The Black-Scholes Model
Liuren Wu
Zicklin School of Business, Baruch College
Fall, 2007
(Continuous time finance primer)
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 1 / 57 Outline
1 Brownian motion
2 Ito’s lemma
3 BSM
5 Measure change
6 Dynamic hedging Delta Vega Gamma
7 Static hedging
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 2 / 57 The Black-Scholes-Merton (BSM) model
Black and Scholes (1973) and Merton (1973) derive option prices under the following assumption on the stock price dynamics,
dSt = µSt dt + σSt dWt
The binomial model: Discrete states and discrete time (The number of possible stock prices and time steps are both finite). The BSM model: Continuous states (stock price can be anything between 0 and ∞) and continuous time (time goes continuously). Scholes and Merton won Nobel price. Black passed away. BSM proposed the model for stock option pricing. Later, the model has been extended/twisted to price currency options (Garman&Kohlhagen) and options on futures (Black). I treat all these variations as the same concept and call them indiscriminately the BSM model.
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 3 / 57 Primer on continuous time process
dSt = µSt dt + σSt dWt
The driver of the process is Wt , a Brownian motion, or a Wiener process.
Wt generates random “continuous movements:” Movements arrive continuously, & the sizes of the movements are random and center around zero. Contrast: Merton (1976) proposes a stock price process with both continuous movements (Wt ) and compound Poisson jumps: The jump arrives randomly over time (say on average once every two years). Given one jump occurring, the jump size is randomly distributed. It is appropriate to use Brownian motion to capture daily random price movements, and use the compound Poisson jumps to rare but extreme events (such as market crashes, credit events, etc).
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 4 / 57 Properties of a Brownian motion
dSt = µSt dt + σSt dWt
The process Wt generates a random variable that is normally distributed with mean 0 and variance t, φ(0, t).
The process is composed of independent normal increments dWt ∼ φ(0, dt).
I “d” is the continuous time-time limit of the discrete time difference (∆).
I ∆t denotes a finite time step (say, 3 months), dt denotes an extremely thin slice of time (smaller than 1 milisecond).
I It is so thin that it is often referred to as instantaneous. I Similarly, dWt = Wt+dt − Wt denotes the instantaneous increment (change) of a Brownian motion. By extension, increments over non-overlapping time periods are
independent: For (t1 > t2 > t3), (Wt3 − Wt2 ) ∼ φ(0, t3 − t2) is independent
of (Wt2 − Wt1 ) ∼ φ(0, t2 − t1).
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 5 / 57 Properties of a normally distributed random variable
dSt = µSt dt + σSt dWt
If X ∼ φ(0, 1), then a + bX ∼ φ(a, b2). If y ∼ φ(m, V ), then a + by ∼ φ(a + bm, b2V ).
Since dWt ∼ φ(0, dt), the instantaneous price change 2 2 dSt = µSt dt + σSt dWt ∼ φ(µSt dt, σ St dt). dS 2 The instantaneous return S = µdt + σdWt ∼ φ(µdt, σ dt). 1 dS 2 The annualized instantaneous return dt S ∼ φ(µ, σ ).
I Under the BSM model, µ is the annualized mean of the instantaneous return — instantaneous mean return. 2 I σ is the annualized variance of the instantaneous return — instantaneous return variance.
I σ is the annualized standard deviation of the instantaneous return — instantaneous return volatility.
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 6 / 57 Geometric Brownian motion
dSt /St = µdt + σdWt The stock price is said to follow a geometric Brownian motion. µ is often referred to as the drift, and σ the diffusion of the process. Instantaneously, the stock price change is normally distributed, 2 2 φ(µSt dt, σ St dt). Over longer horizons, the price change is lognormally distributed. The log return (continuous compounded return) is normally distributed over all horizons: 1 2 d ln St = µ − 2 σ dt + σdWt . (By Ito’s lemma, chap 11). 1 2 2 I d ln St ∼ φ(µdt − 2 σ dt, σ dt). 1 2 2 I ln St ∼ φ(ln S0 + µt − 2 σ t, σ t). 1 2 2 I ln ST /St ∼ φ µ − 2 σ (T − t), σ (T − t) .
1 2 µt− 2 σ t+σWt 1 2 Integral form: St = S0e , ln St = ln S0 + µt − 2 σ t + σWt
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 7 / 57 Aggregating normally distributed random variables
If yi is independent of one another and yi ∼ φ(m, V ) for all i, then PN i=1 yi ∼ φ(Nm, NV ). Mean and variance increase with√ time periods N, volatility increases with square root of time periods, N. 1 2 2 Application: Instantaneous return d ln St ∼ φ(µdt − 2 σ dt, σ dt) has identical and independent normal distribution (iid normal), the aggregated return over finite time period (say, ln ST /St ) remains normal with mean and variance proportional to the time period (T − t).
If yi is independent but not identical: yi ∼ φ(mi , Vi ) with (mi , Vi ), the PN distribution of i=1 yi is unknown (no longer normal). 2 2 I Example: The instantaneous price change dSt ∼ φ(µSt dt, σ St dt), independent but not identical (it varies with St level). The aggregated price change is no longer normally distributed.
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 8 / 57 Examples on time aggregation
Suppose daily returns are iid normal with mean 0.05% and standard deviation is 1.5%. What is the distribution of the aggregated return over 1 year (252 business days)?
I The aggregated 1-year return is also normally distributed, and has a mean of 0.05% × 252 = 12.6%. 2 I The annual return has a variance of (1.5%) × 252 = 0.0567. √ I The annual return has a volatility of 1.5% × 252 = 23.81%. Pay attention to the aggregation on volatility.
I Another example: Suppose the annual return volatility for a stock is 20%, under the iid normal assumption, what’s the monthly return volatility? √ Remember: Wt has a volatility of t.
I Which of the following pairs are identical in distribution? (A) (2Wt , W2t ), (B) (4Wt , W2t ), (C) (4 + 4Wt , 2 + W2t ), (D) (4 + 9Wt , 4 + W3t ).
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 9 / 57 Normal versus lognormal distribution
dSt = µSt dt + σSt dWt , µ = 10%, σ = 20%, S0 = 100, t = 1.
2 0.02
1.8 0.018 S =100 1.6 0.016 0 1.4 0.014 ) 0 t /S
t 1.2 0.012
1 0.01
0.8 PDF of S 0.008 PDF of ln(S 0.6 0.006
0.4 0.004
0.2 0.002
0 0 −1 −0.5 0 0.5 1 50 100 150 200 250 ln(S /S ) S t 0 t
The earliest application of Brownian motion to finance is Louis Bachelier in his dissertation (1900) “Theory of Speculation.” He specified the stock price as following a Brownian motion with drift:
dSt = µdt + σdWt
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 10 / 57 Simulate 100 stock price sample paths
dSt = µSt dt + σSt dWt , µ = 10%, σ = 20%, S0 = 100, t = 1.
0.05 200
0.04 180 0.03 160 0.02
0.01 140
0 120 Stock price Daily returns −0.01 100 −0.02 80 −0.03
−0.04 60 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Days days
1 2 Stock with the return process: d ln St = (µ − 2 σ )dt + σdWt . Discretize to daily intervals dt ≈ ∆t = 1/252. Draw standard normal random variables ε(100 × 252) ∼ φ(0, 1). √ 1 2 Convert them into daily log returns: Rd = (µ − 2 σ )∆t + σ ∆tε. P252 R Convert returns into stock price sample paths: St = S0e d=1 d .
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 11 / 57 1 2 µ versus µ − 2σ
Suppose we have daily data for a period of several months Arithmetic mean: If you compute daily percentage return ∆S/S, and take the simple sample average on the return. After annualization, it should be close to µ. " N # 252 1 X ∆Sd µb = 1 N Sd d=1 Geometric mean: If you start at day 1 and compound continuously (or daily, 1 it should be close), the return per year is close to µ − 2 σ. h i1/N 252 ∆S1 ∆S2 ∆SN I Daily: 1 + 1 + ··· 1 + − 1 1 S1 S2 SN n h io n h io 252 1 S1 S2 SN 252 SN I Continuous: ln ··· = ln 1 N S0 S1 SN −1 N S0
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 12 / 57 Example
Suppose that returns in successive years are: 15%, 20%, 30%, -20% and 25%. (0.15+0.20+0.30−0.20+0.25) Arithmetic mean: 5 = 0.14. Geometric mean: [(1 + 0.15) (1 + 0.20) (1 + 0.30) (1 − 0.20) (1 + 0.25)]1/5 − 1 = 0.124. Comments:
I Geometric mean is normally lower than arithmetic mean due to something called “concavity (convexity) adjustment.” I If Rt ∼ φ(m, V ), then E[Rt ] = m, but
R m+ 1 V R 1 E[e t ] = e 2 , ln E[e t ] = m + V 2
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 13 / 57 Ito’s lemma on continuous processes
For a generic continuous process xt ,
dxt = µx dt + σx dWt ,
the transformed variable y = f (x, t) follows,
1 dy = f + f µ + f σ2 dt + f σ dW t t x x 2 xx x x x t Example 1: dSt = µSt dt + σSt dWt and y = ln St . We have 1 2 d ln St = µ − 2 σ dt + σdWt √ √ Example 2: dvt = κ (θ − vt ) dt + ω vt dWt , and σt = vt .
1 1 1 1 dσ = σ−1κθ − κσ − ω2σ−1 dt + ωdW t 2 t 2 t 8 t 2 t
2 Example 3: dxt = −κxt dt + σdWt and vt = a + bxt . ⇒ dvt =?.
Liuren Wu The Black-Scholes Model Option Pricing, Fall, 2007 14 / 57 Ito’s lemma on jumps
For a generic process xt with jumps, R dxt = µx dt + σx dWt + g(z)(µ(dz, dt) − ν(z, t)dzdt) , R = (µx − Et [g]) dt + σx dWt + g(z)µ(dz, dt), where the random counting measure µ(dz, dt) realizes to a nonzero value for a given z if and only if x jumps from xt− to xt = xt− + g(z) at time t. The process ν(z, t)dzdt compensates the jump process so that the last term is the increment of a pure jump martingale. The transformed variable y = f (x, t) follows,