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: range of risk free interest rates: -1.99
weighted average risk-free interest rate: -2.58
expected life of the shares: 6
range of expected volatility of underlying stock price: -27.85
weighted average expected volatility of underlying stock price: -28.51
expected annual dividends: n/a
2007: range of risk free interest rates: -4.98
weighted average risk-free interest rate: -5.02
expected life of the shares: 6
range of expected volatility of underlying stock price: -27.53
weighted average expected volatility of underlying stock price: -28.22
expected annual dividends: n/a
2006: range of risk free interest rates: -5.01
weighted average risk-free interest rate: -5.08
expected life of the shares: 6
range of expected volatility of underlying stock price: -29.6
weighted average expected volatility of underlying stock price: -29.6
expected annual dividends: 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. .

Questions:
what was the change in value of the weighted average risk-free interest rate between 2007 and 2008?
what is the percent change?

Answers:
-2.44
-48.6%