american tower corporation and subsidiaries notes to consolidated financial statements 2014 ( continued ) from december 1 through may 31 of each year .
during the 2008 , 2007 and 2006 offering periods employees purchased 55764 , 48886 and 53210 shares , respectively , at weighted average prices per share of $ 30.08 , $ 33.93 and $ 24.98 , 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 common stock .
the weighted average fair value for the espp shares purchased during 2008 , 2007 and 2006 were $ 7.89 , $ 9.09 and $ 6.79 , respectively .
at december 31 , 2008 , 8.8 million 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: .

2008
2007
2006
Range of risk free interest rates
1.99%—3.28%
4.98%—5.05%
5.01%—5.17%
Weighted average risk-free interest rate
2.58%
5.02%
5.08%
Expected life of the shares
6 months
6 months
6 months
Range of expected volatility of underlying stock price
27.85%—28.51%
27.53%—28.74%
29.60%
Weighted average expected volatility of underlying stock price
28.51%
28.22%
29.60%
Expected annual dividends
N/A
N/A
N/A

13 .
stockholders 2019 equity warrants 2014in january 2003 , the company issued warrants to purchase approximately 11.4 million shares of its common stock in connection with an offering of 808000 units , each consisting of $ 1000 principal amount at maturity of ati 12.25% ( 12.25 % ) senior subordinated discount notes due 2008 and a warrant to purchase 14.0953 shares of the company 2019s common stock .
these warrants became exercisable on january 29 , 2006 at an exercise price of $ 0.01 per share .
as these warrants expired on august 1 , 2008 , none were outstanding as of december 31 , in august 2005 , the company completed its merger with spectrasite , inc .
and assumed outstanding warrants to purchase shares of spectrasite , inc .
common stock .
as of the merger completion date , each warrant was exercisable for two shares of spectrasite , inc .
common stock at an exercise price of $ 32 per warrant .
upon completion of the merger , each warrant to purchase shares of spectrasite , inc .
common stock automatically converted into a warrant to purchase shares of the company 2019s common stock , such that upon exercise of each warrant , the holder has a right to receive 3.575 shares of the company 2019s common stock in lieu of each share of spectrasite , inc .
common stock that would have been receivable under each assumed warrant prior to the merger .
upon completion of the company 2019s merger with spectrasite , inc. , these warrants were exercisable for approximately 6.8 million shares of common stock .
of these warrants , warrants to purchase approximately 1.8 million and 2.0 million shares of common stock remained outstanding as of december 31 , 2008 and 2007 , respectively .
these warrants will expire on february 10 , 2010 .
stock repurchase programs 2014during the year ended december 31 , 2008 , the company repurchased an aggregate of approximately 18.3 million shares of its common stock for an aggregate of $ 697.1 million , including commissions and fees , pursuant to its publicly announced stock repurchase programs , as described below. .

Question: Analyse this data from a financial earnings document. what was the percentage change in the weighted average risk-free interest rate from 2007 to 2008
Choices:
-0.17311
-0.48606
-0.94574
-8.5584
-0.18839
Correct choice index: 1