american tower corporation and subsidiaries notes to consolidated financial statements 2014 ( continued ) atc mexico stock option plan 2014as of december 31 , 2006 , the company maintained a stock option plan for its atc mexico subsidiary ( atc mexico plan ) which was terminated in february 2007 . the atc mexico plan provided for the issuance of options to officers , employees , directors and consultants of atc mexico , however there was no option activity and no outstanding options as of and for the years ended december 31 , 2006 and 2005 . atc south america stock option plan 2014as of december 31 , 2006 , the company maintained a stock option plan for its atc south america subsidiary ( atc south america plan ) which was terminated in february 2007 . the atc south america plan provided for the issuance of options to officers , employees , directors and consultants of atc south america . during the year ended december 31 , 2004 , atc south america granted options to purchase 6024 shares of atc south america common stock to officers and employees , including messrs . gearon and hess , who received options to purchase an approximate 6.7% ( 6.7 % ) and 1.6% ( 1.6 % ) interest , respectively . such options were issued at one time with an exercise price of $ 1349 per share . the exercise price per share was at fair market value on the date of issuance as determined by the board of directors with the assistance of an independent financial advisor performed at the company 2019s request . the fair value of atc south america plan options granted during 2004 were $ 79 per share as determined by using the black-scholes option pricing model . options granted vested upon the earlier to occur of ( a ) the exercise by or on behalf of mr . gearon of his right to sell his interest in atc south america to the company , ( b ) the exercise by the company of its right to acquire mr . gearon 2019s interest in atc south america , or ( c ) july 1 , 2006 . these options expired ten years from the date of grant . in october 2005 , in connection with the exercise by mr . gearon 2019s of his right to require the company to purchase his interest in atc south america , all options granted pursuant to the atc south america stock option plan vested in full and were exercised . upon exercise of these options , the holders received 4428 shares of atc south america ( representing a 7.8% ( 7.8 % ) interest ) , net of 1596 shares retained by the company to satisfy employee tax withholding obligations . ( see note 11. ) employee stock purchase plan 2014the company also maintains an employee stock purchase plan ( espp ) for all eligible employees . under the espp , shares of the company 2019s class a common stock may be purchased during bi-annual offering periods at 85% ( 85 % ) of the lower of the fair market value on the first or the last day of each offering period . employees may purchase shares having a value not exceeding 15% ( 15 % ) of their gross compensation during an offering period and may not purchase more than $ 25000 worth of stock in a calendar year ( based on market values at the beginning of each offering period ) . the offering periods run from june 1 through november 30 and from december 1 through may 31 of each year . during the 2007 , 2006 and 2005 , offering periods , employees purchased 48886 , 53210 and 50119 shares , respectively , at weighted average prices per share of $ 33.93 , $ 24.98 and $ 15.32 , respectively . the fair value of the espp offerings is estimated on the offering period commencement date using a black-scholes pricing model with the expense recognized over the expected life , which is the six month offering period over which employees accumulate payroll deductions to purchase the company 2019s class a common stock . the weighted average fair value for the espp shares purchased during 2007 , 2006 and 2005 were $ 9.09 , $ 6.79 and $ 5.15 , respectively . at december 31 , 2007 , 3895402 shares remain reserved for future issuance under the plan . key assumptions used to apply this pricing model for the years ended december 31 , are as follows: .
based on the the pricing model what was the percentage change in the weighted average risk-free interest rate from 2005 to 2007
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