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 .
[['', '2016', '2015', '2014'], ['average risk-free interest rate', '1.1% ( 1.1 % )', '1.4% ( 1.4 % )', '1.5% ( 1.5 % )'], ['expected dividend yield', 'none', 'none', 'none'], ['expected volatility', '33% ( 33 % )', '30% ( 30 % )', '31% ( 31 % )'], ['expected life ( years )', '4.5', '4.6', '4.6'], ['fair value per share', '$ 31.00', '$ 18.13', '$ 11.75']]
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Question: what is the percentage difference in the fair value per share between 2015 and 2016?
Answer: 71%