the risk factors themselves, is the subject of the next section. By way of illustration, Table 4.1 compares the prices of European options and their sensitivities—obtained through AMC with 10,000 paths—to their analytical Black-Scholes values. The errors are typically less than the difference in price resulting from a change of one volatility point. Table 4.1 Performance of AMC for price and Greeks on one-year European options. S = 100, r = 2.95%, σ = 20% Type

BSPrice

AMCPrice

BSDelta

AMCDelta

BSGamma

AMCGamma

Call @ 105

7.106

7.100

0.501

0.494

0.0199

0.0198

Call @ 100

9.388

9.386

0.597

0.588

0.0193

0.0193

Call @ 95

12.151

12.153

0.693

0.682

0.0176

0.0176

Put @ 95

4.389

4.366

−0.307

−0.301

0.0176

0.0167

Put @ 100

6.481

6.454

−0.402

−0.394

0.0193

0.0184

Put @ 105

9.054

9.024

−0.499

−0.489

0.0199

0.0189

4.5.3 Removing Correlation As we saw above, sensitivities to a risk driver ξ (i) computed through regression implicitly contain sensitivity also to other risk drivers that are correlated to ξ (i) . While this is useful in the case where we compute only one sensitivity, since it can give more information on the risk of the trade (and therefore better hedges), it is an undesirable effect if we want to use the sensitivities as a tool to explain daily changes of profit and loss (P&L) of a business. It also becomes undesirable as soon as we are interested in sensitivities to more than one risk driver. With some assumptions it is possible, however, to remove this correlation effect and to produce decorrelated sensitivities, that is, sensitivities with respect to a chosen risk driver while keeping all remaining risk drivers constant. To this end, suppose we have m simulated values of each of the n risk drivers ξ (i) , i = 1, . . . , n, ˆ be the m × n matrix defined by9 and let X (i) Xˆ j,i = Δξj ,

i = 1, . . . , n, j = 1, . . . , m,

(4.60)

where the ˆ indicates a sampled (simulated) value and the superscript indicates the ˆ is a sample of size m from the distribusample index. By this, the i’th column of X (i) tion of Δξ , the incremental change in the i’th risk driver. In what follows below, 9 See

Sect. 2.3.1 to clarify notation.

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97

we make the reasonably accurate assumption that all incremental moves X in the risk drivers (which happen over a very short time period ε) are normally distributed with mean zero.10 Now consider the estimation of a linear model for Y ≡ ΔV in terms of the risk drivers Ξ , ˆ = Xα ˆ + εˆ , Y

(ˆε ∼ N (0, Σ)),

(4.61)

ˆ is a vector containing a sample of size m from the distribution of ΔV . The where Y least-squares estimate of α in this model is given by ˆ T X) ˆ −1 X ˆ T Y. ˆ α = (X

(4.62)

Compare this to the corresponding estimate we would get for α if a univariate regression were to be performed on just one of the risk drivers ξ (i) , namely ˆ TX ˆ −1 ˆ T ˆ α˜ j = (X j j ) Xj Y,

j = 1, . . . , n,

(4.63)

ˆ Putting together the estimates for α˜ = ˆ j is the j ’th column of X. where X (α˜ 1 , . . . , α˜ n ), we have ˆ T Y, ˆ α˜ = D−1 X

(4.64)

where D is a diagonal matrix whose (i, i)’th entry is a multiple of an unbiased estimator of the variance of ξ (i) , namely ˆi ·X ˆ i =: (m − 1)Var(ξ (i) ). Di,i = X

(4.65)

All this means that α˜ and α are related by ˆ T X)α. ˆ α˜ = D−1 (X

(4.66)

ˆ TX ˆ has entries In the above note that X ˆ T X] ˆi ·X ˆ j =: (m − 1)Cov(ξ ˆ i,j = X (i) , ξ (j ) ), [X

(4.67)

and is therefore simply a multiple of an unbiased estimate of the covariance matrix ˆ are equal. ˆ TX of Ξ . In particular, the diagonal elements of D and X ˜ as estimates derived from two different linear modThe significance of α and α, els for Y , is that the components of α represent partial derivatives to the ξ (i) while ˜ obtained by regression on ξ (i) alone, represent full derivatives with rethose of α, spect to ξ (i) . The expression (4.66) therefore allows us to obtain the true partial derivatives in terms of the correlated sensitivities computed from regression. To evaluate the accuracy of this methodology and the effect of correlation, consider the following simple example: suppose that we enter into a trade which pays 10 Recall that we have defined our framework so that all risk drivers are derived from simulated Brownian Motions.

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us 100 mEUR on October 20, 2015 and in which we pay 100 mGBP on October 20, 2010. Table 4.2 summarises the deltas11 (in terms of percentage point moves) obtained by bumping, regression and decorrelation, expressed in USD, along with the difference in computational time required for the three methods. We can see that correlated deltas are significantly different from the de-correlated ones. This example also suggests that the accuracy of the decorrelated deltas compared with the deltas obtained via numerical differentiation is acceptable, given the benefit of the higher computational speed achieved. In practice, before using this methodology on a large portfolio, it is necessary to carefully assess its accuracy for different types of products and over different time horizons. One possibility is to predict portfolio movements using sensitivities, and compare the results either with historical valuations, or with full revaluation of the portfolio.12 Table 4.2 Comparison between deltas computed by finite difference and by regression using correlated and decorrelated method. Simulation has been experimented on a desktop Intel Core 2.13 GHz machine Risk Driver

EUR Rates EURUSD FX Rate

Delta Finite Difference

Correlated

Decorrelated

−6,933,027

−1,989,992

−7,177,312

824,200

278,850

892,464

GBP Rates

2,128,061

−1,881,813

2,355,130

GBPUSD FX Rate

−973,972

−620,948

−1,046,550

Computation Time

4.27 s

1.06 s

1.09 s

4.6 Extensions American Monte Carlo valuation techniques have been analysed in various papers. We have already mentioned the Longstaff-Schwartz [76] and the Tilley [103] algorithm. Haugh & Kogan [59] and Rogers [91] introduced a dual method for pricing American options, providing an upper bound for the price of the option. Andersen & Broadie [2], Broadie & Glasserman [20], and Broadie & Cao [19] further develop this methodology obtaining both an upper and lower bound for the Bermudan option price.

11 For 12 In

interest rate deltas, we have considered a parallel-shift type of delta.

the financial industry this procedure is often called P&L explain.

Part II

Architecture and Implementation

Chapter 5

Computational Framework

In Part I we described a general framework that allows the specification of models for different asset classes, and we showed how the AMC valuation technique gives the possibility of computing price distributions, hence estimating counterparty exposure. Our goal is now to show how this mathematical framework can be naturally translated into a computational framework that will enable the computation of exposure in a systematic way for all types of products across the asset classes we provided models for. The basic ideas we highlight in this chapter will lead to the description of a basic software architecture, which can be used to address typical integration problems that large financial institutions face. The motivation for many of the challenges we consider in this and the following chapters, as well as many of the choices we take, will become clearer in Part IV, where the computation, controlling, and hedging of exposure, will be done at counterparty and not just at trade level.

5.1 AMC Implementation and Trade Representation Recall the basic principles of the AMC algorithm. Products are described by defining the cashflows Xt the holder a product P is entitled to, the cashflows Yt of an alternative product Q, which could replace P on a predefined set of exercise dates T (which could include T ∞ ), and the exercise strategy τE . In practice the cashflows X and Y will be defined on a set of dates S = {T1 , T2 , . . . , Tn }, called event dates, which in general contain T . With this information we can fully describe the product and then, using the AMC backward induction process, estimate prices along different scenario paths. To proceed a step further, at this point one remark is key. When we use the AMC algorithm we do not need to explicitly define products. A product is implicitly described via (i) the cashflows X, paid before exercise, and the cashflows Y of the exercise portfolio, paid on the event dates S , G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0_5, © Springer-Verlag Berlin Heidelberg 2009

101

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(ii) the set of exercise dates T , and (iii) the type of exercise. If we find a way to describe and process this information systematically, we have provided a computational framework that gives the capability of computing exposure for all types of products without knowing a priori the product type.1 Consider the cashflows X and Y defined on S . In general they are expressed as functions of financial quantities, such as Libor rates, FX values, or stock prices. These functions can be relatively complex, depending on the nature of the product. To be able to describe them we need to have basic building blocks, which we can combine to obtain the desired results. These building blocks, which we call statistics, are functions of the simulated paths and can be combined together using mathematical functions. We could have for example a statistic that extracts the Libor rate observed at a certain date t over a time interval (T1 , T2 ), or a statistic that computes the swap rate, or the average stock price over a period of time, and functions that e.g. compare values, provide their max or min, and multiply or divide them. With the description of cashflows X and Y , and with a mechanism to choose at predefined dates T if we want to exercise or continue, we can now compute counterparty exposure for generic products. From a computational point of view we simply need to compare values computed via the backward induction steps with predefined cashflows. We have turned the problem from defining individual products, to describing features of products.

5.1.1 Examples To clarify these points consider again the examples we have shown in the previous chapter. Non-exercisable products: To define products that cannot be exercised we need to describe only X. As a concrete example consider an up-and-out option on a stock struck at K and knocking out at H . This product tracks a stock price and its running maximum. If its value does not exceed the barrier H it will pay at maturity the difference between the stock price and the strike K, provided that its value is positive. Mathematically we can describe its payoff as XT = (ST − K)+ 1maxu∈[0,T ] Su ≤H .

(5.1)

To describe (S −K)+ we need a statistic that, given a Brownian path of the stock, extracts the value of the stock price S at maturity T , and two mathematical functions, one performing differences and one finding the max between values. To describe the second part of the payoff (related to the barrier), we need a statistic that computes 1 . . . provided that the underlying model is appropriate to describe the features of that class of products.

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103

the extremum value of the stock price between 0 and t , and an indicator function that returns zero or one depending on the extremum and the barrier value. If we implement these statistics we can compute the payoff of this product at maturity for each underlying path of the stock, and then rely on AMC to estimate intermediate prices from maturity to trade inception, and thus to compute counterparty credit exposure. Other common examples of non-exercisable products are vanilla swaps. In this case the value of the cashflows X defined at coupon dates Ti are α(Lt,Ti−1 ,Ti − c), with α being the day count fraction, c the fixed rate, and L the statistic that provides the Libor rate paid over a time interval. The computational mechanism is the same, using a statistic that provides the Libor rate, describes the payoff at each payment date and then uses AMC to evaluate intermediate prices. Cancellable Swaps: As we have seen, cancellable swaps are products with intrinsic optionality. We need to describe at each t in T not only X, but also Y . When we exercise, however, we only need to replace the value of X with Y . Physically Settled European Swaptions: In a physically-settled European swaption, the event dates Ti are the union of the swaption exercise date and swap coupon dates. X is equal to zero and Y is equal to the swap cashflows. The optionality is with physical settlement, meaning that upon exercise we replace the cashflows X with Y , entering into a swap.

5.1.2 Expression Trees We have seen that products can be described via their payoff cashflows X and Y and that their description can be done systematically using statistics as building blocks. We can now consider how to have a mechanism to produce computing code that generates the expressions needed to describe payoffs. The standard way to build and evaluate expressions is to use trees that are generated according to predefined rules, i.e. a predefined grammar. Consider as an example the expression tree of the up-and-out option as described in (5.1). At maturity, i.e. at the event date, we will have to evaluate an expression tree as shown in Fig. 5.1.

5.2 A Portfolio Aggregation Language We can define products via their expression tree and then, using this information, drive the AMC algorithm to compute price distributions. In Chap. 6 we describe some basic implementation principles. Our goal now is to add an abstraction layer in order to (i) easily compute exposure of trades that usually are described via termsheets, (ii) de-couple trade description from implementation of the analytics, and

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Fig. 5.1 An expression tree for an up-and-out option. We have used the following statistics (see Chap. 6 for more details): INDEX to extract stock information, EXTREMUM to obtain the extremum value of a path, INDICATORBELOW to compute the indicator function. In addition we have used mathematical functions to perform differences and multiplication between values and to compute the max between values. The hierarchy of operators is given by the grammar that generates the tree

(iii) bring trades from existing booking systems into a single unified booking representation. The technical solution for these requirements is the definition of a programming language to describe products, which acts as an interface between statistics and analytics. As the main goal is to allow a portfolio view, we have called this language Portfolio Aggregation Language (PAL). By defining an appropriate syntax and grammar, and by using a lexer and parser, we can then generate expression trees, which in turn will call the statistics defined in the analytics. There are many tools available in the market to automatically generate parsers from given grammars. Classical examples are Lex and Yacc [74], Flex and Bison [73], or ANTLR [84]. PAL is designed with two competing goals in mind. It has to be (i) Simple enough to describe different types of trades in a clear and concise way. In other words, the syntax has to allow trade description in a way that is close to business language. (ii) Flexible enough to accommodate various levels of trade complexity and allow translation from other different booking systems across the firm into one single language. To respond to these requirements we have designed PAL with the following main technical features. (i) It has the typical declarative statement of a procedural language. For example it is possible to define numerical or logical (boolean) variables, arithmetic operations between them (with the usual precedence rules), and loops.

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105

(ii) There are some predefined types such as vectors or date schedules. (iii) There are custom and built-in functions: examples are Exp, Max, Log. (iv) It has some object-oriented capabilities to define, for example objects of type ‘Instrument’ (this allows the possibility of having analytical pricing for selected products). (v) It has some typical financial construct. For example you can say that you will Pay or Receive a certain amount at certain dates. (vi) It defines a context that specifies how computation is performed. For example it is possible to define the quoting currency or the fact that a counterparty is collateralised. The following examples clarify these concepts.

5.2.1 PAL Examples The code snippet below shows how a simple interest-rate swap can be described in a succinct way. Table 5.1 Vanilla swap Schedule = From 2009/09/30 to 2019/06/30 every 3 months; Notional = 100 mm EUR; DcFr = DCF(now-3m, now, "ACT/ACT"); // Day Count Fraction Receive Notional * (ir:eur3m on Now - 3m)* DcFr on Schedule; Pay Notional * 3.5%* DcFr on Schedule;

Even if there are some elements typical of programming languages, the syntax is close enough to a typical termsheet description: we can define a schedule using dates, we can specify what parties A and B respectively pay and receive, and we can use typical business terminology. The same swap in arrears will be written as, Table 5.2 Floating leg of a vanilla swap in arrears Receive Notional * (ir:eur3m on Now - 0m)* DcFr on Schedule;

If we want to make our swap callable, we add the following line of code, which defines the cashflows Y and the dates when they need to be paid. Table 5.3 Swap callable at each payment date specified in the swap schedule Long Callable on Schedule into (Receive 0 EUR on Schedule);

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If we have the option to enter into a swap, i.e. if we want to have a trade with physical exercise, we can write the code below, where the non-intrinsic feature of the callability statement is explicit.

Table 5.4 Physically settled swaption Notional = 100mm USD; Schedule = From 2009/09/30 to 2019/06/30 every 6 months; Date = 2014/09/30; DcFr = DCF(now-6m, now, "ACT/365"); Swap = Receive Notional*(IR:USD6M on now-6m)*DcFr on Schedule; Swap = Pay Notional * 3%* DcFr on Schedule; // The settlement is physical Long Callable on Date into Swap with Physical Settlement NonIntrinsic;

Consider now the example of a non-callable product, e.g. the barrier for which we have also shown the expression tree in Fig. 5.1. The PAL code could be written as follows,

Table 5.5 FX up and out option Notional = 100mm EUR; Strike = 1.0; Barrier = 1.5; Receive Notional * max(fx:gbpeur - Strike, 0.0) * (maximum(fx:gbpeur, 2009/10/20, 2010/10/20) < Barrier ? 1.0 : 0.0) on 2010/10/20;

An example of a CDS on a counterparty characterised by a ‘Credit Curve’, is given in the table below. We use here two constructs, ‘creditloss’ and ‘creditevents’. The first corresponds to the loss suffered by the underlying instrument and it is used to describe the protection leg of the CDS. The second indicates simply the event of default and therefore does not take into account recovery rate. It is used to define the payment leg of the CDS. More details are given in Chap. 6, where we describe how these quantities are taken from underlying simulated processes, and in Chap. 10, where we focus on credit derivatives. We have mentioned that PAL allows also to use other typical programming language features, such as loops, matrix operations and some typical objectoriented construction. These features can significantly help the booking of complex

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107

Table 5.6 CDS start = 2004/07/20; end = 2009/09/20; Schedule = From (start+3m) to end every 3 months; Notional = 100 mm EUR; DcFr = DCF(now-3m, now, "ACT/ACT"); // Day Count Fraction Receive Notional * creditloss(cr:"CreditCurve", Now - 3m, now) on Schedule; Pay Notional * 0.0043 * (1.0-creditevents(cr:"CreditCurve", start, now))*DcFr on Schedule;

transactions, described usually in long termsheets. Below is the example of a trade using a ‘for loop’ to describe repetitive payments, and the definition of a new product, a swap, which can be used later within the program. Table 5.7 For Loop example Notional = 100mm USD; DcFr1 = DCF(now-6m, now, "ACT/ACT"); DcFr2 = DCF(now-1Y, now, "ACT/ACT"); ScheduleRec = From 2009/01/01 to 2020/01/01 every 1y; SchedulePay = From 2009/01/01 to 2020/01/01 every 6m; Maturity=2020/01/01; Receive Notional * 0.04 * DcFr2 on ScheduleRec; Pay Notional * (((USD 10y)-(USD 2y))>0 ? ((USD 6M)+0.007)*DcFr1 : 0) on SchedulePay; DD=2005/04/22; S=0; For (i=1;i0 ? ((USD 6M on DD)+0.007)*DcFr1 : 0); } Pay Notional * Max(0,0.3-S) on Maturity;

The code snippet above represents a complex interest-rate trade, where party A pays a fix rate every year, and party B pays Libor plus spread or zero every six months, depending on the difference between two points of the swap curve, the 10 years and the 2 years points. In addition, at maturity, a cumulative coupon is paid. This is computed within the ‘for loop’ depending again on the two points of the curve.

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For very simple products, whose valuation is model-independent (such as for example standard interest-rate swaps), it is possible to use functions, which we call ‘Instruments’ (see below) to define an analytical pricing function. This gives the possibility of combining AMC with other pricing techniques. We will see at the end of this chapter how to use ‘Instruments’ to define more complicated products. Table 5.8 Instrument example Instrument Swap(Notional, Currency, Fix, FromDate, ToDate, Freq) { DcFr = DCF(now-Freq, now, "ACT/ACT"); Float = Libor(Currency, now - Freq, now - Freq, now); Schedule = From (FromDate + Freq) to ToDate every Freq; Receive Notional * Fix * DcFr Currency on Schedule; Pay Notional*(Float on now - Freq)*DcFr Currency on Schedule; } Buy Swap(100 mm, CHF, 3%, 2009/06/30, 2020/06/30, 6 m);

5.3 The Concept of Scenarios We have mentioned on many occasions that the classical Monte Carlo framework used to compute counterparty exposure is via scenario generation and then pricing using analytical formulas or approximations. Sometimes these scenarios are generated in a centralised location and then sent to various engines that perform the pricing step. AMC can also be considered as a pricing approximation, even if sophisticated and thus enabling the valuation of complex transactions. In this sense we could think of scenario generation and AMC valuation as two separated sub-systems. However, because we want simulation and pricing treatment to be generic, it is not possible to know beforehand which financial quantities will need to be extracted from the simulation. Therefore it is essential to be able to retrieve any financial quantity efficiently from the basic stochastic drivers.

5.4 The Concept of Super-Product With the computational framework we have described and the definition of the PAL language, we can now make a step further, and define new types of products, whose payoffs, i.e. X and Y cashflows, depend on the price distribution of an already computed product. We call these products Super-Products to highlight the fact that these are products built with the results of the computation performed for another product.

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109

5.4.1 An Example of Super-Products: The C-CDS An example of a super-product is the contingent credit default swap (C-CDS). We will analyse this product in detail in Chap. 14, where we show how to compute credit valuation adjustments and how to hedge counterparty credit exposure. In this context it is sufficient to note that a C-CDS is an OTC derivative between two counterparties, A and B, say. Assume A has a derivative portfolio with a third counterparty C, and that it enters into a C-CDS with B. In case of default of C, under the C-CDS contract B will pay to A the positive value (as seen from A’s perspective), Vt+ , of the portfolio. In other words a C-CDS corresponds to the protection leg of a CDS paying at each point in time the value of an underlying transaction. The construction of the C-CDS is performed as follows: (i) first the price distribution Vt of the derivative portfolio is computed, (ii) then defaults of counterparty C are simulated, and, (iii) finally the cashflows of the C-CDS are created combining the default values with the price distribution. The advantage of a generic computational framework, where only cashflows are relevant and valuation is performed via AMC are clear. We can effortlessly create a new product whose payoff depends on another product. In Table 5.9 we show a PAL example of a C-CDS. We use several concepts described before: object-oriented features, the idea of ‘Instrument’ and the construct used to define CDSs. We can see in particular that the C-CDS corresponds, as mentioned above, to the protection leg of a CDS. Table 5.9 CCDS Instrument ccds(objInstrument) { receive max(0, objInstrument(currentdate)) * creditevents(objInstrument.cpty,previousdate,currentdate)) on objInstrument.schedule); }

Chapter 6

Implementation

The previous chapter introduced a computational framework within which complicated payoffs can be specified and then simulated to obtain the price distributions required for credit exposure estimation. Trade specification is based on quantities we called statistics, which can be thought of as functions that return some financial quantity, given a simulated scenario. We will use these statistics later in Part III to specify various products. This chapter is dedicated to a more detailed analysis of various statistics. We describe their implementation, the practical issues that arise, and the solutions we adopted. Since simulation is at the heart of our framework, we describe also various Monte Carlo schemes for simulating SDEs. We end the chapter by analysing the different types of errors introduced in the various steps of the modelling.

6.1 Spot and Forward Statistics The first type of statistics we consider are those that extract spot and forward values directly from simulated scenarios. These statistics are relatively simple in the sense of being deterministic functions of the simulated Brownian Motion processes. Typical examples are values of stock and foreign exchange rates, bond prices, and Libor rates. Most of these statistics can have a common signature and can be differentiated by the type of underlying they are applied to. Thus, we define a generic statistic (called INDEX), which has as argument the type of underlying (e.g. EQ, FX, or IR), and a symbol identifying a specific instance of the underlying (e.g. the IBM stock, the USDGBP exchange rate, the USD3Y three years swap rate). Having defined the underlying type and its instance, the observation date is expressed as a lag relative to the payoff date. Examples are given in Table 6.1. The generic way we have implemented the INDEX statistic is as follows. Different underlying types will correspond to different martingales, each of which are deterministic functions of simulated Brownian Motion paths. All that the INDEX statistic needs to do is to look up the correct martingale for each underlying type that is asked for. G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0_6, © Springer-Verlag Berlin Heidelberg 2009

111

112

6 Implementation

Table 6.1 At payoff date, return the value of index SYMBOL, of type TYPE, observed at the payoff date minus lag LAG. Examples: (i) The 3-year USD swap rate is defined as USD3Y, and its type, “interest rate”, as IR. In a 3Y USD CMS swap contract, the swap rate is called as INDEX(IR, USD3Y, 0M). (ii) To define a vanilla interest rate swap, paying quarterly, the 3-month libor rate fixed 3 months ago is accessed as INDEX(IR, USD3M, 3M). (iii) The IBM stock is called as INDEX(EQ, IBM, 0M). (iv) USDGBP FX rate is called as INDEX(FX, USDGBP, 0M). (v) USDGBP 1 year forward is called as INDEX(FX, USDGBP, -1Y)

Spot and forward

Date

Payoff

Ti

INDEX(TYPE, SYMBOL, LAG)

The reader may have noticed one practical problem that needs to be dealt with, and will affect computation of all statistics. When simulating the basic martingales for different asset classes, the simulation will need to happen on a finite number of prespecified time-points t ∈ {t0 , t1 , . . . , tn }. Of course the tj cannot be chosen a priori to incorporate all payoff dates of the portfolio that needs to be computed, first because there might be too many such dates, and second because to ensure scenario consistency one needs to be able to compute price distributions of new portfolios starting from the same basic Brownian Paths. We solve this problem by an interpolation method which we describe in a later section in this chapter.

6.1.1 Libor Rates and Bond Prices Libor rates and bond prices occur frequently and play an important role in a large proportion of portfolios. For this reason we have defined specialised statistics to aid in creating payoffs that depend on these quantities. Libor Rates are a specific case of an INDEX in our computational framework, but we have nevertheless defined a specialised statistic that returns simulated Libor rates. Recall that if t < [T1 , T2 ], the Libor rate Lt,[T1 ,T2 ] observed at time t for the period [T1 , T2 ], is a simple function of bond prices, that is, 1 Lt,[T1 ,T2 ] = T2 − T1

Dt,T1 D0,T1 Mt,T1 1 −1 = −1 , Dt,T2 T2 − T1 D0,T2 Mt,T2

(6.1)

where we have used the representation of bond prices D(t, T ) =

D(0, T ) M(t, T ) D(0, t) M(t, t)

(6.2)

in terms of the basic martingales M. Table 6.2 shows the syntax for both Libor and bond price statistics.

6.1 Spot and Forward Statistics

113

Table 6.2 At payoff date, return the Libor rate for a specified currency and tenor observed at payoff date minus lag. Example: the 6-months EUR Libor rate fixed in arrears is called LIBOR(EUR, 6M, 0M). Similarly, ZEROBOND(EUR, 10Y, 1Y) returns the price of a 10-year bond as observed 1 year before the current payoff date Date

Payoff

Libor

Ti

LIBOR(CURRENCY, TENOR, LAG)

ZeroBond

Ti

ZEROBOND(CURRENCY, MATURITY, LAG)

6.1.2 Annuity An annuity pays a unit coupon at regular points in time. In other words, it pays the difference between a coupon bond and a zero bond. The implementation of this statistic is similar to the previous one. At,T1 ,...,Tn =

n

(Ti − Ti−1 ) Dt,Ti =

i=1

n

(Ti − Ti−1 ) D0,Ti

i=1

Mt,Ti . Mt,t

(6.3)

Table 6.3 shows the usage of the ANNUITY function. Table 6.3 At payoff date, return the annuity of a swap with specified tenor and payment frequency. Example: the current annuity of a 3-years USD swap with monthly payments is specified as ANNUITY(USD, 3Y, 12, 0M)

Annuity

Date

Payoff

Ti

ANNUITY(CURRENCY, MATURITY, TIMESPERYEAR, LAG)

6.1.3 Swap Rate The swap rate is the coupon that the fixed-rate leg of a swap would have to pay for the swap to have zero net present value. Said in another way, it is the coupon that an annuity would need to pay in order to have value equal to a floating-rate leg. Thus, the SWAPRATE function can be evaluated as st,T1 ,...,Tn =

Dt,T0 − Dt,Tn D0,T0 Mt,T0 − D0,Tn Mt,Tn = . At,T1 ,...,Tn At,T1 ,...,Tn Mt,t

(6.4)

Again, the right side above involves only quantities that have already been considered. A specific example of usage of SWAPRATE is shown in Table 6.4.

114

6 Implementation

Table 6.4 At payoff date, return the par rate of a swap with specified tenor and payment frequency. Example: the par rate of a 3-year USD swap with monthly payments is called SWAPRATE(USD, 3Y, 12, 0M)

Swap Rate

Date

Payoff

Ti

SWAPRATE(CURRENCY, TENOR, TIMESPERYEAR, LAG)

6.2 Path Dependent Statistics We now turn to statistics whose evaluation cannot be done in simple deterministic fashion from simulated Brownian Motion paths. Examples of such statistics are the maximum attained by a process, the average value of a process, and the time spent by a process within a given range. In principle, if one could simulate the basic Brownian Motions on a fine-enough grid, such statistics would be a sampling exercise. However, since the size of portfolios and time horizons involved do not allow the luxury of arbitrarily small time steps, we need to find estimators of the above quantities that remain accurate when the simulation time step is relatively large.

6.2.1 Extremum Extremum is the maximum or minimum value reached on a given scenario path between two observable dates. This statistic is typically used to describe barrier features. The parameters we need for its implementation are its type and symbol, and some time parameters to define where to perform the observation. Table 6.5 At payoff date, return either the maximum (ISMAX = true) or the minimum (ISMAX = false) observed value of an index over an observation period starting at T start = Tpay − lag − tenor, and finishing at T end = Tpay − lag

Extremum

Date

Payoff

Ti

EXTREMUM(TYPE, SYMBOL, LAG, TENOR, ISMAX)

The implementation of EXTREMUM can be performed using some properties of Brownian motions. Consider an N-Brownian motion W for which we need to estimate the maximum and minimum values on an interval [t, T ], and suppose that we know the values, Wt = a and WT = b. The idea is to consider maxima and minima as random variables, defined by, Wmax = max Wu u∈[t,T ]

and Wmin = min Wu . u∈[t,T ]

(6.5)

We can write (see the Technical Note 6.4.4), √ √ T − t √ a+b + K + eK πΦ(− 2K) (6.6) E (Wmax | Wt = a, WT = b) = 2 2

6.2 Path Dependent Statistics

115

a+b − E (Wmin | Wt = a, WT = b) = 2

√ √ T − t √ K + eK π Φ(− 2K) , (6.7) 2

where K=

(b − a)2 . 2(T − t)

(6.8)

Once we have simulated the extremum of the underlying stochastic driver, as long as the quantity X (e.g. stock price, libor rate. . . ) can be expressed as monotonic function f of a single Brownian motion, we can derive the maximum (resp. minimum) of X as being either f (Wmax ) (resp. f (Wmin )) if f > 0 or f (Wmin ) (resp. f (Wmax )) if f < 0.1

6.2.2 Average Knowing Xmax and Xmin on any interval allows also to compute quantities that are path dependent, such as average values, or days within a range. A simple way of doing so for any interval [T1 , T2 ] is – From XT1 and XT2 obtain Xmax and Xmin . – Interpolate the path (T1 → T2 ) by imposing for example that max and min occur at (T2 − T1 )/4 and 3(T2 − T1 )/4, 3(T2 − T1 ) T2 − T 1 = Xmax , and X = Xmin . (6.9) X 4 4 Polynomial interpolation can now be used to obtain the path of X on [T1 , T2 ] from the four values given. Table 6.6 At payoff date, return the average value of any index over an observation period starting at T start = Tpay − lag − tenor, and finishing at T end = Tpay − lag

Average

Date

Payoff

Ti

AVERAGE(TYPE, SYMBOL, LAG, TENOR)

Asian options require to simulate the average value ΠAverage of X on a time interval [T1 , T2 ]: T2 1 ΠAverage = Xu du. (6.10) T2 − T1 T1 1 Note

that this approximation gives a bias due to Jensen’s inequality. If needed, an approximation for E(fmax ) on the Brownian bridge could be implemented. This, however, becomes model dependent.

116

6 Implementation

6.2.3 In Range Fraction Range accrual products usually pay an exotic coupon, which will be the product of a fixed coupon c with the proportion of days a given underlying has remained within a range [Klower , Kupper ]. T2 c ΠI nRange = 1Xu ∈[Klower ,Kupper ] du, ∈ [0, c]. (6.11) T2 − T1 T1

Table 6.7 At payoff date, return n/N , where n is the number of days where the index is in the range [LowerRange, UpperRange], and N is the total number of days in the observation period. The observation period starts at T start = Tpay −lag −tenor, and finishes at T end = Tpay −lag

Average

Date

Payoff

Ti

INRANGEFRACTION(TYPE, SYMBOL, LAG, TENOR, LOWERRANGE, UPPERRANGE)

6.2.4 Credit Loss To compute credit derivatives we need to be able to compute credit losses occurring in a given time interval. They are defined as follows, n j =1 Nj (1 − Rj )1Ti 0 between foreign currency bonds and the exchange rate is not respected. To assess the impact of independence, we consider the following example transaction. Fix a time horizon T , say, and consider a contingent claim CT = χT ,

(6.59)

that simply pays one unit of the foreign currency at T . At each t , CT should have value D˜ t,T χt , the time-t price of a foreign T -bond expressed in the reference currency. Now consider the option to enter, at time s < T , a portfolio that is long CT and short D˜ s,T χs . Because D˜ s,T χs is the no-arbitrage time-s value of the claim, the option should be worthless. If we assume independence, this is not guaranteed, because the FX rate simulation happens independently of the individual bonds (and also because of Monte Carlo and AMC regression error). As an illustrating example, we fixed s to be the end of the year 2015,2 and priced the option described above for values T shown in Table 6.10, and for four different currencies. We first computed the prices of the option using the independence assumption—results of this are shown in the third column in Table 6.10. For comparison, we re-did the computation by performing the simulation differently ensuring the interest-rate parity (which entails lifting the independence assumption) that 2 Time

of computation is April 09.

132

6 Implementation

Table 6.10 Value, as a percentage of notional, of an option to enter into a portfolio that is long an FX forward and short a bond in the foreign currency. The theoretical value of the option is zero. The table compares AMC values of the option under the assumption of independence between FX rates and interest rates, to the values obtained when that assumption is lifted. The time T is the maturity of the bond and forward. The option’s exercise time is fixed at 2015. In the last column we also report the estimated value of the underlying at time of expiry of the option, computed under the assumption of independence between rates and FX. The underlying has theoretical value zero by construction. The notional amount is 100 million units of the payment currency (computation performed April 09) T

USD

GBP

EUR

JPY

FX and Rates Independent

FX and Rates not Independent

Value of Option Underlying

2020

0, then the time-zero value of the close-out risk per share of stock faced at default is given by E Nt−1 (St − St−δ )+ , (14.20) therefore hedging the close-out risk on a stock forward involves hedging the cliquettype payoff appearing in the expectation in the equation written above.

14.9 Case Study To highlight the importance of dynamically hedging CVA, consider the following stylised example. Suppose that in June 2008 we had entered into a 20-year EUR/USD forward contract of 500 million EUR notional, in which we receive USD

14.9 Case Study

227

and pay EUR. To value the trade at par, the contract would have been struck at around 1.65 USD per EUR. Assume now that the counterparty we are facing had at inception a CDS curve trading at 300 bps flat. The initial CVA for this transaction would have been in the order of 10 mUSD. Leaping one year forward to June 2009, because of the fall in the EUR/USD exchange rate, the forward contract is now worth about 47 mUSD in our favour. Figure 14.3 shows the difference in exposure profiles generated on both dates. As we can see, the EPE profile computed in June 2009 is substantially higher than that computed a year before.

Fig. 14.3 PFE and EPE profiles computed June 08 and June 09. The two set of profiles are superimposed and the time axis is referred to the 2008 computation. The 2009 EPE and PFE profile start at the one-year point. Note that their initial value is within the PFE profile computed one year earlier, but due to the new market condition, the new profiles are outside the bounds computed in 2008

It is also interesting to note that the present value of the trade in June 2009 lies within the PFE confidence level computed in 2008.6 Assuming that the counterparty spread stayed at 300 bps, the resulting CVA would have gone up to roughly 24 mUSD. Had we only hedged the risk relating to the counterparty’s CDS curve, we would therefore face a loss of about 14 mUSD. Figure 14.4 shows the difference in CCDS profiles generated on both dates. Assume now that at inception of the trade in 2008, we had decided to also hedge the EUR/USD risk. To do so, we would have needed to choose an instrument which is liquidly traded and that does not add additional counterparty risk, such as one-year EUR/USD futures. In June 2008, the EUR/USD delta for the CVA of our fictional 6 Note that the EUR/USD exchange rate saw its greatest historical absolute fall between 2008 and 2009.

228

14 Pricing Counterparty Credit Risk

Fig. 14.4 CVA EPE and PFE profiles computed in June 08 and 09. The initial points of the two sets of profile corresponds to the CVA computed at inception and one year later

transaction was of the order of −37 mUSD, meaning that for every 0.01 move in the EUR/USD exchange rate, the CVA would increase by 370 kUSD. Hence, being long 37 mEUR notional worth of one-year futures should in theory eliminate the currency risk. The table below summarises the result of our hedging strategy from inception until June 2009. As we can see, the hedging strategy would have resulted in a profit of roughly 1 mUSD, as opposed to an un-hedged loss of 14 mUSD.7 Table 14.1 Summary of hedging strategy including CDS and FX hedges CVA

June 2008

June 2009

−9,983,526

−24,061,324

0

14,870,065

9,983,526

10,185,207

0

993,948

Hedge Cash Net

Of course, it is unrealistic to assume that the counterparty spread would have remained unchanged during a one year period. Assume now that in fact the CDS spread would have increased from 300 bps to 400 bps. In this case, the CVA computed in 2009 would no longer be 24 mUSD, but 27.7 mUSD. The credit delta8 computed in 2008 was of the order of 20 kUSD per basis point. While the maturity of the underlying portfolio is 20 years, assume that the only liquid maturity for the counterparty CDS is five years, for which the credit delta would be of the order of 7 Note 8 In

that we have assumed that the deposit rate at which the cash grows is 2% per annum.

other words, the counterparty spread delta of the CVA.

14.9 Case Study

229

400 USD per basis point on a one million USD notional, meaning that in order to hedge the counterparty spread risk we should enter into a 5 year CDS of roughly 50 mUSD notional. In June 2009, this position would have yielded a profit in the region of 1.6 mUSD, partially offsetting the loss. We can see now how, even in this stylised example, a simple market hedge can considerably improve P&L resulting from changes in CVA. An un-hedged position would have resulted in a net loss of almost 18m USD, while the full hedge we described would have reduced this loss to about 1 mUSD. It is worth noting that a static hedge involving solely the counterparty CDS would have resulted in a loss of roughly 16 mUSD. From this and the previous examples we can see that replicating a C-CDS involves hedging a hybrid product, which has market, (e.g. FX or interest rate), and credit components. Ignoring for example the FX risk would clearly undermine any hedging strategy. It is interesting to note that in a classical set-up, where the CVA is computed statically using simply the EPE profile and the spread of the counterparties, the market risk components of the hedge are difficult to compute, as they require the EPE sensitivities to market risk factors. They involve, however, the usage of instruments, which are in general traded on exchanges. The credit component can be computed more easily, but on the other side it involves CDS products, which are still mainly traded as OTC transactions and are not available for all names and all maturities.9 This involve the usage of credit indices and the finding of curves which can be used as proxies for illiquid names. In addition to the risks we have already mentioned, we need to consider the so called vega-risk deriving from movements of implied volatility. This can be substantial especially when the portfolio is dominated by FX positions. To appropriately hedge this risk we need to include in the C-CDS computation volatility as a stochastic driver. This can be performed by implementing a stochastic volatility model, as described in Chap. 3.

9 To reduce the counterparty risk in general CDS hedges are traded with fully collateralised counterparties. There are extensive discussions to standardise CDSs and trade them on exchanges.

Concluding Remarks

Our goal in this book was to model counterparty credit exposure for all types of transactions. We saw that by appropriately choosing the fundamental quantities to model we can approach the problem in a modular way, dividing features and conquering products. Price distributions are obtained using American Monte Carlo (AMC) techniques, allowing a valuation framework where modularity and flexibility are key. With the introduction of a booking language, PAL, we added a further layer of de-coupling and abstraction, enabling a system architecture that could address most of the problems faced by a counterparty exposure system dealing with large diverse portfolios. The natural next step was to investigate how to manage counterparty exposure, both in static and dynamic ways. This led to the introduction of the so-called contingent credit default swap product, C-CDS, which replicates the cost of protection. We now summarise the steps needed to compute and hedge credit exposure. (i) Translation. First of all, all trades within the portfolio should be understood by the valuation engine. This means that each trade needs to be translated into the common trade representation language. (ii) Portfolio Valuation. Once the first stage is completed, it is possible to model the underlying risk drivers, which have been recognised via the common trade representation language, and value each trade, along with its future price distributions. All trades are then aggregated together, including possible netting rules or break clauses, to finally arrive at the future distributions of the portfolio. If a collateral agreement exists, its logic should then be applied to the portfolio distributions. (iii) C-CDS Valuation. The credit valuation adjustment, CVA, can be valued using the modified EPE profile of the portfolio and the counterparty credit spread curve. Using C-CDSs, however, we can compute not only the value of CVA, but also the CVA future price distribution. (iv) Sensitivities Computation and Replication. As a final step, sensitivities can be computed from the C-CDS distribution, using either a regression-based approach, or a full revaluation (known in the industry as ‘bumping’ method), starting the process again from step (ii). G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0, © Springer-Verlag Berlin Heidelberg 2009

231

232

Concluding Remarks

(v) Post-processing. For purposes of risk control (e.g. to compute regulatory capital or compare PFE with limits), a post-processing of the price distribution may be needed. Examples of this include stress-testing and accounting for rightway/wrong-way risk. The techniques we described can also be applied to other problems that large financial companies need to address. Examples are (i) computing the value of the so called own credit of a company, (ii) valuing debt valuation adjustments (DVA) of portfolios of transactions, (iii) addressing the problem of valuing the cost of funding and cost of collateral, (iv) computing potential values of transactions in different scenarios, (v) determining the value of risk weighted assets and of regulatory capital, or (vi) investigating various hedging strategies. All these problems deserve a thorough analysis which could be the subject of further research. It is interesting to note here that any solution to these questions will require, as fundamental feature, the capability of computing future distributions of prices. This is the feature at the heart of our work. A final remark to conclude. What we described in this work is only a brief overview of the problem we try to solve. As we highlighted throughout this book, in many occasions we accepted compromises in our implementation and highlighted shortcuts. Many points can be improved, further explored and changed. We think, however, that at a general level, the framework and the ideas we provide are a viable solution to the modelling, pricing and hedging of counterparty credit exposure for large portfolios of different products.

Appendix A

Approximations

We summarise here some useful approximations of counterparty exposure computation, often used by practitioners. While they cannot provide satisfactory results in general, they may serve as a sanity check for more complex computations, and to help intuition. In some cases in the computation of Expected Positive Exposure (EPE) for some types of products, they are based on pricing information and give exact valuation. Some of the formulae we present are general and others can be used only for specific products. We consider here what we found useful in our day-to-day work.

A.1 Maximum Likely Exposure In general, the Potential Future Exposure profile (PFE) of a given product is a function of time. We call its maximum value Maximum Likely Exposure (MLE). In the following sections we provide some MLE estimate for simple products.

A.1.1 MLE of Equity and FX Products MLE values can be easily approximated in the case of options or forwards on assets that can be modelled as Geometric Brownian Motions assuming constant volatility and interest rate. Under these assumptions in fact the exposure profile reaches its maximum at maturity of the trade, where its value coincides with its intrinsic value. Thus, to compute the MLE, what is necessary is to estimate the potential value of the asset at maturity. Consider for example an option on a stock S with Black-Scholes volatility σ , interest rate r, and strike K. The maximum value of the exposure at maturity T (within a 97.5% confidence interval) is given by 1

MLE = Se(r− 2 σ

2 )T +1.96σ

√

T

− K.

G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0, © Springer-Verlag Berlin Heidelberg 2009

(A.1) 233

234

A Approximations

If we assume zero interest rate, stock returns normally distributed, and at the money products (S = K), we can simplify this formula as follows, √ MLE = 1.96Sσ T . (A.2) The main problem in these valuations is the choice of volatility. If the volatility is assumed to be constant, it is necessary to estimate the value that will best fit the terminal asset distribution. If the choice is to use implied volatilities, the atthe-money volatility is often the most suitable one to use. In practice if implied volatilities are not available historical volatilities are used.

A.1.2 MLE of Swaps Throughout this book we have seen several PFE profiles of interest-rate swaps. In general, when the product is vanilla, they show a typical bell shape, which starts from zero, increases over time and then decreases to reach zero again at maturity. This shape is driven by two factors, the declining duration (time to maturity) and the increasing variance of the swap. Assume that, at any time t, the duration is proportional to the remaining life of the swap via a constant A0√< 1, and that the interest-rate volatility increases with the square root of time, σN t.1 We can write the volatility of the swap as √ VolSwap = A0 (T − t)σN t. (A.3) The peak exposure, i.e. the MLE, is reached at about one third of the life of the trade. We can see this by simply taking the first derivative of the volatility with respect to time, and imposing its value to be zero. ∂VolSwap T 1 = 0 ⇐⇒ −A0 + A0 (T − t) = 0 ⇐⇒ t = . ∂t 2t 3

(A.4)

Using this result and assuming that the price distribution of an at-the-money swap is normally distributed, we can estimate the price distribution of a swap at time T /3, 2 T Z, (A.5) SwapDistributiont=T /3 ≈ A0 T σN 3 3 where Z ∼ N(0, 1). If we want to value the MLE, i.e the peak PFE exposure at 97.5% confidence interval, we need to substitute Z with 1.96. The present value of EPE can be computed by taking the expectation of the positive part of this distribution. Doing this we obtain √ √ 1 EPEPt V ≈ √ A0 (T − t)σN t ≈ 0.4A0 (T − t)σN t. 2π 1σ N

(A.6)

is the volatility of a normal distribution. It is related to the log-normal (Black-Scholes) volatility σ of the swap rate via the level of interest rate, σN ≈ rσ .

A.2 Expected Positive Exposure

235

A.2 Expected Positive Exposure The Expected Positive Exposure (EPE) computation is strongly related to pricing. In general, under pricing measure assumptions, the EPE of a transaction at time t is the price of an option to enter in the transaction at time t. This is a very useful result, as it allows to approximate EPE computations using price information.

A.2.1 EPE and CVA of Equity Options As a first example consider an option on a stock or an FX currency. Under simplified assumptions, EPE can be written as EPEt = E[Vt+ ],

(A.7)

where Vt is the price distribution at time t. In the case where Vt is always nonnegative, as for example for options, this equation becomes EPEt = E[Vt+ ] = E[Vt ] = E[E[e−r(T −t) (ST − K)+ |Ft ]] = V0 ert ,

(A.8)

where we have assumed constant interest rates and volatility. Thus, EPEt = V0 ert .

(A.9)

In other words the EPE of an option at time t is the option premium increased at the risk-free rate. The CVA can be computed as the discounted EPE multiplied by the spread (assumed to be constant) multiplied by time to maturity, CVA ≈ V0 s0 T .

(A.10)

This formula holds for any product whose price distribution is non-negative and which does not pay intermediate cashflows. For example it can be used to compute CVA of a cash-settled swaption, while it cannot be applied in the case of a physically-settled swaption.

A.2.2 Relation between MLE, EPE If we assume zero interest rate we can approximate the price V0 of an at-the-money (S = K) option as, V0 = SΦ(d1 ) − KΦ(d2 ),

(A.11)

236

A Approximations

where Φ is the cumulative normal distribution, and √ √ ln(S/K) σ T σ T ± d1/2 = =± . √ 2 2 σ T

(A.12)

Using the following approximation Φ(x) =

1 1 + √ x + O(x 3 ), 2 2π

√ and assuming σ T 1 the above equation becomes, √ √ √ √ σ T σ T 1 − KΦ − ≈ √ Sσ T ≈ 0.4Sσ T . V0 = SΦ 2 2 2π

(A.13)

(A.14)

We have seen that the EPE of an option can be computed as the option premium growing at risk free rate. Thus √ EPE ≈ 0.4σ S T . (A.15) We can now compute a relation between EPE and the 97.5% MLE. Recall that, √ MLE ≈ 1.96σ S T . (A.16) Thus, we obtain, EPE ≈ 0.2. (A.17) MLE In other words, if the distribution of the portfolio is normal and centered around zero, then the 97.5% MLE is roughly five time larger than the EPE.

A.3 CVA of Swaps The EPE value at time t of a swaps portfolio is often computed by practitioners as the value of a swaption, i.e. the value of an option to enter into a (portfolio of) swaps. This valuation is correct, however, only if the modified value of the EPE, as defined in Chaps. 12 and 14, is used. Often this valuation methodology is called swaption approach. We can evaluate approximation of the CVA of a swap as follows. CVA

swap

≈ 0

T

EPEPu V s0 du ≈ s0

4 = s0 0.4A0 σN T 5/2 , 15

T

√ 0.4A0 (T − u)σN udu

0

(A.18)

A.3 CVA of Swaps

237

where s0 is the CDS spread and EPEP V is the present value of the EPE. Recalling (A.5) we can approximate the peak value of the discounted EPE profile as 2 V ≈ 0.4A0 σN T 3/2 √ , EPEPmax 3 3 and thus,

(A.19)

√ 6 3 V EPEPmax . (A.20) CVA ≈ s0 T 15 Noting that the maximum value of the EPE profile of an at-the-money swap occurs at t = T /3 and using the ‘swaption approach’ we defined earlier, we get, T 2 ,T , (A.21) CVA ≈ s0 T Swaption 3 3 √ where we have approximated 6 3/15 with 2/3, and Swaption( T3 , T ) is the value of an option to enter at time T /3 into a swap of maturity T .

Appendix B

Results from Stochastic Calculus and Finance

This book is concerned with the pricing and hedging of risk borne by financial institutions when entering into transactions with other counterparties. Such risk arises from the random nature of the prices of products transacted as well as the possibility that the counterparty defaults, but its pricing and replication uses the same concepts as for other kinds of financial derivatives. This appendix collects a few technical results that we will need throughout. We start by giving definitions for the basic stochastic processes we use, and then recall the concept of change of measure. We give also a brief overview of the fundamental theorem of asset pricing, which allows us to characterise the hedging portfolio for a traded derivative from martingale representation. Derivation and analysis of these results can be found in standard finance books, such as Baxter & Rennie [10], Hunt & Kennedy [64], Karatzas & Shreve [68], Rogers & Williams [93, 94], Shreve [98], and Williams [106].

B.1 Brownian Motion and Martingales All our processes are defined relative to a filtered probability space (Ω, F , (Ft )t≥0 , P), where (Ft )t≥0 is a filtration in F . The basic process we work with is Brownian Motion. Definition 1 A process W ≡ (Wt )t≥0 on (Ω, F , P) is called Brownian Motion if (i) W0 (ω) = 0, for all paths ω ∈ Ω; (ii) for each ω ∈ Ω, Wt (ω) is a continuous function of t; (iii) for each t, h ≥ 0, Wt+h − Wt is independent of Wt , and has a Gaussian distribution with mean 0 and variance h. Brownian Motion is an example of a martingale, the most important class of processes. Definition 2 A process M is called a martingale with respect to (Ft )t≥0 if G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0, © Springer-Verlag Berlin Heidelberg 2009

239

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B Results from Stochastic Calculus and Finance

(i) M is adapted, that is Mt is Ft -measurable; (ii) E[|Mt |] < ∞; (iii) if s ≤ t, then E[Mt | Fs ] = Ms . M is a supermartingale (resp. submartingale) if we replace equality in (iii) above by ≤ (resp. ≥). For proving general results, the class of martingales is not the right notion to work with, and one needs to consider local martingales. While all martingales are also local martingales, the converse is true only if certain conditions hold. The distinction will not be important for our purposes in this book. At the heart of most of what we do is the idea of looking at various processes in a measure different from that of the given probability triple (Ω, F , P). Indeed, if Z is a non-negative random variable (that is, F -measurable) then ˜ ) := E[Z1F ]/E[Z], P(F

F ∈F

(B.1)

defines a new probability measure P˜ on F for which ˜ ] = 0. P[F ] = 0 ⇒ P[F

(B.2)

The last implication allows us to make the following definition: Definition 3 A probability measure P˜ on (Ω, F ) is said to be absolutely continuous with respect to P, denoted P˜ P, if for all F ∈ F , (B.2) is true. If both P˜ P and P P˜ are true, then P and P˜ are said to be equivalent. In this case, P and P˜ have the same sets of measure zero. The converse to (B.1) is given by the Radon-Nikodym theorem. Theorem 1 Let P˜ P be a probability measure that is absolutely continuous with respect to P. Then P˜ can be characterised as in (B.1) for some non-negative random variable Z, which is then called the Radon-Nikodym derivative of P˜ with respect to P, and we write Z≡

d P˜ . dP

(B.3)

The context in which we will most often see measure-change at work is when changing the drift of a Brownian Motion process. Given a P-Brownian Motion W , if the process γ ≡ (γt )t≥0 is such that t 1 t 2 γs dWs − γs ds (B.4) ζt := exp 2 0 0 is a martingale, then there exists a unique probability measure P˜ such that t Wt − γs ds 0

(B.5)

B.2 Replication of Contingent Claims: Martingale Representation

241

˜ is a P-Brownian Motion. Moreover, the Radon-Nikodym derivative of P˜ relative to P is given on every Ft by d P˜ = ζt . (B.6) dP Ft ˜ W has a drift of γ . Equivalently, The above says that under P, ˜ W˜ is a P-martingale ⇐⇒ ζ W˜ is a P-martingale.

(B.7)

The change-of-measure technique is an indispensable device for simplifying calculations by removing from a process an unwanted drift term. We use it also to study price distributions under probability measures different to the ones in which they are simulated (see also Chap. 13).

B.2 Replication of Contingent Claims: Martingale Representation Consider an economy that puts at our disposal a number of assets St = (1) (n) (i) (St , . . . , St ), so that St is the time-t price of the i’th asset. There is a market for trading these assets. Thus, at any time t , a market participant, of wealth Vt (1) (n) say, will have a proportion of wealth allocated to a portfolio θ t = (θt , . . . , θt ), with the remainder held in some deposit account, so that Vt = ϕt Bt + θ t · St ,

(B.8)

where Bt is the value at t of one unit invested in the deposit account at time zero, and ϕt Bt is the wealth not invested in S. Because any value kept in the deposit account grows at some positive rate, it is more useful to express asset prices in terms of B, writing V˜t ≡ Bt−1 Vt , S˜ t ≡ Bt−1 St . The wealth equation (B.8) then becomes V˜t = ϕt + θ t · S˜ t ,

(B.9)

so that, as we expect, in any time interval where the holdings ϕ and θ are kept constant, the growth in discounted wealth V˜ derives only from growth in the discounted ˜ assets S. Of course, funds may be switched between the holdings in S and the deposit account, but it is natural to suppose that no new wealth can be injected, in which case the portfolio of holdings (ϕ, θ ) is said to be self-financing. The consequence of V being self-financing is then that t ˜ ˜ θ u · d S˜ u , (B.10) Vt = V0 + 0

so that the discounted wealth is the integral of the portfolio holdings against the discounted asset price process.

242

B Results from Stochastic Calculus and Finance

The fundamental theorem of asset pricing, formalised by Harrison & Kreps [57] and Harrison & Pliska [58], and formulated in more general setting in the work of Delbaen & Schachermayer (for example, [34] and [35]), states that arbitrage is excluded if and only if there is some equivalent martingale measure under which discounted asset price processes are martingales. This implies that the price of a contingent claim can be computed as the expectation in the martingale measure of the discounted payoff of that claim. If the market is also complete, so that all claims can be replicated perfectly,1 then the martingale measure (and hence the market price for any claim) is unique. Now if P˜ is a measure under which S˜ is a martingale, and Y = f (S˜ T ) is a con˜ the discounted time-t price of Y , π˜ t,T , say, being the price of a tingent claim on S, traded asset, is itself a P˜ martingale. It follows that π˜ has a representation as ˜ −1 Y |Ft ]. π˜ t,T = Bt−1 πt,T = E[B T

(B.11)

In the absence of any other condition enforcing a unique price for the claim Y , there will be potentially as many prices π˜ for Y as there are market agents, each price reflecting that agent’s own risk aversion. If the market is complete, however, there is a price-enforcing mechanism: the price of Y will be the cost V0Y of setting up a portfolio worth V Y (0) = ϕ0Y + θ 0 · S0

(B.12)

V Y (T ) = Y

(B.13)

at time zero and

at time T . The existence of a unique process θ Y that makes the wealth equation (B.10) true is a consequence of the martingale property of the price processes π˜ t,T = V˜ Y (t) and St and the martingale representation theorem (see Rogers & Williams [94]). Theorem 2 Let X be a local martingale on the filtered probability space (Ω, F , (Ft ), P), and assume that (Ft ) is the filtration generated by X. Then, any local martingale M adapted to (Ft ) has a representation as t Mt = M0 + Hu dXu (B.14) 0

where H is previsible with respect to (Ft ). Moreover, H is unique up to sets of measure zero. ˜ Because the claim price process π˜ t,T and the asset price process S are both Pmartingales, the martingale representation theorem shows the existence of a strategy with which to hedge the claim Y by trading in the assets S. this we mean that for every time-T claim Y one can find a portfolio V˜tY = V˜0 + such that VT = Y . 1 By

t 0

θ u · d S˜ u

B.3 Change of Numeraire

243

B.3 Change of Numeraire In writing the wealth equation (B.10) we defined S˜ t ≡ Bt−1 St and V˜ ≡ Bt−1 Vt by expressing the prices of assets and the wealth V in units of the deposit account. One says that the deposit account is being used as numeraire. There is nothing that keeps us from using as numeraire the value of a different asset, and in fact changing numeraire is a powerful modelling and computational technique. Geman and Jamshidian were the first to employ this idea. Suppose X is the price of any traded asset (scaled by its time-zero value); for reasons that will soon become obvious, we need to assume Xt > 0 for each t. Then, because by ˜ definition of P˜ all discounted assets are P-martingales, we have that Xt ˜ is a P-martingale. Bt

(B.15)

This allows us to define a new measure, PX say, whose Radon-Nikodym derivative is given for every t by Xt dPX ζt = = (B.16) . Bt d P˜ Ft It then follows, for any given process M, that ˜ Bt−1 Mt is a P-martingale ⇐⇒ Xt−1 Mt is a PX -martingale, so for any claim Y maturing at T we can write the equivalent expressions

−1

−1 B (T ) X X (T ) F Ft , πt,T = E˜ = E Y Y t −1 −1 B (t) X (t)

(B.17)

(B.18)

where the first expectation happens under P˜ and the second under PX . For example, if one takes for X the price process of the bond maturing at time T , the price of any claim Y received at T is

−1 B (T ) T ˜ F = D Y Y Ft , (B.19) E πt,T = E t t,T B −1 (t) where Dt,T is the observed time-t price of the T -bond, so that DT ,T ≡ 1, and where the expectation is now in the T -forward measure in which asset prices discounted by the T -bond are martingales. The price of Y can now be computed as the expectation of Y in the T -forward measure. An in-depth account of martingale theory and stochastic processes, which we have used here, is Rogers & Williams [94]. Our description of self-financing portfolios closely follows the article of Rogers [92], which shows how ideas of economic equilibrium lead directly to the existence of equivalent martingale measures.

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Index

A ABS, see Asset backed securities Absolute return swap, see Asset swap, absolute return ABX, 13, 215 Accelerated shares re-purchase (ASR), 176, 177 Advance, see Interest-rate swap, in advance Aggregation, see Risk aggregation AMC, see American Monte Carlo American Monte Carlo (AMC), 19, 20, 79, 80, 88–94, 101, 135 backward induction, 20, 85–88, 90, 94, 101 bias correction, 92 bundling, 88–92, 130 continuation value, 86, 88–91 error, 91, 125, 129 granularity bias, 90 observables, 82, 129, 218 regression, 92, 93, 130 Architecture, 18, 20, 21, 101, 131 component, 139 conceptual view, 137 design principles, 136 logical view, 137, 139 physical view, 137, 142 requirements, 136 Arrears, see Interest-rate swap, in arrears ASR, see Accelerated shares re-purchase Asset backed securities, 215 Asset swap, 166 absolute return, 166 relative return, 167

B Backtest, 128, 129, 187 Backward induction, see American Monte Carlo (AMC), backward induction Barrier option, see Equity-FX option, barrier Base correlation, 65 Basel II accord, 128, 186, 201 Bermudan option, see Interest rate option and equity-FX option, Bermudan Bernoulli distribution, 72, 74 BGM, see Libor market model Bi-modal price distribution, 172 Black model, 53 Black-Scholes formula, 16 Black-Scholes model, 16, 53 Booking language, 20 Booking system, 21, 139 Break clause, 150, 189, 223 Brownian Motion, 28–30, 239–241 Bullet bond, 175, 176 Bundling, see American Monte Carlo (AMC), bundling Business requirements, 136 C C-CDS, see Contingent credit default swap Calibration, 45, 47 correlation, 69, 75 to caps, 47 to CDO tranches, 69 to floors, 47 to options on CDSs, 65, 66 to swaptions, 48 to variance swaps, 54

G. Cesari et al., Modelling, Pricing, and Hedging Counterparty Credit Exposure, Springer Finance, DOI 10.1007/978-3-642-04454-0, © Springer-Verlag Berlin Heidelberg 2009

249

250

Call option, see Equity-FX option, call Call spread overlay (CSO), 180 Callability, 5, 24 Callable daily range accrual (CDRAN), 178 Cancellability, 150, 189 Cancellable swap, see Interest-rate swap, cancellable Cap, see Interest-rate option, cap Capital, 10, 175, 185, 186, 232 Caplet, see Interest-rate option, caplet Cash bond, 11 Cash settlement, 81, 156 CDO, see Collateral debt obligation CDRAN, see Callable daily range accrual CDS, see Credit default swap CDS curve, 39 CDX, 215 CEV, see Constant Elasticity of Variance Change of measure, 26, 27, 34, 202, 241 Characteristic function, 59, 60, 63 Cliquet, 168 Close-out risk, 8, 13, 183, 184, 194, 195 CMBX, 215 Coherent measure, 204 Collateral, 8, 183, 184, 188, 191, 192, 194, 195, 199 Collateral agreement, 13, 184, 190–194, 223 Collateral debt obligation (CDO), 3, 43, 126, 172, 173 attachment / detachment point, 43, 172 tranche, 43, 65, 69, 73, 171–173 Commodities, 159 Conservatorship, 189 Constant Elasticity of Variance (CEV) model, 55 Constant maturity swap, see Interest-rate swap, constant maturity Contingent claim, 241 Contingent credit default swap (C-CDS), 109, 135, 216–226 PAL example, 109 Continuation value, see American Monte Carlo (AMC), continuation value Contribution, 196, 197, 199 Convertible bond, 179 Convexity, see Volatility, convexity Copula, 39, 42 Gaussian, 42, 69, 73

Index

Corporate bond, 42 Correlation between Brownian Motions, 62 between drivers, 36 between reference names, 42 between volatility and asset price, 58 calibration, 75, 126 Gaussian copula, 42, 69 instantaneous, 37, 38, 58 product, 65 structure, 65 Cost of funding, 232 Counterparty credit exposure approximations, 233 basic concepts, 3 cross-gammas, 94, 220 definition, 10, 23–25, 80 hedging, 10, 216–220 profile, 5 sensitivities, 4, 9, 79, 94–97, 221–223 simulation, 17 Credit charge, see Credit valuation adjustment Credit crisis, 173, 184 Credit default swap (CDS), 3, 8, 39, 171, 172, 215, 220, 225 PAL example, 106 payment leg, 106, 171, 172 protection leg, 106, 109, 171, 216, 217 Credit derivative, 3, 27, 38, 65, 116, 171– 173, 185, 195 Credit exposure, see Counterparty credit exposure Credit loss, 42, 65, 116, 171 Credit model, 38–41, 65, 66 Credit Officer, 184 Credit protection, 15 Credit quantification, vii, 15 Credit risk, 8 Credit spread, 8, 15, 38, 65 Credit spread overlays (CSO), 179, 180 Credit support annex (CSA), 14, 188, 192– 194 Credit valuation adjustment (CVA), 4, 8, 14, 109, 190, 195, 213, 216, 217 approximation, 235 sensitivities, 221–223 Cross-currency swap, 153, 188, 212

Index

Cross-gammas, see Counterparty credit exposure, cross-gammas CSA, see Credit support annex CSO, see Call spread overlay CVA, see Credit valuation adjustment D Debt valuation adjustment (DVA), 186, 190, 232 Default correlation, 42 dependence, 69 probability, 38–42 stochastic probability, 41 time, 38–42, 66–69, 216 Delta, see Counterparty credit exposure, sensitivities Digital option, see Equity-FX option, digital Disaggregation, see Risk allocation Distance-to-default, 206, 212 DJ Euro Stoxx, 167–169 Domestic currency, see Reference currency Dupire formula, 63 DVA, see Debt valuation adjustment E EAD, see Exposure at default EPE, see Expected positive exposure Equity model, 37, 38, 52–64 Equity tranche, see Collateral debt obligation (CDO), tranche Equity-FX option American, 159, 160, 162 Asian, 159, 160, 164 barrier, 160, 164 Bermudan, 159, 160, 162 call, 15, 160, 162, 187, 210 digital, 160, 162, 178 European, 159, 160 PAL examples, 106 put, 160, 162, 187, 206 vanilla, 162 Equivalent martingale measure, 242 Error analysis, see American Monte Carlo (AMC), error ES, see Expected shortfall Euler allocation, see Risk allocation, Euler Euler scheme, 117–119

251

European option, see Interest rate option, European Expected positive exposure (EPE), 4, 12, 185 approximation, 235 modified, 127, 186, 216, 225 reverse, 186 Expected shortfall (ES), 9, 12, 162, 172, 185 Expected tail loss, see Expected shortfall Exposure at default (EAD), 186 Exposure profile, see Counterparty credit exposure Expression tree, 103 F Fannie Mae, 188 Fast Fourier Transform (FFT), 59, 62 Fat tail, 52 FFT, see Fast Fourier Transform Filtration, 11, 24, 239 Flesaker-Hughston framework, 27 Floorlet, see Interest-rate option, floorlet Foreign currency, 33–37 Forward contract, 159, 160, 169 strip, 159 Freddie Mac, 188 Functional requirements, 136 FX model, 33–37, 52–64 G Gamma, see Counterparty credit exposure, sensitivities Gas price, 206 Gaussian copula, see Copula, Gaussian Gaussian model, 39 Geometric Brownian Motion, 161, 233 Gold price, 204, 205 Grace period, 195 Grammar, 103, 104 Granularity bias, see American Monte Carlo (AMC), granularity bias Grid, 131, 142 H Haircut, 192, 195 Heath-Jarrow-Morton framework (HJM), 31, 126 Hedging, see Counterparty credit exposure, hedging

252

Heston model, 58–60, 62 HJM, see Heath-Jarrow-Morton framework Hull-White model, 31, 45, 50–52 Hybrid model, 75 Hybrid product, 20, 75, 183, 229 I Implied volatility, see Volatility, implied Inflation index, 159 Inflation model, 37, 63 Initial margin, 13, 191, 195 Instantaneous forward rate, 50 Intensity process, 41 Interest-rate model, 27–33, 45–52 Interest-rate option, 156 Bermudan, 8, 83 cap, 47 caplet, 47, 156, 225 digital, 156 European, 8 floorlet, 156 Interest-rate swap, 149–152, 234 cancellable, 5, 84, 93, 103, 150, 152 capped, 150, 152 constant maturity, 154 floored, 150, 152 in advance, 150 in arrears, 105, 150 PAL examples, 105 range accrual, 155 steepener, 45, 126, 154 vanilla, 4, 5, 151 International Swap Dealer Association (ISDA), 188 Intrinsic exercise, 81 ITraxx, 3, 215, 222 J Jump model, 60 K Kurtosis, see Volatility, kurtosis L Laplace transform, 60 Large homogeneous portfolio, 73 Lévy process, 52, 60 Lexer, 104 Libor market model (BGM), 32, 126

Index

Libor rate, 26, 29 Limits, 183, 188, 190 Local currency, see Reference currency Local volatility model, 52–56, 60 Longstaff-Schwartz algorithm, 89 Loss distribution, 73 Loss product, see Credit derivative Loss simulation, 42 M Margin call, 14, 184, 191 Markov process, 27 Martingale, 17, 26, 239–243 Martingale interpolation, 122 Maximum likely exposure (MLE), 233 MBS, see Mortgage backed securities Mean, 4 Measure, see Pricing measure and physical measure Merton model, 206 Mezzanine tranche, see Collateral debt obligation (CDO), tranche Milstein scheme, 118, 119 Minimum transfer amount, 14, 191 MLE, see Maximum likely exposure Model risk, 125 Modified EPE, see Expected positive exposure, modified Monoline, 3, 14 Monte Carlo, 11, 17–20, 108, 111, 117–121, 183, 186, 196 Mortgage backed securities (MBS), 13 Municipality, 175 N Nested simulations, 8, 144 Netting, 12 Netting / no-netting agreement, 188 Nominal rate, 159 Number of paths, 131 Numeraire, 11, 24–26, 243 Numeraire measure, 25 O Object-oriented, 105, 106 Observables, see American Monte Carlo (AMC), observables Oil price, 203–206, 212

Index

Option, see Equity-FX option and interest rate option OTC, see Over the counter Over the counter (OTC), 3, 109, 229 Own credit, 232 P P&L, see Profit and loss (P&L) PAL, see Portfolio aggregation language Parser, 104 Partial differential equation (PDE), 19 Payment leg, see Credit default swap (CDS), payment leg Payoff language, 173 PDE, see Partial differential equation Percentile, see Quantile PFE, see Potential future exposure Physical measure, 25, 201, 202 Physical settlement, 81, 156 Portfolio aggregation language (PAL), 20, 21, 103–108, 135 Portfolio manager, 139 Potential future exposure (PFE), 4, 12, 184 approximation, 234 Price distribution, 4, 5, 23, 101, 125 Pricing function, 19 Pricing measure, 11, 185–187, 201 Probability measure, 240 Probability space, 24, 239 Procedural language, 104 Profit and loss (P&L), 18, 96, 98, 221 volatility, 216, 217 Programming language, 104–106 Protection, see Credit protection Protection leg, see Credit default swap (CDS), protection leg Put option, see Equity-FX option, put Q Quantification unit, 141 Quantile, 4, 184, 195 Quanto adjustment, 38 R Radon-Nikodym derivative, 34, 202, 203, 208, 240 Range accrual, see Interest-rate swap, range accrual Rating, 173

253

Rating migration, 65 Rational lognormal model, 27 Real measure, see Physical measure, 11 Real rate, 64, 159 Real-world measure, see Physical measure Recovery rate, 9, 171–173, 215, 216 Recursion, 72 Reference currency, 27, 188 Reference measure, 27 Rehypotecation, 190 Relative return swap, see Asset swap, relative return Repudiation risk, 189 Retail price index (RPI), 37, 159 Right-way/wrong-way exposure, 14, 180, 201, 205–213, 224 Risk aggregation, 12, 187–190 Risk allocation, 13, 195–198 Euler, 196–198 naive, 196 Risk Control, 184, 187 Risk measure, 12, 184–187, 196 Risk mitigation, 190–194 Risk weighted assets (RWA), 10, 185, 232 Risk-neutral measure, see Pricing measure Risk-on-risk, see Right-way/wrong-way exposure RPI, see Retail price index RWA, see Risk weighted assets S Scenario, 17, 18, 23, 108, 125 consistency, 13, 19, 23, 75, 183, 187, 189 generation, 11, 141 SDE, see Stochastic differential equation Senior tranche, see Collateral debt obligation (CDO), tranche Sensitivities, see Counterparty credit exposure, sensitivities Separable volatility, see Volatility, separable Sharpe ratio, 203, 221 Shifted Libor rate, 33 Short rate, 30, 50 Simulation, 17 Sinking fund, 175, 176 Skew, see Volatility, skew Smile, see Volatility, smile State of the world, 141 State-price density, 27

254

Statistics, 102, 116 Steepener, see Interest-rate swap, steepener Stochastic differential equation (SDE), 28, 31, 117–121 Stochastic volatility model, 52, 53, 58, 60, 62, 63 Straddle, 169 Stress scenario, 201 Stress test, 10, 204, 205 Structured products, 175 Submartingale, 240 Subprime mortgage, 13 Super-product, 108, 135, 218 Super-senior tranche, see Collateral debt obligation (CDO), tranche Supermartingale, 27, 240 Survival probability, 41, 66, 67 Swap, see Interest-rate swap Swaption, 48, 156, 236 cash settled, 81, 156 European, 84, 94, 103 PAL example, 106 physically settled, 81, 84, 103, 156 Swaption approach, 236 T Tail risk, 185 Target redemption swap, 169 TCP-IP, 142 Threshold, 13, 191 Tilley algorithm, 88 Tranche, see Collateral debt obligation (CDO), tranche Translator, 21, 139

Index

external, 139 internal, 141 U UML, see Unified modeling language Unified modeling language (UML), 139 V Value-at-risk, 10 Variance swap, see Calibration, to variance swaps Vasicek model, 50 Vega risk, 229 Volatility, 17 at-the-money, 234 convexity, 55, 58 historical, 234 implied, 47, 53, 229, 234 instantaneous, 38 kurtosis, 63 separable, 30, 31, 45–47 skew, 52, 54–56, 58, 60, 63 smile, 54, 63 Volume weighted average price, 177 VWAP, see Volume weighted average price W West Texas Intermediate (WTI), 204, 212 Wrong way risk, see Right-way/wrong-way exposure WTI, see West Texas Intermediate Y Yield curve, 27, 29, 45

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