CIR 2 - Modeling Correlated Defaults Flashcards
Two main Credit Default models analyzed in Blum 2
- Bernoulli Model (L): Moody’s KMV / Risk Metrics / Most Internal Bank Models
- Poisson Model (L’ ): CreditRisk+
Total defaults for Bernoulli loss statistic L
Loss Percentage for Bernoulli loss statistic L
Properties of Bernoulli Loss Statistic
Meaning of Uniform default probability p in (bernoulli loss statistic)
Means the probability of default of each counterparty is the same, pi = p for all i.
Overview of General Bernoulli Mixture Model
- Specifies explicit dependencies b/w counterparties L’s
- Loss probabilites are random variables w/ distribution F
- Conditional on P, L’s are independent
General Bernoulli Mixture Model distribution and moments
Covariance derivation under the General Bernoulli Mixture Model
Uniform Default Probability and Uniform Correlation
- called uniform portfolios
- Works best for portfolios with all exposures are of approximately the same size and type of risk
Correlation formula for uniform Bernoulli portfolio
Correlation cases for uniform Bernoulli portfolio
- A correlation of 0 happens if and only if there is no randomness at all regarding P. In this case, there is a binomial distribution with default probability p bar
- A correlation of 1 happens with the “rigid” behavior that either:
- All counterparties default
- Or all counterparties survive simultaneously
- Most realistic scenarios woud have a correlation strictly between 0 and 1
Stability of Poisson Models
The sum of independent poisson models is poisson where you just add the underlying parameter values
Overview of General Poisson Mixture Model
Correlation formula for uniform Poisson portfolio
Dispersion ratio of a random variable
- ratio of the variance devided by the mean
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Overview of common credit models
Overview of Log-return model in Moody’s KMV and RiskMetrics
One-year default probability of firm i conditional on composite factor (Moody’s KMV and RiskMetrics)
Overview of Composite Factor in Log-return model (Moody’s KMV and RiskMetrics’)
- Composite factor captures the level of general market risk, or nonidiosyncratic
risk - Broken into indices (e.g. industry and country indices)
- Weighted average of inices
Overview of CreditRisk+
- analyzes risk factors by sector indexed by the variable s (in contrast with Moody’s which is a factor model)
- each sector has a default intensity following gamma distribution
- Sectors’ default intensities are independent
- two firms are correlated if and only if there is at least one sector where
both firms have a positive sector weight- two firms admit a common source of systematic default risk
- Default risk of firm i is modeled with a mixed Poisson L’i with random intensity lamdai following a gamma distribution
Default intensity formula of Firm i in CreditRisk+ model
- s is the sector index
- ms is the total number of sectors
- wis is the weight in firm i and sector s
- lamdai is the mean default intensity of firm i
- lamda(s) is the mean default intensity parameter for sector s
- Lamdai is the default intensity of Firm i
- Lamda(s) is the default intensity for sector s (random realization)
Overview of Credit Portfolio View (CPV)
- Rating-based approach to modeling dependence of default and migration probabilities on the economic cyle
- Often focuses on migrantion matrices
- Obeserved migration matrix in a given year is conditional
- Can average conditional to get unconditional migration matrices
- Can construct a migration matrix using a 3 Step Shift Algorithm
Step Shift Algorithm to Construct a Migration Matrix in CPV
m¯is is the probability of moving from credit state i to j by the end of the year
Overview of CPV Macro
- Focuses on time series of empirical data using macroeconomic regression
- Macroeconomic variables drive the distribution of default probabilities and migration matrices for each risk segment
- Examples of macroeconomic data:
- GDP
- Unemployement Rate
- Interest Rates
- Currency Exchange Rates