Calibrating the Black-Derman-Toy Model
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AppliedMathematical Finance 8,27–48 (2001) Calibrating the Black–Derman– Toy model: some theoretical results PHELIM P.BOYLE, KEN SENG TAN andWEIDONG TIAN Universityof Waterloo,Ontario, Canada, N2L 3G1 ReceivedJune 2000 TheBlack– Derman– Toy (BDT) modelis a popularone-factor interest rate model that is widely used by practitioners. Oneof itsadvantages is that the model can be calibratedto both the current market term structure of interestrate and thecurrent term structure of volatilities.The input term structure of volatilitycan be eitherthe short term volatility or theyield volatility. Sandmann and Sondermann derived conditions for the calibration to be feasible when the conditionalshort rate volatility is used.In this paper conditions are investigated under which calibration to the yield volatilityis feasible. Mathematical conditions for this to happen are derived. The restrictions in this case are more complicatedthan when the short rate volatilities are used since the calibration at each time step now involves the solutionof two non-linear equations. The theoretical results are illustrated by showing numerically that in certain situationsthe calibration based on the yield volatility breaks down for apparently plausible inputs. In implementing the calibrationfrom period n to period n +1,thecorresponding yield volatility has to lie within certain bounds. Under certaincircumstances these bounds become very tight. For yield volatilities that violate these bounds, the computed shortrates for the period ( n, n +1)either become negative or else explode and this feature corresponds to the economicintuition behind the breakdown. Keywords: interestrate models, Black– Derman– T oymodel, volatility, short term, yield 1.Introduction Themodern approach to the modelling of stochastic interest rates started with the classic paper by Vasicek(1977). The early models postulated a stochasticdifferential equation for theevolution of theshort rate of interest and, by invoking no-arbitrage arguments, developed expressions for theprices ofpure discount bond and other securities of interest such as options. Other examples of such modelsinclude Brennan and Schwartz (1979) and Cox, Ingersoll and Ross (1985). One of theproblems withthese models was thatthey did not have enough degrees of freedom to match the model prices ofpure discount bonds with the corresponding market prices. This was anuncomfortable situation Theauthors are gratefulto Baoyan Ding, Fred Guan, Houben Huang, George Lai, David Li, Chonghui Liu, and Ken Vetzal foruseful discussion.Phelim P .Boylethanks the Social Science andHumanities Research Councilof Canada andthe Natural Sciences and EngineeringResearch Councilof Canada,for research support.Ken SengTan acknowledges the research supportfrom the Natural Sciences andEngineering Research Councilof Canada.Weidong Tian thanks CITO (Communicationsand Information Technology Ontario)for research support. AppliedMathematical Finance ISSN1350-486X print/ ISSN1466-4313 online # 2001Taylor & FrancisLtd http://www.tandf.co.uk/journals DOI: 10.1080/13504860110062049 28 Boyle,T anand Tian sincethe model could not be described as arbitrage free whenthe model prices differed from thecurrent marketprices. Oneway to resolve this problem is to makethe parameters of theshort term rate time dependent. In the Vasicekcase, the resulting model is known as the extended V asicekmodel. This approach has been popularizedby Hull and White (1993). Another approach is to model the uncertainty by assuming a stochasticprocess for theevolution of the forward rate.Since this approach starts from thecurrent observedforward rate,the market prices of today’ s zero-couponbonds are built directly into the foundationsof themodel. The rst paperto applythis approach was Hoand Lee (1986) in a discrete-time framework.Subsequently Heath, Jarrow andMorton (1990) (HJM) provideda muchmore extensive and rigorousapproach in continuous-timeframework. The HJM modelis quite general; it canbe calibratedto currentbond prices and option prices. In practice, the model parameters are selected by ttingmodel pricesto the current market prices of the most liquid instruments. From atradingperspective, this approachis useful since the model reproduces market prices over for themost liquid traded securities. 1 For manypricing applications, it is convenient to have a simplebinomial type model that tsthe currentterm structure of bondprices and the current term structure of volatilities.In this connection, the Black,Derman and Toy (1990) (BDT) modelis awidelyused model that can be calibratedto matchthe termstructure of zero-couponbond prices and the term structure of volatilities.In the original BDT paper, theauthors used the yield volatilities as the input term structure of volatility. The yield volatility correspondsto the volatility of the yields on long term bonds. In practical applications, it is often more convenientto use the term structure of shortrate volatilities since they can be directly inferred from the marketprices of interestrate caps. Thus,there are two ways to calibrate the BDT model.The rst oneis the short rate volatility method which uses thecurrent term structure of zerocoupon bond prices; theterm structure of futureshort rate volatilities. Thesecond one is the yield rate volatility method which uses thecurrent term structure of zerocoupon bond prices; theterm structure of yieldson zero coupon bonds. For eachof theseapproaches it is of interestto investigate the conditions under which ttingthe BDT model results in a reasonablecalibration .Our conditionsfor areasonablecalibration are quite weak. W ewillrequire thatall the short rates and all the output volatilities 2 inthe calibrated BDT modelare positive. Sandmann and Sondermann(1993) have analysed the case when the BDT calibrationis based on theshort rate volatilities. Theyprovide necessary and suf cient conditions for thisto happen. Their result is: if the current implied forward ratesare all positive (i.e. the pure discount bond price declines as the time to maturity increases) and theshort rate volatilities are all positive, then it is possible to calibrate the BDT model.Their conditions are simplefrom amathematicalperspective and have an intuitive economic interpretation. 1 Thisapproach has disadvantagesfrom an econometric perspective since, by ttingnew parameters ona dailybasis, we are effectivelyassuming a new modelevery day. 2 Theoutput volatility under the rst approachis theyield volatility as computedfrom the calibrated BDT modelsince theinput volatilityis theshort rate volatility.The output volatility under the second approach is theshort rate volatilityas computedfrom the calibratedBDT modelsince theinput volatility is theyield rate volatility. Calibratingthe Black– Derman– Toy model 29 Thispaper investigates the conditions under which we cancalibrate the BDT modelwhen the yield volatilityis used. W ederivethe precise mathematical conditions which the input data must satisfy so that the BDT modelcan be calibrated.These conditions are less elegant than in the case when the short rate volatility isused. However ,we ndthatit isnot possible to calibrate the BDT modelfor seeminglyplausible input termstructures. T oobtainour conditions we useresults from thetheory of polynomialequations. Theoutline of the rest of the paper is as follows. In the next section, we reviewthe details of the procedurethat is used to calibrate a BDT model.W eexamineboth the original and modi ed BDT models whichcalibrate to the term structure of yieldvolatilities and short rate volatilities respectively. W eshow thatthe calibration equations can be reducedto a systemof polynomialequations so that we candraw on resultsfrom theso-called Quanti er Elimination;an algebraic approach that provides conditions for polynomialsto have real roots. In Section 3, we providea detailedanalysis of thecalibration of athree- periodBDT model.W eprovideboth necessary and suf cient conditions for thecalibration to be feasible. Theconditions are quite complicated even in the three period case and it appears dif cult to extend this typeof analysis to the n periodscase. Hence in Section4, we providea sufcient condition for calibration tobe feasible from the nthstep to the ( n +1)thstep given that the calibration was successfulfor the preceding n periods.In Section 5 we provideseveral examples which illustrate the conditions developed inSection 4. The nalsection concludes the paper. 2.Calibration ofthe BDT model Inthis section, we reviewthe procedure used to calibrate a BDT interestrate model. In this model, a recombiningbinomial lattice is constructed so that it matches the current yield curve and the current yield volatilitycurve. W eassumethe calibrated binomial lattice has N periodsand each period is of size t years.Hence the total time horizon of thebinomial lattice is T = N t years.The recombining nature of thebinomial lattice ensures that at time period n, there are n +1states.W elabelthese states as i = 0, 1, . ., n. Let r(n, i)bethe(annualized) one-period short rate at period n and state i.Theshort rate r(n, i) evolves either to r(n + 1, i)(i.e.down-state) or to r(n + 1, i +1)(i.e. up-state) one period hence with equal risk-neutralprobability. Let Y^(0, n) Y^(n)bethecurrent (market) yield on a n-periodzero-coupon bond (i.e. with maturity n t) and ^Y (n)bethe corresponding current yield volatility. Then the price of an n-periodzero-coupon bond, P^(0, n) P^(n), isgiven as n P^(n) 1 Y^(n) t ¡ (1) ˆ ‰ ‡ Š Similarly,let P (n) and Y (n)denotethe model