Ch 12. Interest Rate and Credit Models

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Ch 12. Interest Rate and Credit Models Ch 12. Interest Rate and Credit Models I. Equilibrium Interest Rate Models II. No-Arbitrage Interest Rate Models III. Forward Rate Models IV. Credit Risk Models • This chapter introduces interest rate models first. Two categories of the interest rate models, the equilibrium and no-arbitrage models, will be introduced. In addition, the forward rate models is also discussed, in which the risk factor is the instantaneous forward rate rather than the instantaneous short rate in the interest rate models. Finally, two classical credit risk models, the reduced-form and structural models, are introduced. I. Equilibrium Interest Rate Models • After the emergence of the Black-Scholes model, some academics try to model the bond price following the lognormal or normal distribution and thus derive the pricing formulae of the bond options following the method in the Black-Scholes model. However, the critical problem of these model is that the bond price is neither lognormally nor normally distributed. • Furthermore, for all bonds and bonds options, their prices are affected by the change of the term structure of interest rates, which implies that they share a common source of risk factor. So it is inappropriate to model the movement of the price of each bond separately, i.e., the price of each bond is assumed to follow an individual stochastic process. • It is known that there exist a relationship between the bond price and the interest rate theoretically. Hence, some academics turn to consider the stochastic process of interest rates. They try to model the short rate process dr, which is by definition the instantaneous interest rate for an infinitesimal period of time. • It is worth noting that any interest rate we can observe in the market is associated with a time to maturity and thus the interest rates we can observe in the market is called the long rates. Unlike the long rate, the short rate is unobservable. 12-1 • To model the short rate, there are two streams of models: the equilibrium and no-arbitrage models. In this section, two famous equilibrium interest rate models are introduced: the Vasicek and Cox-Ingersoll-Ross (CIR) models. • Vasicek model dr = β(µ − r)dt + σdZ, where µ is the long-term mean of r, and β measures the speed of mean reversion. By performing the stochastic integral, we can express r(T ) as follows. −β(T −t) β(T −t) R T β(τ−t) r(T ) = e (r(t) + µ(e − 1) + σ e dZ(τ)); t where r(t) is the level of short rate today. From the above equation, we know that r(T ) is normally distributed, and −β(T −t) Et[r(T )] = µ + (r(t) − µ)e ; σ2 −2β(T −t) vart(r(T )) = 2β (1 − e ): ∗ Denote P to be the current price of a zero coupon bond with 1 dollar payoff at maturity, and P is a function of t (the time point today), r, and T (the maturity date). According to the It^o'sLemma, @P @P 1 2 @2P @P dP (t; r; T ) = ( @t + β(µ − r) @r + 2 σ @r2 )dt + σ @r dZ ≡ P · µ(t; r; T )dt + P · σ(t; r; T )dZ. (Note that the maturity date T is a constant in the bond contract, so it is not necessary to consider the differentiation with respect to T in the It^o'sLemma.) longing σ(t; r; T )P (t; r; T ) shares of P (t; r; T ) Construct a portfolio W by 2 2 1 ; shorting σ(t; r; T1)P (t; r; T1) shares of P (t; r; T2) ) dW = f[σ(t; r; T2)P (t; r; T2)]µ(t; r; T1)P (t; r; T1)−[σ(t; r; T1)P (t; r; T1)]µ(t; r; T2)P (t; r; T2)gdt = r(t)W dt = r(t)[σ(t; r; T2)P (t; r; T2)P (t; r; T1)−σ(t; r; T1)P (t; r; T1)P (t; r; T2)]dt ) µ(t; r; T1)σ(t; r; T2) − r(t)σ(t; r; T2) = µ(t; r; T2)σ(t; r; T1) − r(t)σ(t; r; T1) ) µ(t;r;T1)−r(t) = µ(t;r;T2)−r(t) = λ σ(t;r;T1) σ(t;r;T2) (For zero-coupon bonds with different maturity dates, the ratio of their excess returns over volatilities are identical. It is reasonable since all these bonds share a common risk factor dr and they should be with the same market price of risk, which is, by definition, the (excess return)-volatility ratio of any derivatives based on r.) ) µ(t; r; T ) − r(t) = λσ(t; r; T ). Substitute µ(t; r; T ) and σ(t; r; T ) of dP (t; r; T ) to derive 2 ( @P +β(µ−r) @P + 1 σ2 @ P ) @P @t @r 2 @r2 σ @r P − r(t) = λ P 12-2 @P @P 1 2 @2P ) @t + (β(µ − r) − λσ) @r + 2 σ @r2 = rP . (Based on the Vasicek short rate model, the price of any zero coupon bond should satisfy the above partial differential equation.) ∗ To solve the above partial differential equation, we can derive the theoretical value of any zero coupon bond as follows. P (t; r; T ) = A(t; T )e−B(t;T )r(t); where r(t) is the current short rate level, and 2 ( (B(t;T )−T +t)(β2µ− σ ) σ2B(t;T )2 exp[ 2 + σλ (B(t; T ) − (T − t)) − ] if β 6= 0 A(t; T ) = β2 β 4β , σ2(T −t)3 exp[ 6 ] if β = 0 −β(T −t) 1−e if β 6= 0 B(t; T ) = β . T − t if β = 0 ∗ Note that in the risk-neutral world, the market price of risk, λ, is zero, and the above pricing formula becomes identical to that presented in Hull (2011). ∗ The formula of the zero coupon bond price P (t; r; T ) makes it possible to derive the whole term structure based on the Vasicek short rate process. This is because given the zero coupon bond price P (t; r; T ) and the identity function P (t; r; T ) = exp(−r(t; T )(T − t)), the zero rate corresponding to the maturity date T , r(t; T ), can be derived through r(t; T ) = − ln(P (t; r; T ))=(T − t). ∗ In a word, once you specify the values of the parameters β, µ, and σ and the intial value of the short rate r(t), you can derive a corresponding term structure. The Vasicek model can generate normal, inverted, and humped-shape term structures. ∗ One possible way to calibrate the estimations of β, µ, σ, and r(t) is to employ the least squares approach such that the generated term structure can best fit the real term sturcture in the market. 12-3 The binomial tree to simulate the Vasicek interest rate process: Since the volatility term is constant in the Vasicek model, it is straightforward to apply the method in Cox and Rubinstein (1985) to constructing the corresponding binomial tree. Figure 12-1 r t r r t p 1 β(µ−r) ∆t p(r) = 2 + 2σ . If p(r) is not in [0; 1], consider a smaller ∆t to fix this problem. When ∆t approaches zero, p(r) will approach 0:5. • Cox-Ingersoll-Ross (CIR) model p dr = β(µ − r)dt + σ rdZ −β(T −t) Et[r(T )] = µ + (r(t) − µ)e ; σ2 −β(T −t) −2β(T −t) σ2 −β(T −t) 2 vart(r(T )) = r(t) β [e − e ] + µ 2β (1 − e ) : ∗ In comparison with the Vasicek interest rate process, the only difference is the volatility term, which is from a constant σ to become σpr(t). The modification decreases the volatility of the short rate when the interest rate is low. When the short rate reaches zero (the volatility term approaching zero as well), the positive drift makes the subsequent short rate become positive. Hence, the CIR model remedies the negative interest rate problem in the Vasicek model. ∗ Technically speaking, if σ2 > 2βµ, r(t) can reach zero; if σ2 ≤ 2βµ, the upward drift is sufficiently large to make the origin inaccessible. ∗ It is worth noting that Cox, Ingersoll, and Ross (1985) do not develop their model by simply modifying the Vasicek interest rate process. Instead, they derive this interest rate model based on a general equilibrium asset pricing model. ∗ In addition, the CIR process can be defined as the sum of squared Ornstein-Uhlinbeck processes as follows. X 1 1 r(t) = X2(t), where dX (t) = − αX (t)dt + σdZ (t): i i 2 i 2 i Consequently, the short rate r(t) follows a noncentral Chi-square distribution and thus is always positive. 12-4 ∗ Following the similar procedure in the Vasicek model and the assumption of λ = 0, the partial differential equation of the zero coupon bond based on the CIR model is as follows: @P @P 1 2 @2P @t + β(µ − r) @r + 2 σ r @r2 = rP: Solve the above partial differential equation, the theoretical value of the zero-coupon bond today is P (t; r; T ) = A(t; T )e−B(t;T )r(t), where r(t) is the short rate level today, and 2υe(β+υ)(T −t)=2 2βµ/σ2 A(t; T ) = [ (β+υ)[eυ(T −t)−1]+2υ ] ; 2[eυ(T −t)−1] B(t; T ) = (β+υ)[eυ(T −t)−1]+2υ ; υ = pβ2 + 2σ2: The binomial tree approach to simulate the CIR interest rate process: Because the CIR model is with a non-constant volatility, we need to apply the method in Nelson and Ramaswamy (1990) to construct the corresponding binomial tree.
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