when valuing employee stock options , we stratify the employee population into three homogenous groups that historically have exhibited similar exercise behaviors . these groups are executive officers , directors , and all other employees . we value the stock options based on the unique assumptions for each of these employee groups . we calculate the expected term for our employee stock options based on historical employee exercise behavior and base the risk-free interest rate on a traded zero-coupon u.s . treasury bond with a term substantially equal to the option 2019s expected term . the volatility used to value employee stock options is based on historical volatility . we calculate historical volatility using a simple-average calculation methodology based on daily price intervals as measured over the expected term of the option. .
humana inc . notes to consolidated financial statements 2014 ( continued ) value , or the excess of the market value over the exercise or purchase price , of stock options exercised and restricted stock awards vested during the period . the actual tax benefit realized for the deductions taken on our tax returns from option exercises and restricted stock vesting totaled $ 16.3 million in 2009 , $ 16.9 million in 2008 , and $ 48.0 million in 2007 . there was no capitalized stock-based compensation expense . the stock plans provide that one restricted share is equivalent to 1.7 stock options . at december 31 , 2009 , there were 12818855 shares reserved for stock award plans , including 4797304 shares of common stock available for future grants assuming all stock options or 2821944 shares available for future grants assuming all restricted shares . stock options stock options are granted with an exercise price equal to the average market value of the underlying common stock on the date of grant . our stock plans , as approved by the board of directors and stockholders , define average market value as the average of the highest and lowest stock prices reported by the new york stock exchange on a given date . exercise provisions vary , but most options vest in whole or in part 1 to 3 years after grant and expire 7 to 10 years after grant . upon grant , stock options are assigned a fair value based on the black-scholes valuation model . compensation expense is recognized on a straight-line basis over the total requisite service period , generally the total vesting period , for the entire award . for stock options granted on or after january 1 , 2010 to retirement eligible employees , the compensation expense is recognized on a straight-line basis over the shorter of the requisite service period or the period from the date of grant to an employee 2019s eligible retirement date . the weighted-average fair value of each option granted during 2009 , 2008 , and 2007 is provided below . the fair value was estimated on the date of grant using the black-scholes pricing model with the weighted-average assumptions indicated below: .
what was the percent of the change of the expected volatility from 2008 to 2009
the expected volatility of 2009 is 39.2% ( 39.2 % ) ; the expected volatility of 2008 is 28.2% ( 28.2 % ) ; the expected volatility of 2007 is 28.9% ( 28.9 % ) ;
2009 2008 2007
weighted-average fair value at grant date $ 14.24 $ 17.95 $ 21.07
expected option life ( years ) 4.6 5.1 4.8
expected volatility 39.2% ( 39.2 % ) 28.2% ( 28.2 % ) 28.9% ( 28.9 % )
risk-free interest rate at grant date 1.9% ( 1.9 % ) 2.9% ( 2.9 % ) 4.5% ( 4.5 % )
dividend yield none none none