Black-Scholes-Merton (BSM) Option Pricing Model 40 th Anniversary Conference. The Recovery Theorem

Transcription

1 Black-Scholes-Merton (BSM) Option Pricing Model 40 th Anniversary Conference The Recovery Theorem October 2, 2013 Whitehead Institute, MIT Steve Ross Franco Modigliani Professor of Financial Economics MIT Managing Partner Ross, Jeffrey & Antle LLC

2 The BSM Model The modern theory of derivatives indeed, the core of modern finance began 40 years ago with the BSM model Arguably the BSM model is the most successful model in economics and, in fact, in all of the social sciences BSM s comment that the model might have some implications for corporate finance is eerily similar to the comment of Watson and Crick that their work on DNA might have some implications for biology The BSM model was certainly the impetus for the development of risk neutral pricing by John Cox and myself and of the binomial model by John and me and Mark Rubinstein To fully understand the BSM model it s important to understand it in it s historical context At the time options markets were only embryonic and the goal of models was to derive prices essentially in the absence of large well functioning markets That meant deriving option prices from what was observed The motivation for the work presented here is the inverse problem, i.e., given option prices what can we say about market parameters that are not directly observed? Page 4

3 Forecasting the Equity Markets For equities we don t even ask the market for a single spot forecast like we do with forward interest rates In the equity markets we: Ø Use historical market returns and the historical premium over risk-free returns to predict future returns Ø Build a model (e.g., a dividend/yield model) to predict stock returns Ø Survey market participants and institutional peers Ø Use the martingale measure from options or some ad hoc adjustment to it as though it was the same as the natural probability distribution We use forward rates to extract some forecasts in the fixed income markets We want to see if we can use options to do this for other markets Page 5

4 The Binomial Model In a binomial model stocks follow the simple process: S f 1 - f as bs b < 1 + r < a where f is the natural jump probability and r is the risk free interest rate The absence of arbitrage implies the existence of positive (Arrow Debreu (AD)) prices for pure contingent claims, i.e., digitals that pay $1 in state a or b: where p(a)= ( 1/1+r )π π= (1+r) b/a b and p(b)= ( 1/1+r )(1 π) and 1 π = a (1+r)/a b Page 6

5 The BSM Model The binomial model converges to a lognormal diffusion as the step size gets smaller ds/s = μdt+ σdz The binomial formula for the price of an option also converges to the famous BSM risk neutral differential equation 1/2 σ 2 S 2 2 P/ S 2 +rs P/ S rp+ P/ t =0 yielding the equally famous BSM solution for a call option P(S,t)=SN( d 1 ) K e r(t t) N( d 2 ) where N( ) is the cumulative normal, K is the strike, and d 1 = ln ( S/K ) +(r+( 1/2 ) σ 2 )(T t) /σ T t and d 2 = d 1 σ T t Page 7

6 BSM Solution The Good and the Bad As expected, the BSM solution is also the discounted expected value of the payoff on a call using the risk neutral probability distribution, i.e., under the assumption that the stock drift is the risk free rate Since the expected return on the stock, µ, doesn t enter the formula, option prices are independent of the expected return of the stock This is both a blessing and a curse; It means that we can price derivatives without knowing the very difficult to measure expected return But, conversely, the BSM model is of no use for determining the expected return The challenge, then, is to build a different class of models that will have the ability to link option prices to the underlying parameters of the stock process while at the same time being consistent with risk neutral pricing Page 8

7 The Options Market A put option is insurance against a market decline with a deductible; if the market drops by more than the strike, then the put option will pay the excess of the decline over the strike, i.e., the strike acts like a deductible A call option on the market is a security with a specified strike price and maturity, say one year, that pays the difference between the market and the strike iff the market is above the strike in one year The markets use the BSM formula to quote option prices, and volatility is an input into that formula The implied volatility is the volatility that the stock must have to reconcile the BSM formula with the market price Notice that the market isn t necessarily using the BSM formula to price options, only to quote their prices Page 9

8 The Volatility Surface Surface date: January 6, 2012 Page 10

9 Risk Aversion and Contingent Forward Prices Like any insurance, put prices are a product of three effects: Put price = Discount Rate x Risk Aversion x Probability of a Crash But which is it how much of a high price comes from high risk aversion and how much from a higher chance of a crash? A contingent forward security pays off if the market is at, say, j = 1900, one month from now when the market is currently at, say, i = 1500 and is also a form of insurance, : Contingent Forward Price (i,j) = δ x ϕ(i,j) x f ij where δ is the market s average discount rate, ϕ(i,j) is risk aversion, and f ij is the natural probability of transiting from state i to state j Assuming that risk aversion is a simple ratio: ϕ(i,j) = R(j)/R(i) we have all the ingredients that will allow us to recover the market s distribution for future returns, the f ij, and risk aversion from the contingent forward prices - there are (almost) as many above equations as there are unknowns Page 11

10 Applying the Recovery Theorem A Three Step Procedure Step 1: Step 2: Step 3: Pick a date and use option prices to determine the forward prices Estimate the contingent forward prices Apply the Recovery Theorem and solve for the unknown probabilities and the risk aversions The next slides compare these predictions for a given date with the distribution obtained by bootstrapping monthly historical data Page 12

11 The Forward Price Surface Priced using the SPX volatility surface from April 27, 2011 Page 13

12 Risk Neutral Density May 1, 2009 Page 14

13 Recovered Market Natural Density May 1, 2009 Page 15

14 Recovered Probabilities vs. Historical Probabilities May 1, 2009 Page 16

15 The Bootstrapped (Historical) and Recovered Probabilities On May 1, 2009 for May 1, 2010 Market Historical Recovered Scenario Bootstrap Probabili7es - 50% % % % % % % % % % % % % Page 17

16 Recovered (May 1, 2009) and Historical Statistics Historical Recovered mean 8.40% 9.81% excess return 3.06% 7.13% sigma 15.30% 16.28% Sharpe rf 5.34% 1.08% divyld 3.25% 2.67% ATM ivol 32.84% Page 18

17 Actual Volatility vs. 3 month lagged Recovered Volatility (May 1, 2009 May 1, 2013) Page 19

18 Quarterly Lagged Recovered Regression Adjusted Expected Return vs. Quarterly S&P 500 Return (May 1, 2009 April 1, 2013) Page 20

19 Regression of S&P 500 Quarterly Return on Recovered Quarterly Expected Return (May 1, 2009 April 1, 2013) Regression Statistics Multiple R R Square Adjusted R Standard E Observatio 46 ANOVA df SS MS F ignificance F Regression Residual Total Coefficientstandard Erro t Stat P- value Lower 95%Upper 95%ower 95.0%Upper 95.0% Intercept X Variable Page 21

20 Some Applications and a To Do List We should test the method by recovering the distribution at a series of historical dates, and then compare those forecasts with the actual subsequent outcomes We should also compare the recovered predictions with the historical distributions and with other economic and capital market factors to find potential hedge and/or leading/lagging indicator relationships Extending the analysis to the fixed income markets is already underway by Peter Carr and his coauthors and in joint research by myself and Ian Martin of Stanford Publish a monthly report on the current recovered characteristics of the stock return distribution: Ø The forecast equity risk premium Ø The chance of a catastrophe or a boom Give a final thanks to Bob and Myron and Fischer, too, for giving us all so much to work with Page 22