Théorie Financière. Financial Options

Transcription

1 Théorie Financière Financial Options Professeur André éfarber

2 Options Objectives for this session: 1. Define options (calls and puts) 2. Analyze terminal payoff 3. Define basic strategies 4. Binomial option pricing model 5. Black Scholes formula Tfin 09 Options (1) 2

3 Theory of asset pricing under uncertainty 1950 Markowitz Portfolio o o theory 1960 Sharpe Lintner CAPM 1970 Black Scholes Merton Ross OPM APT Vasiceck Term structure Ross Risk neutral pricing Cox Ross Rubinstein Binomial OPM Arrow State prices Arrow Debreu General equilibrium Lucas Asset Prices Harrison Kreps Martingales 1980 Theoretical developments in the period since 1979, with relatively few exceptions, have been a mopping-up operation Duffie,D. D Dynamic Asset Pricing Theory, 3d ed. Princeton Universiy Press Cochrane Campbell: p = E(MX) Tfin 09 Options (1) 3

4 Definitions A call (put) contract gives gvesto the eowner the right : to buy (sell) an underlying asset (stocks, bonds, portfolios,..) on or before some future date (maturity) on : "European" option before: "American" option ataprice set in advance (the exercise price or striking price) Buyer pays a premium to the seller (writer) Tfin 09 Options (1) 4

5 Terminal Payoff: European call Exerciseoption if, at maturity: Stock price > Exercice price S T > K Profit at maturity Call value at maturity C T =S T -KifS T >K otherwise: C T =0 C T = MAX(0, S T - K) - Premium K S T Striking Stock price price Tfin 09 Options (1) 5

6 Terminal Payoff: European put Exercise option if, at maturity: Stock price < Exercice price < K S T Value / profit at maturity Put value at maturity P T = K - S T if S T < K otherwise: P T = 0 P T = MAX(0, K- S T ) Value Profit K Striking price Premium S T Stock price Tfin 09 Options (1) 6

7 The Put-Call Parity relation A relationship between European put Value at maturity and call prices on the same stock Compare 2 strategies: Strategy 1. Buy 1 share + 1 put At maturity T: S T <K S T >K Share value S T S T Put value (K - S T ) 0 K Total value K S T Put = insurance contract K S T Tfin 09 Options (1) 7

8 Put-Call Parity y( (2) Consider an alternative strategy: Value at maturity Strategy 2: Buy call, invest PV(K) Strategy 2 Call At maturity T: S T <K S T >K Call value 0 S T -K Invesmt K K Total value K S T At maturity, both strategies lead to the same terminal value K Investme Stock + Put = Call + Exercise price K S T Tfin 09 Options (1) 8

9 Put-Call Parity y( (3) Two equivalent e strategies es should have the esame cost S + P = C + PV(K) where S current stock price P current put value C current call value PV(K) present value of the striking price This is the put-call parity relation Another presentation of the same relation: C = S + P - PV(K) A call is equivalent to a purchase of stock and a put financed by borrowing the PV(K) Tfin 09 Options (1) 9

10 Valuing option contracts The intuition t behind the teoptiono pricing formulas can be introduced toducedin a two-state option model (binomial model). LetS be the current price of a non-dividend paying stock. Suppose that, over a period of time (say 6 months), the stock price can either increase (to us, u>1) or decrease (to ds, d<1). Consider a K = 100 call with 1-period to maturity. us = 125 C u = 25 S = 100 C ds = 80 C d = 0 Tfin 09 Options (1) 10

11 Key idea underlying option pricing models It is possible to create ceateaa synthetic call that replicates the future value of the te call option as follow: Buy Delta shares Borrow B at the riskless rate r (5% per annum simple interest) Choose Delta and B so that the future value of this portfolio is equal to the value of the call option. Delta us -(1+rΔt) B = C u Delta B = 25 Delta ds -(1+rΔt) B = C d Delta B = 0 (Δt is the length of the time period (in years) e.g. : 6-month means Δt=0.5) Tfin 09 Options (1) 11

12 No arbitrage condition In a perfect capital market, the value of the call should then be equal to the value of its synthetic reproduction, otherwise arbitrage would be possible: C = Delta S - B This is the Black Scholes formula We now have 2 equations with 2 unknowns to solve. [Eq1]-[Eq2] Delta (125-80) = 25 Delta = Replace Delta by its value in [Eq2] B = Call value: C = Delta S - B = C = Tfin 09 Options (1) 12

13 A closed form solution for the 1-period binomial model C=[p C u + (1-p) C d ]/(1+rΔt) with p =(1+rΔt - d)/(u-d) p is the probability of a stock price increase in a "risk neutral world" where the expected return is equal to the risk free rate. In a risk neutral world : p us + (1-p) ds = 1+rΔt p C u + (1-p) C d is the expected value of the call option one period later assuming risk neutrality The current value is obtained by discounting this expected value (in a risk neutral world) at the risk-free rate. Tfin 09 Options (1) 13

14 Risk neutral pricing illustrated Inour example,the e,t epossible returns are: ae: + 25% if stock up - 20% if stock down In a risk-neutral world, the expected return for 6-month is 5% 0.5= 2.5% The risk-neutral probability should satisfy the equation: p (+0.25%) + (1-p) (-0.20%) = 2.5% p =0.50 The call value is then: C = / = Tfin 09 Options (1) 14

15 Multi-period model: European option For European option, o follow owsame procedure pocedue (1) Calculate, at maturity, - the different possible stock prices; - the corresponding values of the call option - the risk neutral probabilities (2) Calculatelate the expected call value in a neutral world (3) Discount at the risk-free rate Tfin 09 Options (1) 15

16 An example: valuing a 1-year call option Same data as before: e: S=100, K=100, r=5%, u =1.25, d=0.80 Call maturity = 1 year (2-period) Stock price evolution Risk-neutral proba. Call value t=0 t=1 t= p² = p(1-p) = (1-p)² = Current call value : C = / (1.025)² = Tfin 09 Options (1) 16

17 Volatility The value a call option, o is a function of the following ow gvariables: ab 1. The current stock price S 2. The exercise price K 3. The time to expiration date T 4. The risk-free interest rate r 5. The volatility ofthe underlying ingasset σ Note:In the binomial model, uandd capture the volatility (the standard deviation of the return) of the underlying stock Technically, u and d are given by the following formulas: u Δt = e σ 1 d = u Tfin 09 Options (1) 17

18 Option values are increasing functions of volatility The value of a call or of a put option o is in increasing function ucto of volatility ty (for all other variable unchanged) Intuition: a larger volatility increases possibles gains without affecting loss (i (since thevalue of an option isnevernegative) Check: previous 1-period binomial example for different volatilities Volatility u d C P (S=100, K=100, r=5%, Δt=0.5) Tfin 09 Options (1) 18

19 Black-Scholes formula For European call on non dividend de d paying stocks s The limiting case of the binomial model for Δtverysmall C = SN(d 1 )-PV(K) N(d 2 ) Delta In BS: PV(K) present value of K (discounted at the risk-free rate) ln( Delta = N(d( 1 ) PV ( K d N(): cumulative probability of the standardized normal distribution B σ S T ) ) 1 = + 0.5σ T B =PV(K) N(d 2 ) d d σ T 2 = 1 Tfin 09 Options (1) 19

20 Black-Scholes : numerical example 2determinants ts of call value: Moneyness : S/PV(K) Cumulative volatility : Example: S = 100, K = 100, Maturity T = 4, Volatility σ = 30% r =6% Moneyness = 100/(100/ ) = 100/79.2= Cumulative volatility=30% x 4 = 60% d 1 =ln(1.2625)/0.6 + (0.5)(0.60) ) =0.688 N(d 1 ) = d 2 = ln(1.2625)/0.6 - (0.5)(0.60) =0.089 N(d 2 ) = C = (100) (0.754) (79.20) (0.535) = Tfin 09 Options (1) 20

21 Cumulative normal distribution This table shows values for N(x) for x For x<0, N(x) = 1 N(-x) Examples: N(1.22) = 0.889, N(-0.60) = 1 N(0.60) = = In Excell, use Normsdist() function to obtain N(x) Tfin 09 Options (1) 21

22 Black-Scholes illustrated Upper bound Stock price Lower bound Intrinsic value Max(0,S-K) Action Option Valeur intrinséque Tfin 09 Options (1) 22