Credit Curve Bootstrapping

Hazard rate and Survival Probability

The reduced-form model that we use here is based on the work of Jarrow and Turnbull (1995), who characterize a credit event as the first event of a Poisson counting process which occurs at some time t with a probability defined as :

Pr[τ < t + dt | τ ≥ t] = λ(t)dt

the probability of a default occurring within the time interval [t,t+dt) conditional on surviving to time t, is proportional to some time dependent function λ(t), known as the hazard rate, and the length of the time interval dt. We make the simplifying assumption that the hazard rate process is deterministic. By extension, this assumption also implies that the hazard rate is independent of interest rates and recovery rates. From this definition, we can calculate the continuous time survival probability to the time T conditional on surviving to the valuation time t_V by considering the limit  → 0. It can be shown that the survival probability is given by:

Q(t_V, T) = exp (−∫_tV^Tλ(s) ds)

Finally, we assume that the hazard rate function is a step-wise constant function. In the below example, the hazard rate between time 0 and 1Y is h_0, 1 = 1% and the hazard rate between between 1Y and 3Y is h_1, 3 = 2.5%. Therefore we have the 1Y survival probability Q_0, 1 = exp(−h_0, 1 × 1) = 99% and the 3Y survival probability Q_1, 3 = Q_0, 1 * exp(−h_1, 3 × 2) = 91.9%.

Valuing the Premium Leg

The premium leg is the series of payments of the default swap spread made to maturity or to the time of the credit event, whichever occurs first. It also includes the payment of premium accrued from the previous premium payment date until the time of the credit event. Assume that there are n = 1, ..., N contractual payment dates t₁, ..., t_N where t_N is the maturity date of the default swap.

The present value of the premium leg is given by:

$\text{PL PV}(t_{V},t_{N})=S(t_{0},t_{N})\sum_{n=1}^{N}\Delta(t_{n-1},t_{n},B)Z(t_{V},t_{n})\left[Q(t_{V},t_{n})+\frac{1_{PA}}{2}(Q(t_{V},t_{n-1})-Q(t_{V},t_{n}))\right]$

where:

• N is the total number of premiums. n = 1, …, N is the index for these premiums; premium dates are denoted t₁, …, t_N. t_N is the maturity date of the swap
• S(t₀, t_N) is the spread of a Credit Default Swap that matures on t_N
• Δ(t_n − 1, t_n, B) is the day count fraction between premium dates t_n − 1 and t_n in the appropriate basis convention denoted by B (ACT/360 in our model)
• Q(t_V, t_n) is the arbitrage-free survival probability of the reference entity from valuation time t_V to premium time t_n
• Z(t_V, t_n) is the risk-free discount factor from valuation date to the premium date n
• 1_PAequals 1 if premium accrued (PA) are included in the premium leg calculation, 0 otherwise.

Valuing the Default Leg

The default leg (or protection leg) is the contingent payment of (100% - R) on the face value of the protection made following the credit event. R is the expected recovery rate. In pricing the default leg, it is important to take into account the timing of the credit event because this can have a significant effect on the present value of the protection leg especially for longer maturity default swaps. Within the hazard rate approach we can solve this timing problem by conditioning on each small time interval [s, s + ds] between time t_V and time t_N at which the credit event can occur. We calculate the expected present value of the recovery payment as:

DL PV(t_V, t_N) = (1 − R)∫_tV^t_NZ(t_V, s)Q(t_V, s)λ(s)ds

The integral makes this expression tedious to evaluate. It is possible to show that we can, without any material loss of accuracy, simply assume that the credit event can only occur on a finite number M of discrete points per year. In this case, the default leg value can be expressed as:

$\text{DL PV}(t_{V},t_{N})=(1-R)\sum_{m=1}^{M\times t_{N}}Z(t_{V},t_{m})\left(Q(t_{V},t_{m-1})-Q(t_{V},t_{m})\right)$

where:

• M is the number of discrete points per year on which we assume a credit event can happen. t_N is the maturity of the swap. m = 1, …, M × t_N is the index the discrete point on which a credit event can happen between the valuation date and the maturity of the swap. We choose M = 12 (monthly discretization) to keep the computation lightweight. D. O'kane and S. Turnbull (2003) compare the results of a daily discretization (M = 365) and a monthly discretization (M = 12) and shows that monthly intervals has an absolute error which is well inside the bid-offer spread; hence monthly discretization can be used to keep the model fast and simple.
• R is the recovery rate
• Q(t_V, t_m) is the arbitrage-free survival probability of the reference entity from valuation time t_V to premium time t_m
• Z(t_V, t_m) is the risk-free discount factor from valuation date to the date m

Calibration of the Credit Curve

Calibration of the model imply finding an hazard rate (non-cumulative hazard rate) function that matches the market CDS spreads. We make the following assumptions:

• Swap premium payments are made quarterly following a business day calendar
• Hazard rate is a piece-wise constant function of time (i.e. hazard rates are independent from interest rates)
• Recovery rate is constant

The construction of the hazard rate term structure is done by an iterative process called bootstrapping. Let’s assume we have quotes for 1Y, 3Y, 5Y and 7Y for a given issuer. From the 1Y CDS spread s_1Y, we will find the hazard rate λ_0, 1 which equates the present value of the premium leg and of the protection leg. From the 3Y spread s_3Y, we will find the hazard rate λ_1, 3 which equates the present value of the premium leg to the present value of the protection leg. By iterating this process, we obtain the hazard rates: λ_0, 1, λ_1, 3, λ_3, 5, λ_5, 7. We can also easily calculate the survival probabilities from this hazard rate term structure (as we have seen earlier).

RadioShack Corp, 27 May 2014

creditcurve_RSH = bootstrapCDS(yieldcurveTenor, yieldcurveRate, 
                   cdsTenor, cdsSpread_RSH,
                   RR, premiumFrequency, defaultFrequency, accruedPremium)

Pfizer Inc, 27 May 2014

creditcurve_PFE = bootstrapCDS(yieldcurveTenor, yieldcurveRate, 
                   cdsTenor, cdsSpread_PFE,
                   RR, premiumFrequency, defaultFrequency, accruedPremium)