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The
Merton (1973) option pricing
formula generalization the
Black-Scholes (1973) formula so it can
price European options on
stocks or stock indices paying a known
dividend yield.
The yield is expressed as an annual
continuously compounded rate q. Values for a
call price
c or put price p are:
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[1] |
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[2] |
where:
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[3] |
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[4] |
Here, log denotes the natural logarithm, and:
s = the price of the
underlying stock
x = the
strike price
r = the continuously compounded
risk free interest rate
q = the continuously compounded annual dividend yield
t = the time in years until the
expiration of the
option
σ = the
implied volatility for
the underlying stock
Φ = the
standard normal
cumulative distribution function.
Consider a call option on a stock index. The option is struck at
EUR
8000 and expires in .18 years. The index is
trading at EUR 7986 and has 24% (that is .24) implied volatility. The
continuously compounded risk free interest rate is .0293. Based upon recent
dividends, assume an annual dividend yield of q = .0254. Applying
formula [1], the option has
market value EUR 319. Because the option is
out-of-the-money, that value is entirely
time value.
The
Greeks—delta, gamma, vega,
theta and rho—for a call are:
where
denotes the standard
normal probability density function. For a put, the Greeks are:
A shortcoming of the Merton formula is its assumption that
dividends are paid out continuously. For a stock index, this is an imperfect but usually
reasonable approximation. For individual stocks, which typically
distribute dividends in two payments each year, it is more problematic. The
stock's annual yield is immaterial. The quantity q needs to reflect the
dividends that will be earned prior to the option's expiration. If the stock has
no dividend record date prior to the option's expiration, set q = 0.
Otherwise, calculate the stock's dividend yield through expiration and
annualize. Another problem is the fact that the model assumes that the dividend
yield is a known constant. Often a dividend payment will be scheduled during the
life of an option, but the amount of the payment has not yet been announced.
This is an additional source of uncertainty the Merton model can not reflect.

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