pre_text: edwards lifesciences corporation notes to consolidated financial statements ( continued ) 13 . common stock ( continued ) the company also maintains the nonemployee directors stock incentive compensation program ( the 2018 2018nonemployee directors program 2019 2019 ) . under the nonemployee directors program , upon a director 2019s initial election to the board , the director receives an initial grant of stock options or restricted stock units equal to a fair market value on grant date of $ 0.2 million , not to exceed 20000 shares . these grants vest over three years from the date of grant , subject to the director 2019s continued service . in addition , annually each nonemployee director may receive up to 40000 stock options or 16000 restricted stock units of the company 2019s common stock , or a combination thereof , provided that in no event may the total value of the combined annual award exceed $ 0.2 million . these grants generally vest over one year from the date of grant . under the nonemployee directors program , an aggregate of 2.8 million shares of the company 2019s common stock has been authorized for issuance . the company has an employee stock purchase plan for united states employees and a plan for international employees ( collectively 2018 2018espp 2019 2019 ) . under the espp , eligible employees may purchase shares of the company 2019s common stock at 85% ( 85 % ) of the lower of the fair market value of edwards lifesciences common stock on the effective date of subscription or the date of purchase . under the espp , employees can authorize the company to withhold up to 12% ( 12 % ) of their compensation for common stock purchases , subject to certain limitations . the espp is available to all active employees of the company paid from the united states payroll and to eligible employees of the company outside the united states , to the extent permitted by local law . the espp for united states employees is qualified under section 423 of the internal revenue code . the number of shares of common stock authorized for issuance under the espp was 13.8 million shares . the fair value of each option award and employee stock purchase subscription is estimated on the date of grant using the black-scholes option valuation model that uses the assumptions noted in the following tables . the risk-free interest rate is estimated using the u.s . treasury yield curve and is based on the expected term of the award . expected volatility is estimated based on a blend of the weighted-average of the historical volatility of edwards lifesciences 2019 stock and the implied volatility from traded options on edwards lifesciences 2019 stock . the expected term of awards granted is estimated from the vesting period of the award , as well as historical exercise behavior , and represents the period of time that awards granted are expected to be outstanding . the company uses historical data to estimate forfeitures and has estimated an annual forfeiture rate of 6.0% ( 6.0 % ) . the black-scholes option pricing model was used with the following weighted-average assumptions for options granted during the following periods : option awards .
post_text: .
table: 2016 2015 2014
table: Average risk-free interest rate 1.1% 1.4% 1.5%
table: Expected dividend yield None None None
table: Expected volatility 33% 30% 31%
table: Expected life (years) 4.5 4.6 4.6
table: Fair value, per share $31.00 $18.13 $11.75
question: Analyse this data from a financial earnings document. what is the percentage difference in the fair value per share between 2015 and 2016?