Two Alternative Binomial Option Pricing Model Approaches to Derive Black-Scholes Option Pricing Model* CHENG-FEW LEE Department of Finance and Economics, Rutgers Business School Rutgers University, New Brunswick New Jersey, U.S. CARL S. LIN Department of Economics Rutger University, New Brunswick New Jersey, U.S. Abstract In this chapter, we review two famous models on binomial option pricing, Rendleman and Barter (RB, 1979) and Cox, Ross, and Rubinstein (CRR, 1979). We show that the limiting results of the two models both lead to the celebrated Black-Scholes formula. From our detailed derivations, CRR is easy to follow if one has the advanced level knowledge in probability theory but the assumptions on the model parameters make its applications limited. On the other hand, RB model is intuitive and does not require higher level knowledge in probability theory. Nevertheless, the derivations of RB model are more complicated and tedious. For readers who are interested in the binomial option pricing model, they can compare the two different approaches and find the best one which fits their interests and is easier to follow. 1. Introduction The main purpose of this chapter is to review two famous binomial option pricing model: Rendleman and Barter (RB, 1979) and Cox, Ross, and Rubinstein (CRR, 1979). First, we will give an alternative detailed derivation of the two models and show that the limiting results of the two models both lead to the celebrated * Section 3 of this chapter is essentially drawing from the paper by Lee et al.(2004). 1 Black-Scholes formula. Then we will make comparisons of the two different approaches and analyze the advantages of each approach. Hence, this chapter can help to understand the statistical aspects of option pricing models for Economics and Finance professions. Also, it gives important financial and economic intuitions for readers in statistics professions. Therefore, by showing two alternative binomial option pricing models approaches to derive the Black-Scholes model, this chapter is useful for understanding the relationship between the two important optional pricing models and the Black-Scholes formula. 2. The Two-State Option Pricing Model of Rendleman and Bartter In Rendleman and Bartter (1979), a stock price can either advance or decline during the next period. Let H T and H T represent the returns per dollar invested in the stock if the price rises (the + state) or falls (the - state), respectively, from time T-1 to time T. And VT and VT the corresponding end-of-period values of the option. Let R be the riskless interest rate, Rendleman and Bartter (1979) show that the price of the option can be represented as a recursive form PT 1 V (1 R HT ) VT ( HT 1 R) ( HT HT )(1 R) 2 that can be applied at any time T-1 to determine the price of the option as a function of its value at time T. 2.1 The Discrete Time Model From the above equation, the value of a call option at maturing date T-1 is given by WT 1 WT (1 R H ) WT ( H (1 R)) ( H H )(1 R) (2.1) WT1 (1 R H ) WT1 ( H (1 R)) ( H H )(1 R) (2.2) Similarly, WT 2 Substituting (2.1) into (2.2) can get, WT 2 (WT (1 R H ) WT ( H (1 R)))(1 R H ) ( H H )2 (1 R)2 (WT (1 R H ) WT ( H (1 R)))( H (1 R)) ( H H )2 (1 R)2 (2.3) Noting that WT WT , so (2.3) can be simplified as: WT 2 (WT (1 R H )2 2WT ( H (1 R)))(1 R H ) WT ( H (1 R)) 2 ( H H )2 (1 R)2 (2.4) We can use this recursive form to get W0 : T Since after T periods, there are ways that a sequence of (T) pluses can occur, 0 3 T T ways that (T-1) pluses can occur, ways that (T-2) pluses can occur, and so 1 2 on…. Hence, by Binomial Theorem W0 can be represented as: W0 T [ WT...... (1 R H )T ( H (1 R ))0 0 T WT...... (1 R H )T 1 ( H (1 R ))1 1 T WT..... (1 R H )T 2 ( H (1 R )) 2 2 T .... (1 R H )1 ( H (1 R ))T 1 WT T 1 T WT...... (1 R H )0 ( H (1 R ))T ] T (2.5) [( H H )(1 R )]T Next to determine the value of the option at maturity. Suppose that stock increases i T i times and declines (T i ) times, then the price of the stock will be S0 H H i on the expiration date. So the option will be exercised if T i S0 H H i X The maturity value of the option will be WT S0 H i 4 T H i X (2.6) Let a denote the minimum integer value of i in (2.6) for which the inequality is satisfied. l n X ( ) T lHn ( S 0 a 1 INT l nH l H n ) (2.7) where INT [] is the integer operator. i.e., taking natural logarithm of RHS of (2.6), ln S0 i ln H (T i ) ln H ln X i ln H i ln H ln( X ) T ln H S0 X ) T ln H S0 i ln H ln H ln( Hence, the maturing value of the option is given by T WT S0 H H i WT 0 if i i if i X a (2.8) a Substituting (2.8) into (2.5), then the generalized option pricing equation for the discrete time is T W0 T i (S H i a i 0 T H X ) (1 R H i ( H H )T (1 R 2.2 The Continuous Time Model For (2.9), we can write is as: 5 )H ( (R1 i T ) T i )) (2.9) T T T i T i i T i i T i ( S H H )(1 R H ) ( H (1 R )) 0 X (1 R H ) ( H (1 R)) i i i a i a W0 T ( H H ) (1 R )T T T T S0 [ H (1 R H )]i [ H ( H 1 R )]T i ia i ( H H )T (1 R)T T T X (1 R H )i ( H (1 R))T i ia i ( H H )T (1 R)T i T (1 R H ) H ( H 1 R ) H S0 i a i (1 R )( H H ) ( H H )(1 R ) T X (1 R)T T (1 R H ) ia i ( H H ) T i ( H 1 R) (H H ) T i T i (2.10) Since (1 R H ) H ( H 1 R) H (1 R H ) ( H 1 R) + , 1 1 , (1 R)( H H ) ( H H )(1 R) (H H ) (H H ) therefore, can interpret it as “pseudo probability”. Let (1 R H ) H (1 R H ) and , we can restate (2.10) as: (1 R)( H H ) (H H ) W0 S0 B(a, T , ) X B ( a, T , ) (1 R)T (2.11) where B (a, T , ()) is the cumulative binomial probability function, the number of successes will fall between a and T after T trials. As T , W0 S0 N ( Z1 , Z1) X N ( Z 2 , Z 2 ) (1 R)T (2.12) where N ( Z , Z ) is the probability of a normally distributed random variable with zero mean and variance 1 taking values between a lower limit Z and a upper limit Z . 6 And by the property of binomial pdf, Z1 a T T T , Z1 T (1 ) T (1 ) Z2 a T T T , Z 2 T (1 ) T (1 ) Thus, W0 S0 (lim Z1 , lim Z1) T T X N (lim Z 2 , lim Z 2 ) T T lim(1 R)T (2.13) T Let 1 R e r T , then lim(1 R)T er . And since lim Z1 lim Z 2 , the T T T remaining things to be determined are lim Z1 and lim Z 2 . T T From equation (10) and (11) of the text in the Rendleman and Bartter (1979), ( T T ( T T H e H e ) ) (1 ) (1 ) substituting H and H into (2.7), so Z1 a T T (1 ) X ln( ) T S0 1 1 INT T (1 ) T T (1 ) In the limit, the term 1 INT [] will be simplify to [] . So, 7 ln( Z1 X ) S0 T (1 ) 1 T (1 ) X ) T S0 1 (1 ) T (1 ) (1 ) ln( ln( X ) S0 (1 ) (1 ) T T 1 1 T (1 ) T (1 ) 1 T T (1 ) T (1 ) T 1 T (1 ) T (1 ) T (1 ) X ) T T (1 ) S0 T 1 (1 ) T (1 ) T (1 ) (1 ) ln( X ) S0 T ( ) (1 ) (1 ) (1 ) ln( Substituting H , H and 1 R e (2.14) r T into , 8 (1 R H ) H (1 R )( H H ) 1 r ( ) ( ) T 1 T T T T e e e 1 r ( ) ( ) T T 1 T T T e e e r ( T T e e r ( ) T e e T ( e T ) ) ( e e ( T T ) T T ) 1 e 1 e ( ) ( ) T 1 T T 1 ) 1 ) r eT 1 1 1 ( e T ) 1 ( e ( e T ( ( T ) 1 T ) 1 1 r ( T )( 1 ) 1 e T T 1 ( T ) ( ) e T 1 e Now expanding in Taylor’s series in T, 9 1 o( ) 1 T 1 1 1 1 T ( ) ( ) o( ) 1 2T 1 T 1 1 r 2( ) 2 O( ) 2 1 T 1 1 1 1 T ( ) ( ) o( ) 1 2T 1 T T 1 r 2( ) 2 1 2 1 T 1 1 2 1 1 ( ) ( )( ) o( ) 1 2 T 1 T 1 1 o( ) T 1 1 1 1 ) ( )( ) o( ) ( 1 2 T 1 T where, H 1 T T 1 3! T T 1 T T 1 1 2! T T 1 2 3 1 1 2 1 1 o 2T T and H 1 T T 1 3! T T 1 T T 1 1 2! T T 1 2 3 1 1 2 1 o 2T 1 T 1 1 where o( ) denotes a function tending to zero more rapidly than .(when we T T 10 expanding in Taylor’s series in T, the rest of the terms tending to zero more rapidly than 1 1 so regard them as a function o( ) .) Hence, T T lim T 1 1 1 1 (1 ) 1 1 (1 ) 2 and, 11 lim T ( ) T 1 lim T [ T T 1 r 2( ) 2 2 1 1 1 2 1 1 ( ) ( )( ) o( ) 1 2 T 1 T 1 1 o( )] T 1 1 1 1 ) ( )( ) o( ) ( 1 2 T 1 T 1 r 2( ) 2 2 1 lim[ T T 1 1 2 1 1 ( ) ( )( ) o( ) 1 2 T 1 T T 1 1 T o( )] T 1 1 1 1 ) ( )( ) o( ) ( 1 2 T 1 T 1 T ( lim T ( 1 1 2 1 1 ) ( )( ) o( ) T 1 2 T 1 T 1 1 2 1 1 ) ( )( ) o( ) 1 2 T 1 T 1 r 2( ) 2 1 2 1 T o( ) 2 T 1 1 1 1 ( ) ( )( ) o( ) 1 2 T 1 T 1 2 1 1 ( ) (r 2 ( ) 2) 2 1 2 1 1 ( ) 1 1 2 (1 ) 2 2 1 ( ) r 2( ) 2 2 (1 ) 2 1 1 (1 ) 1 2 1 2 1 ( ) r 2( ) 2 1 2 1 2 1 (1 ) 1 2 1 1 2 1 ( ) ( ) r 2( ) 2 1 2 1 2 1 2 1 (1 ) 12 After canceling terms, 1 (1 )(r 2 ) 2 lim T ( ) T Now substituting lim for and lim T ( ) for T T ln( lim Z1 T ln( X ) S0 (1 ) (1 ) T ( ) into (2.14), 1 2 (1 )(r 2 ) (1 ) X 1 )r 2 S0 2 Similarly, ln( lim Z 2 X 1 )r 2 S0 2 T Since N ( Z , ) N (, Z ) , let D1 lim Z1 , D2 lim Z 2 , the continuous time T T version of the two-state model is obtained: w0 S0 N (, D1 ) Xe r N (, D2 ) ln( D1 X 1 )r 2 S0 2 D2 D1 The above equation is identical to the Black-Scholes model. 13 3. The Binomial Option Pricing Model of Cox, Ross and Rubinstein In this section we will concentrate on the limiting behavior of the binomial option pricing model proposed by Cox, Ross and Rubinstein (CRR, 1979). 3.1 The Binomial Option Pricing Formula of CRR Let S be the current stock price, K the option exercise price, R 1 the riskless rate. It is assumed that the stock follows a binomial process, from one period to the next it can only go up by a factor of u with probability p or go down by a factor of d with probability (1 p ) . After n periods to maturity, CRR showed that the option price C is: C 1 Rn n n! k!(n k )! p k (1 p ) n k Max[0, u k d n k S K ] . (3.1) k 0 An alternative expression for C, which is easier to evaluate, is n! u k d nk K n n! p k (1 p) n k ] n [ p k (1 p) n k ] R R k m k!(n k )! k m k!( n k )! K SB(m; n, p ) n B(m; n, p). R n C S[ (3.2) n where B(m; n, p ) n C k p k (1 p) n k and m is the minimum number of upward k m stock movements necessary for the option to terminate in the money, i.e., m is the minimum value of k in (3.1) such that u m d n -m S - X 0. 14 3.2 Limiting Case We now show that the binomial option pricing formula as given in Equation (3.2) will converge to the celebrated Black-Scholes option pricing model. The Black-Scholes formula is C S N (d1 ) e rt X N (d1 t ) (3.3) where d1 S ) XR t t 2 t log( (3.4) 2 = the variance of stock rate of return t = the fixed length of calendar time to expiration date, such that h t . n We wish to show that Equation (3.2) will coincide with Equation (3.3) when n . In order to show the limiting result that the binomial option pricing formula converges to the continuous version of Black-Scholes option pricing formula, we suppose that h represents the lapsed time between successive stock price changes. Thus, if t is the fixed length of calendar time to expiration, and n is the total number of periods each with length h, then h t . n As the trading frequency increases, h will get closer to zero. When h 0 , this is equivalent to n . 15 Let R̂ be one plus the interest rate over a trading period of length h. Then, we will have Rˆ T Rt (3.5) t for any choice of n. Thus, Rˆ R n , which shows that R̂ must depend on n for the total return over elapsed time t to be independent of n. Also, in the limit, (1 r ) n tends to e rt as n . Let S* be the stock price at the end of the nth period with the initial price S. If there are j up periods, then log S* u j log u (n j ) log d j log( ) n log d S d (3.6) where j is the number of upward moves during the n periods. Since j is the realization of a binomial random variable with probability of a success being q, we have expectation of log (S*/S) E (log S* u ) [q log( ) log d ]n ˆn, S d (3.7) and its variance Var (log S* u ) [log( )] 2 q(1 q)n ˆ 2 n. S d (3.8) Since we divide up our original longer time period t into many shorter subperiods 16 of length h so that t hn , our procedure calls for making n longer, while keeping the length t fixed. In the limiting process we would want the mean and the variance of the continuously compounded log rate of return of the assumed stock price movement to coincide with that of actual stock price as n . ̂ 2 n respectively. Let the actual values of ̂ n and Then we want to choose u, d, and q in such a manner that ˆ n t and ˆ 2 n 2 t as n . ue d e q t n It can be shown that if we set , t n (3.9) , 1 1 t , 2 2 n then ˆ n t and ˆ 2 n 2 t as n . In order to proceed further, we need the following version of the central limit theorem. Lyapounov’s Condition. Suppose X 1 , X 2 , are independent and uniformly bounded with E ( X i ) 0 , Yn X 1 X n, and s 2 E (Yn2 ) Var (Yn ). If lim n n 1 k 1 2 n s E Xk 2 0 for some 0, then the distribution of to the standard normal as n . Theorem 1. If 17 Yn converges sn p log u ˆ (1 p) log d ˆ 3 ˆ 3 n 3 0 as n (3.10) then S* log( ) ˆn S Pr z N ( z) ˆ n (3.11) where N(z) is the cumulative standard normal distribution function. Proof. See Appendix. It is noted that the condition (3.10) is a special case of the Lyapounov’s condition which is stated as follows. When 1, we have the condition (3.10). This theorem says that when the fixed length t is divided into many subperiods, the log rate of return will approach to the normal distribution when the number of subperiods approached infinity. For this theorem to hold, the condition stated in Equation (3.10) has to be satisfied. We next show that this condition is indeed satisfied. We will next show that the binomial option pricing model as given in Equation (3.2) will indeed coincide with the Black-Scholes option pricing formula as given in Equation (3.3). Observe that Rˆ n is always equal to R t , as evidenced from Equation (3.5). Thus, comparing the two option pricing formulae given in Equations 18 (3.2) and (3.3), we see that there are apparent similarities. In order to show the limiting result, we need to show that as n , B(m; n, p ) N ( x) and B(m; n, p) N ( x t ). In this section we will only show the second convergence result, as the same argument will hold true for the first convergence. From the definition of B (n, pm) , it is clear that 1 B(m; n, p) Pr( j m 1) j np m 1 np Pr( ). np(1 p) np(1 p) (3.12) Recall that we consider a stock to move from S to uS with probability p and dS with probability (1-p). During the fixed calendar period of t=nh with n subperiods of length h, if there are j up moves, then S* u log j log( ) n log d . S d (3.13) The mean and variance of the continuously compounded rate of return for this stock are ̂ P and ˆ P2 where u d u d ˆ P p log( ) log d and ˆ P2 [log( )] 2 p(1 p) . From Equation (3.13) and the definitions for ̂ P and ˆ P2 , we have 19 j np np(1 p) log( S* ) ˆ P n S . ˆ P n (3.14) Also, from the binomial option pricing formula we have X ) n Sd m 1 u log( ) d X u [log n log d ] / log , S d log( where is a real number between 0 and 1. From the definitions of ̂ P and ˆ P2 , it is easy to show that m 1 np np(1 p) log( X u ) ˆ P n log( ) S d . ˆ P n Thus from Equation (3.12) we have log 1 B(m; n, p ) Pr( S* X u ˆ P n log ˆ P n log( ) S S d ). ˆ P n ˆ P n (3.15) We will now check the condition given by Equation (3.10) in order to apply the central limit theorem. p Now recall that rˆ d , ud t n with rˆ r , and d and u are given in Equation (3.9). 20 We have p e t log r n t n e t n t n e e 3 t t 1 2t 1 log r [1 ] O(n 2 ) n n 2 n 3 t t 1 [1 ] O(n 2 ) n n 1 log r 2 1 1 2 ] t O( n 1 ). [ 2 2 n (3.16) Hence, the condition given by Equation (10) is satisfied because p | log u ˆ p | 3 (1 p) | log d ˆ p | 3 ˆ 3p n (1 p) 2 p 2 np(1 p) 0, as n . Finally, in order to apply the central limit theorem, we have to evaluate ̂ p n , u d ̂ 2p n and log( ) as n . It is clear that 1 2 u d ˆ p n (log r 2 )t , ˆ 2p n 2 t and log( ) 0 . Hence, in order to evaluate the asymptotic probability in Equation (3.12), we have log( X u X 1 ) ˆ p n log( ) log( ) (log r 2 )t S d z S 2 . ˆ p n t Using the fact that 1 N ( z ) N ( z ) , we have, as n B(m; n, p) N ( z ) N ( x t ). 21 Similar argument holds for B (m; n, p ' ) , and hence we completed the proof that the binomial option pricing formula as given in equation (3.2) includes the Block-Scholes option pricing formula as a limiting case. 4. Comparison of the Two Approaches From the results of last two sections, we show that both RB and CRR models lead to the celebrated Black-Scholes formula. The following table shows the comparisons of the necessary mathematical and statistical knowledge and assumptions for the two models. Model Mathematical and Probability Theory Knowledge Rendleman and Bartter (1979) Cox, Ross and Rubinstein (1979) Basic Algebra Taylor Expansion Binomial Theorem Central Limit Theorem Properties of Binomial Distribution Basic Algebra Taylor Expansion Binomial Theorem Central Limit Theorem Properties of Binomial Distribution Lyapounov’s Condition 1. The distribution of returns of the The Assumption Advantage stock follows a binomial stock is stationary over time and the process from one period to the next stock pays no dividends.(Discrete it can only go up by a factor of “u” Time Model) with probability “p” or go down by a factor of “d” with probability “1-p”. 2. The mean and variance of In order to apply the Central Limit logarithmic returns of the stock are Theorem, “u”, “d”, and “p” are held constant over the life of the needed to be chosen. option.(Continuous Time Model) 1. Readers who have undergraduate 1. Readers who have advanced level 22 and level training in mathematics and knowledge in probability theory can Disadvantage probability theory can follow this approach. 2. The approach of RB is intuitive. But the derivation is more complicated and tedious than the approach of CRR. follow this approach; but for those who don’t, CRR approach may be difficult to follow. 2. The assumption on the parameters “u”, “d”, “p” makes CRR approach more restricted than RB approach. Hence, like we indicate in the table, CRR is easy to follow if one has the advanced level knowledge in probability theory but the assumptions on the model parameters make its applications limited. On the other hand, RB model is intuitive and does not require higher level knowledge in probability theory. However, the derivation is more complicated and tedious. For readers who are interested in the binomial option pricing model, they can compare the two different approaches and find the best one which fits their interests and is easier to follow. Appendix The Binomial Theorem n n ( x y )n k 0 x k y n k k Lindberg-Levy Central Limit Theorem 23 If x1 ,..., xn are a random sample from a probability distribution with finite mean 1 n and finite variance 2 and x ( ) i 1 xi , then n d n ( xn ) N 0, 2 Proof of Theorem 1. Since 3 p log u ˆ p log u p log 3 u u log d p (1 p )3 log d d 3 And 3 3 u u (1 p) log d ˆ (1 p ) log d p log log d p 3 (1 p ) log , d d 3 We have p log u ˆ (1 p) log d ˆ p(1 p)[(1 p) 2 p 2 ] log 3 3 u d 3 . Thus p log u ˆ (1 p ) log d ˆ 3 3 ˆ 3 n p (1 p )[(1 p ) 2 p 2 ] log u d 3 u ( p (1 p ) log( ))3 n d 2 2 (1 p ) p 0 as n . np (1 p ) Hence the condition for the theorem to hold as stated in Equation (3.10) is satisfied. 24 References Amram, M. and N. Kulatilaka, Real Options, Oxford University Press, USA, 2001. Banz R. and M. 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