+Normative vs positive models
Normative: explain how prices should behave (ex-ante)
Positive: explain how prices actually behave (ex-post)
Theoretical vs Empirical models
Theoretical: based on assumptions about behavior/processes
Empirical: based on observed behavior/data (better at dealing with complexity)
Applied vs abstract models
Applied: Address real-world problems
Abstract: Address hypothetical situations
Cross-Sectional vs Time-Series models
Cross-Sectional models: describe behavior across many assets for a single point in time.
Time-Series models: describe behavior across for a single asset over a long period of time.
Interest Rate Term Structure Modeling (equilibrium and arbitrage-free models)
Equilibrium Models: assume short-term interest rate follows random process (e.g. Vasicek, Cox-Ingersoll Ross)
-> these don’t actually explain bond prices!
Arbitrage-Free Models: Currently observed term structure of rates is used to calibrate model (e.g. Ho and Lee, Black Dermot Toy (BDT))
Vasicek’s interest rate model
Mean-reverting process for short-term rate of interest
R(t+1) = Rt + κ (μ-Rt) + σε(t+1)
Issues:
- Not calibrated to yield curve and cannot explain bond prices
- Allows rates to go negative
Cox, Ingersoll and Ross interest rate model
Slight adjustment to Vasicek’s model:
R(t+1) = Rt + κ (μ-Rt) + √Rt*σε(t+1)
As Rt approaches 0, and √Rt approaches 0 (faster), process gets less volatile and mean-reversion term dominates.
Issues:
- Not calibrated to yield curve and cannot explain bond prices
Ho and Lee interest rate model
R(t+1) = Rt + θt + σε(t+1)
θt = time-dependent change in the rate: ensures the model explains empirically observed yield curves
Black-Derman-Toy (BDT) model
Adverse selection in the context of credit risk
Information asymmetry: borrower knows more than lender about their credit quality
Moral hazards in the context of credit risk
Lender faces consequences of risks taken by borrower
Expected credit loss
Exp. credit loss = P(default) * LGD
LGD = loss given default
Loss given default (LGD)
LGD = EAD * (1-RR)
EAD = exposure at default (principal + interest)
RR = recovery rate
Recovery rate
RR = PV (recovered cash) / EAD
EAD = exposure at default (principle + interest)
Three approaches to credit risk modeling
Structural: relates default to capital structure (e.g. Merton, KMV)
Reduced-form: models default as a random event (looks at the data of the results as opposed to what is structurally happening)
Empirical: credit scoring based on historical data
Describing a credit-risky bond under Merton’s structural model
Credit risky bond = Rf bond - put (on the company’s assets)
= (So + P - C) - P
= So - C
= covered call
Four important properties of the Merton Model regarding credit risky bonds:
1) Impact of an increase in maturity?
2) Impact of an increase in vol?
3) Impact of an increase in leverage?
4) Impact of an increase in the risk-free rate?
1) Maturity ↑, P(default) ↑, Credit spread ↑ initially and then ↓
2) Asset Vol ↑, P(default) ↑, Credit spread ↑
3) Leverage ↑, P(default) ↑, Credit spread ↑
4) Rf rate ↑, RoA ↑, P(default) ↓, Credit spread ↓
Advantages and disadvantages for using structural models to price credit risk bonds
✔
- Uses liquid market data
- Prices securities with different seniority and/or conversion options
❌
- Equity prices can be irrational
- Not accurate for short term or high quality bonds
- Data on liabilities can be unreliable
Expected time to default
1/λ
where λ = ‘default intensity’
Fair credit spread using a recovery rate (RR)
λ(1-RR)
where λ = ‘default intensity’
Jarrow-Turnbull reduced-form model
Duffie-Singleton reduced-form model
Empiric Credit Models
Credit scoring using historic data
Five input variables for Altman’s Z-score (Empirical credit model)
When Repaid Everyone Must Smile
The higher the value the less likely for there to be a default.
Working capital = Current assets - current liabilities
TA = Total Assets.