Fi8000 Option Valuation II

1
Fi8000Option Valuation II

• Milind Shrikhande

2
Valuation of Options

• Arbitrage Restrictions on the Values of Options
• Quantitative Pricing Models
• Binomial model
• A formula in the simple case
• An algorithm in the general
• Black-Scholes model (a formula)

3
Binomial Option Pricing Model

• Assumptions
• A single period
• Two dates time t0 and time t1 (expiration)
• The future (time 1) stock price has only two
  possible values
• The price can go up or down
• The perfect market assumptions
• No transactions costs, borrowing and lending at
  the risk free interest rate, no taxes

4
Binomial Option Pricing ModelExample

• The stock price

• Assume S 50,
• u 10 and d (-3)

Su55
SuS(1u)
S50
S
Sd48.5
SdS(1d)
5
Binomial Option Pricing ModelExample

• The call option price

• Assume X 50,
• T 1 year (expiration)

Cu 5 Max55-50,0
Cu MaxSu-X,0
C
C
Cd 0 Max48.5-50,0
Cd MaxSd-X,0
6
Binomial Option Pricing ModelExample

• The bond price

• Assume r 6

1.06
(1r)
1
1
1.06
(1r)
7
Replicating Portfolio

• At time t0, we can create a portfolio of N
  shares of the stock and an investment of B
  dollars in the risk-free bond. The payoff of the
  portfolio will replicate the t1 payoffs of the
  call option
• N55 B1.06 5
• N48.5 B1.06 0
• Obviously, this portfolio should also have the
  same price as the call option at t0
• N50 B1 C
• We get N0.7692, B(-35.1959) and the call option
  price is C3.2656.

8
A Different Replication

• The price of 1 in the up state

• The price of 1 in the down state

0
1
qd
qu
1
0
9
Replicating Portfolios Using the State Prices

• We can replicate the t1 payoffs of the stock and
  the bond using the state prices
• qu55 qd48.5 50
• qu1.06 qd1.06 1
• Obviously, once we solve for the two state prices
  we can price any other asset in that economy. In
  particular we can price the call option
• qu5 qd0 C
• We get qu0.6531, qd 0.2903 and the call option
  price is C3.2656.

10
Binomial Option Pricing ModelExample

• The put option price

• Assume X 50,
• T 1 year (expiration)

Pu 0 Max50-55,0
Pu MaxX-Su,0
P
P
Pd 1.5 Max50-48.5,0
Pd MaxX-Sd,0
11
Replicating Portfolios Using the State Prices

• We can replicate the t1 payoffs of the stock and
  the bond using the state prices
• qu55 qd48.5 50
• qu1.06 qd1.06 1
• But the assets are exactly the same and so are
  the state prices. The put option price is
• qu0 qd1.5 P
• We get qu0.6531, qd 0.2903 and the put option
  price is P0.4354.

12
Two Period Example

• Assume that the current stock price is 50, and
  it can either go up 10 or down 3 in each
  period.
• The one period risk-free interest rate is 6.
• What is the price of a European call option on
  that stock, with an exercise price of 50 and
  expiration in two periods?

13
The Stock Price

• S 50, u 10 and d (-3)

Suu60.5
Su55
SudSdu53.35
S50
Sd48.5
Sdd47.05
14
The Bond Price

• r 6 (for each period)

1.1236
1.06
1.1236
1
1.06
1.1236
15
The Call Option Price

• X 50 and T 2 periods

CuuMax60.5-50,010.5
Cu
CudMax53.35-50,03.35
C
Cd
CddMax47.05-50,00
16
State Prices in the Two Period Tree

• We can replicate the t1 payoffs of the stock and
  the bond using the state prices
• qu55 qd48.5 50
• qu1.06 qd1.06 1
• Note that if u, d and r are the same, our
  solution for the state prices will not change
  (regardless of the price levels of the stock and
  the bond)
• quS(1u) qdS (1d) S
• qu (1r)t qd (1r)t (1r)(t-1)
• Therefore, we can use the same state-prices in
  every part of the tree.

17
The Call Option Price

• qu 0.6531 and qd 0.2903

Cuu10.5
Cu
Cud3.35
C
Cd
Cdd0
Cu 0.653110.5 0.29033.35 7.83 Cd
0.65313.35 0.29030.00 2.19 C
0.65317.83 0.29032.19 5.75
18
Two Period Example

• What is the price of a European put option on
  that stock, with an exercise price of 50 and
  expiration in two periods?
• What is the price of an American call option on
  that stock, with an exercise price of 50 and
  expiration in two periods?
• What is the price of an American put option on
  that stock, with an exercise price of 50 and
  expiration in two periods?

19
Two Period Example

• European put option - use the tree or the
  put-call parity
• What is the price of an American call option - if
  there are no dividends
• American put option use the tree

20
The European Put Option Price

• qu 0.6531 and qd 0.2903

Puu0
Pu
Pud0
P
Pd
Pdd2.955
Pu 0.65310 0.29030 0 Pd
0.65310 0.29032.955 0.858 PEU
0.65310 0.29030.858 0.249
21
The American Put Option Price

• qu 0.6531 and qd 0.2903

Puu0
Pu
Pud0
PAm
Pd
Pdd2.955
22
American Put Option

• Note that at time t1 the option buyer will
  decide whether to exercise the option or keep it
  till expiration.
• If the payoff from immediate exercise is higher
  than the option value the optimal strategy is to
  exercise
• If Max X-Su,0 gt Pu(European) gt Exercise

23
The American Put Option Price

• qu 0.6531 and qd 0.2903

Puu0
Pu
Pud0
P
Pd
Pdd2.955
Pu Max 0.65310 0.29030 , 50-55 0 Pd
Max 0.65310 0.29032.955 , 50-48.5
50-48.5 1.5 PAm Max 0.65310 0.29031.5
, 50-50 0.4354 gt 0.2490 PEu
24
Determinants of the Valuesof Call and Put Options
Variable C Call Value P Put Value
S stock price Increase Decrease
X exercise price Decrease Increase
s stock price volatility Increase Increase
T time to expiration Increase Increase
r risk-free interest rate Increase Decrease
Div dividend payouts Decrease Increase
25
Black-Scholes Model

• Developed around 1970
• Closed form, analytical pricing model
• An equation
• Can be calculated easily and quickly (using a
  computer or even a calculator)
• The limit of the binomial model if we are making
  the number of periods infinitely large and every
  period very small continuous time
• Crucial assumptions
• The risk free interest rate and the stock price
  volatility are constant over the life of the
  option.

26
Black-Scholes Model

• C call premium
• S stock price
• X exercise price
• T time to expiration
• r the interest rate
• s std of stock returns
• ln(z) natural log of z
• e-rT exp-rT (2.7183)-rT
• N(z) standard normal
• cumulative probability

27
The N(0,1) Distribution
pdf(z)
N(z)
µz0
z
28
Black-Scholes example

• C ?
• S 47.50
• X 50
• T 0.25 years
• r 0.05 (5 annual rate
• compounded
• continuously)
• s 0.30 (or 30)

29
Black-Scholes Example

• C ?
• S 47.50
• X 50
• T 0.25 years
• r 0.05 (5 annual rate
• compounded
• continuously)
• s 0.30 (or 30)

30
Black-Scholes Model

• Continuous time and therefore continuous
  compounding
• N(d) loosely speaking, N(d) is the risk
  adjusted probability that the call option will
  expire in the money (check the pricing for the
  extreme cases 0 and 1)
• ln(S/X) approximately, a percentage measure of
  option moneyness

31
Black-Scholes Model

• P Put premium
• S stock price
• X exercise price
• T time to expiration
• r the interest rate
• s std of stock returns
• ln(z) natural log of z
• e-rT exp-rT (2.7183)-rT
• N(z) standard normal
• cumulative
• probability

32
Black-Scholes Example

• P ?
• S 47.50
• X 50
• T 0.25 years
• r 0.05 (5 annual rate
• compounded
• continuously)
• s 0.30 (or 30)

33
The Put Call Parity

• The continuous time version (continuous
  compounding)

34
Stock Return Volatility

• One approach
• Calculate an estimate of the volatility using the
  historical stock returns and plug it in the
  option formula to get pricing

35
Stock Return Volatility

• Another approach
• Calculate the stock return volatility implied by
  the option price observed in the market
• (a trial and error algorithm)

36
Option Price and Volatility

• Let s1 lt s 2 be two possible, yet different
  return volatilities
• C1, C2 be the appropriate call option prices and
• P1, P2 be the appropriate put option prices.
• We assume that the options are European, on the
  same stock S that pays no dividends, with the
  same expiration date T.
• Note that our estimate of the stock return
  volatility changes. The two different prices are
  of the same option, and can not exist at the same
  time!
• Then,
• C1 C2 and P1 P2

37
Implied Volatility - example

• C 2.5
• S 47.50
• X 50
• T 0.25 years
• r 0.05 (5 annual rate
• compounded
• continuously)
• s ?

38
Implied Volatility - example

• C ?
• S 47.50
• X 50
• T 0.25 years
• r 0.05 (5 annual rate
• compounded
• continuously)
• s 0.30 (or 30)

39
Implied Volatility - example

• C 2.5 gt 2.0526
• S 47.50
• X 50
• T 0.25 years
• r 0.05 (5 annual rate
• compounded
• continuously)
• s lt or gt 0.3?

40
Implied Volatility - example

• C ?
• S 47.50
• X 50
• T 0.25 years
• r 0.05 (5 annual rate
• compounded
• continuously)
• s 0.40 (or 40)

41
Implied Volatility - example

• C ?
• S 47.50
• X 50
• T 0.25 years
• r 0.05 (5 annual rate
• compounded
• continuously)
• s 0.35 (or 35)

42
Application Portfolio Insurance

• Options can be used to guarantee minimum returns
  from an investment in stocks.
• Purchasing portfolio insurance (protective put
  strategy)
• Long one stock
• Buy a put option on one stock
• If no put option exists, use a stock and
• a bond to replicate the put option
  payoffs.

43
Portfolio Insurance Example

• You decide to invest in one share of General
  Pills (GP) stock, which is currently traded for
  56. The stock pays no dividends.
• You worry that the stocks price may decline and
  decide to purchase a European put option on GPs
  stock. The put allows you to sell the stock at
  the end of one year for 50.
• If the std of the stock price is s0.3 (30) and
  rf0.08 (8 compounded continuously), what is the
  price of the put option?
• What is the CF from your strategy at time t0?
• What is the CF at time t1 as a function of
  0ltSTlt100?

44
Portfolio Insurance Example

• What if there is no put option on the stock that
  you wish to insure? - Use the BS formula to
  replicate the protective put strategy.
• What is your insurance strategy?
• What is the CF from your strategy at time t0?
• Suppose that one week later, the price of the
  stock increased to 60, what is the value of the
  stocks and bonds in your portfolio?
• How should you rebalance the portfolio to keep
  the insurance?

45
Portfolio Insurance Example

• The BS formula for the put option
• -P -Xe-rT1-N(d2)S1-N(d1)
• Therefore the insurance strategy (Original
  portfolio synthetic put) is
• Long one share of stock
• Long X1-N(d2) bonds
• Short 1-N(d1) stocks

46
Portfolio Insurance Example

• The total time t0 CF of the protective put
  (insured portfolio) is
• CF0 -S0-P0
• -S0-Xe-rT1-N(d2)S01-N(d1)
• -S0N(d1) -Xe-rT1-N(d2)
• And the proportion invested in the stock is
• wstock-S0N(d1) /-S0N(d1) -Xe-rT1-N(d2)

47
Portfolio Insurance Example

• The proportion invested in the stock is
• wstock S0N(d1) /S0N(d1) Xe-rT1-N(d2)
• Or, if we remember the original (protective put)
  strategy
• wstock S0N(d1) /S0 P0
• Finally, the proportion invested in the bond is
• wbond 1-wstock

48
Portfolio Insurance Example

• Say you invest 1,000 in the portfolio today
  (t0)
• Time t 0
• wstock 560.7865/(562.38) 75.45
• Stock value 0.75451,000 754.5
• Bond value 0.2455 1,000 245.5
• End of week 1 (we assumed that the stock price
  increased to 60)
• Stock value (60/56)754.5 808.39
• Bond value 245.5e0.08(1/52) 245.88
• Portfolio value 808.39245.88 1,054.27

49
Portfolio Insurance Example

• Now you have a 1,054.27 portfolio
• Time t 1 (beginning of week 2)
• wstock 600.8476/(601.63) 82.53
• Stock value 0.82531,054.27 870.06
• Bond value 0.1747 1,054.27 184.21
• I.e. you should rebalance your portfolio
  (increase the proportion of stocks to 82.53 and
  decrease the proportion of bonds to 17.47).
• Why should we rebalance the portfolio? Should we
  rebalance the portfolio if we use the protective
  put strategy with a real put option?

50
Practice Problems

• BKM Ch. 21 7-10, 17,18
• Practice set 36-42.