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Black & Scholes : a first attempt
Issued on July 08 2011 par Strategies Options

This is 'the' standard option pricing model
Probably the most known formula in finance.


I - Some assumptions :

1- no tax
2- no transaction cost
3- underlying can be short sold
4- volatility and interest rates are known
5- the market is open 24/7




II - 2 variables and 5 parameters

Variables
t the starting date
S the spot

Parameters
K the strike
r continuously compounded annualized interest rate
q continuously compounded annualized dividend rate
T expiry (in years)
σ the annualized volatility



III - Pricing

For a maturity of τ = T - t ,

C = exp ( - q.τ ) . S . N( d1 ) - exp ( - r.τ ) . K . N( d2)


With
d1 = [ Ln( S/K ) + ( ( r-q + 0.5σ² ).τ )] / ( σ√τ )
d2 = [ Ln( S/K ) + ( ( r-q - 0.5σ² ).τ )] / ( σ√τ ) = d1 - ( σ√τ )

N(.) Cumulative Normal Denstity
N( d1 ) = ∫ [ ((1 / ( √2п )) . exp( -z²/2 ) ] dz, derived between –inf and d1


It follows for the put,

P = - exp ( - q.τ ) . S . N( - d1 ) + exp ( - r.τ ) . K . N( - d2 )


Example :
S = 100, K = 100, r = 5%, σ = 30%, q = 0, T = 1 year

d1 = [ Ln( 100 / 100 ) + ( ( 5% + 0.5.(30%)² ). 1 )] / ( 30%√1 ) = 0.316667
d2 = [ Ln( 100 / 100 ) + ( ( 5% - 0.5.(30%)² ).1 )] / ( 30%√1 ) = 0.016667

N(d1) = 0.624252
N(d2) = 0.506649

C = exp ( - (0).(1) ) . 100 . 0.624252 - exp ( - 5%.(1) ) . 100 . 0.506649
C = 62.4252 - (0.9512).(100).(0.506649)
C = 62.4252 - 48.193918
C = 14.231255

N( - d1) = 0.375748
N( - d2) = 0.493351

P = - exp ( - (0).(1) ) . 100 . (0.375748) + exp ( - 5%.1 ) . 100 . (0.493351)
P = - 37.5748 + (0.9512).(100).(0.493351)
P = -375748 + 46.929024
P = 9.354197



IV - Graphs

For a call:


For a put:



Next : Black & Scholes : A Standard Option Pricing Model ( Part 1 )
Black & Scholes: The Greeks

Previous: Les Modèles : Besoin D'un Cadre Pour évaluer Les Produits Dérivés


Pdf connexes :

- Understanding N(d1) and N(d2) : Risk-Adjusted Probabilities in the Black-Scholes Model
- Black-Scholes Option Pricing Model


OPTIONS PRICING MODEL - INDEX
OPTIONS PRICING MODEL - INDEX
OPTIONS PRICING MODEL - CHAPTER I
OPTIONS PRICING MODEL - CHAPTER II
OPTIONS PRICING MODEL - CHAPTER III

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