This excerpt taken from the PNC 10-K filed Mar 2, 2009.
OPTION PRICING ASSUMPTIONS
For purposes of computing stock option expense, we estimated the fair value of stock options primarily by using the Black-Scholes option-pricing model. Option pricing models require the use of numerous assumptions, many of which are very subjective.
We used the following assumptions in the option pricing models to determine 2008, 2007 and 2006 stock option expense: