5.4 Binomial
Option Pricing: Limiting Results

hat happens to the binomial option pricing model when the
period of time becomes smaller and smaller?
In an important result, Cox, Ross, and Rubinstein (1979) demonstrate that
the limit in the multiperiod binomial option pricing model converges upon the
BlackScholes option pricing model, which was derived from a continuous time
framework.
This result illustrates the power of the binomial approach
for modeling option pricing problems. By
the appropriate choice of u, d, and
probability p, different processes can
be converged upon given different assumed structures, which makes this both a
useful theoretical approach and a numerical methods tool.
In the case of the BlackScholes model, this results when the
up and downticks and probability are modeled as:
where n is the number of periods.
Under this structure, the limiting process converges upon the
BlackScholes option pricing model (see Technical
Appendix to Chapter 5: Limiting Results). This option pricing model for a European call option is
where
The nice property that emerges from the binomial derivation
of this pricing model is that the model itself can be interpreted in terms of
the riskneutral probability distributions.
That is, the first term is the expected present value of the stock
conditional on the option finishing inthemoney. The second term is the present value of the expected exercise
cost. In both cases, expectations
are taken with respect to riskneutral probabilities.
It is interesting to note the variables required to value the
option in this closed form solution. These
are: the volatility of the stock
price, the riskfree rate of interest, the time to maturity, the strike price,
and the stock price. The expected
return on the underlying stock is not required because only the riskneutral
probabilities are used to value the option.
Numerical Solutions
The general binomial approach can be applied to find
numerical solutions to a wide range of very complicated option pricing problems.
For example, the problem of valuing an American option can be solved in a
straightforward application of the binomial option pricing model by testing for
the value of early exercise at each node. The
present value of the American option can then be computed as the expected value
over all paths taken with respect to riskneutral probabilities.
In Chapter 6, The BlackScholes Option Pricing Model, we introduce the continuous time approach that BlackScholes originally used to derive their option pricing model.