pre_text: marathon oil corporation notes to consolidated financial statements stock appreciation rights 2013 prior to 2005 , we granted sars under the 2003 plan . no stock appreciation rights have been granted under the 2007 plan . similar to stock options , stock appreciation rights represent the right to receive a payment equal to the excess of the fair market value of shares of common stock on the date the right is exercised over the grant price . under the 2003 plan , certain sars were granted as stock-settled sars and others were granted in tandem with stock options . in general , sars granted under the 2003 plan vest ratably over a three-year period and have a maximum term of ten years from the date they are granted . stock-based performance awards 2013 prior to 2005 , we granted stock-based performance awards under the 2003 plan . no stock-based performance awards have been granted under the 2007 plan . beginning in 2005 , we discontinued granting stock-based performance awards and instead now grant cash-settled performance units to officers . all stock-based performance awards granted under the 2003 plan have either vested or been forfeited . as a result , there are no outstanding stock-based performance awards . restricted stock 2013 we grant restricted stock and restricted stock units under the 2007 plan and previously granted such awards under the 2003 plan . in 2005 , the compensation committee began granting time-based restricted stock to certain u.s.-based officers of marathon and its consolidated subsidiaries as part of their annual long-term incentive package . the restricted stock awards to officers vest three years from the date of grant , contingent on the recipient 2019s continued employment . we also grant restricted stock to certain non-officer employees and restricted stock units to certain international employees ( 201crestricted stock awards 201d ) , based on their performance within certain guidelines and for retention purposes . the restricted stock awards to non-officers generally vest in one-third increments over a three-year period , contingent on the recipient 2019s continued employment , however , certain restricted stock awards granted in 2008 will vest over a four-year period , contingent on the recipient 2019s continued employment . prior to vesting , all restricted stock recipients have the right to vote such stock and receive dividends thereon . the non-vested shares are not transferable and are held by our transfer agent . common stock units 2013 we maintain an equity compensation program for our non-employee directors under the 2007 plan and previously maintained such a program under the 2003 plan . all non-employee directors other than the chairman receive annual grants of common stock units , and they are required to hold those units until they leave the board of directors . when dividends are paid on marathon common stock , directors receive dividend equivalents in the form of additional common stock units . total stock-based compensation expense total employee stock-based compensation expense was $ 43 million , $ 66 million and $ 78 million in 2008 , 2007 and 2006 . the total related income tax benefits were $ 16 million , $ 24 million and $ 29 million . in 2008 and 2007 , cash received upon exercise of stock option awards was $ 9 million and $ 27 million . tax benefits realized for deductions during 2008 and 2007 that were in excess of the stock-based compensation expense recorded for options exercised and other stock-based awards vested during the period totaled $ 7 million and $ 30 million . cash settlements of stock option awards totaled $ 1 million in 2007 . there were no cash settlements in 2008 . stock option awards during 2008 , 2007 and 2006 , we granted stock option awards to both officer and non-officer employees . the weighted average grant date fair value of these awards was based on the following black-scholes assumptions: .
post_text: .
table: 2008 2007 2006
table: Weighted average exercise price per share $51.74 $60.94 $37.84
table: Expected annual dividends per share $0.96 $0.96 $0.80
table: Expected life in years 4.8 5.0 5.1
table: Expected volatility 30% 27% 28%
table: Risk-free interest rate 3.1% 4.1% 5.0%
table: Weighted average grant date fair value of stock option awards granted $13.03 $17.24 $10.19
question: Analyse this data from a financial earnings document. by what percentage did the company's weighted average exercise price per share increase from 2006 to 2008?