Time-changed Birth Processes, Random Thinning, and Correlated Default Risk

Book excerpt: Credit risk pervades all nancial transactions. The credit crisis has indicated the need for quantitative models for valuation, hedging, rating, risk management and regulatory monitoring of credit risk. A credit investor such as a bank granting loans to rms or an asset manager buying corporate bonds is exposed to correlated default risk. A portfolio credit derivative is a nancial security that allows the investor to transfer this risk to the credit market. In the rst part of this thesis, we study the valuation and risk analysis of portfolio derivatives. To capture the complex economic phenomena that drive the pricing of these securities, we introduce a time-changed birth process as a probabilistic model of correlated event timing. The self-exciting property of a time-changed birth process captures the feedback from events that is often observed in credit markets. The stochastic variation of arrival rates between events captures the exposure of rms to common economic risk factors. We derive a closed-form expression for the distribution of a time-changed birth process, and develop analytically tractable pricing relations for a range of portfolio derivatives valuation problems. We illustrate our results by calibrating a tranche forward and option pricer to market rates of index and tranche swaps. A loss point process model such as a time-changed birth process is speci ed without reference to the portfolio constituents. It is silent about the portfolio constituent risks, and cannot be used to address applications that are based on the relationship between portfolio and component risks, for example constituent risk hedging. The second part of this thesis develops a method that extends the reach of these models to the constituents. We use random thinning to decompose the portfolio intensity into the sum of the constituent intensities. We show that a thinning process, which allocates the portfolio intensity to constituents, uniquely exists and is a probabilistic model for the next-to-default. We derive a formula for the constituent default probability in terms of the thinning process and the portfolio intensity, and develop a semi-analytical transform approach to evaluate it. The formula leads to a calibration scheme for the thinning processes, and an estimation scheme for constituent hedge sensitivities. An empirical analysis for September 2008 shows that the constituent hedges generated by our method outperform the hedges prescribed by the Gaussian copula model, which is widely used in practice.