during 2012 , the company granted selected employees an aggregate of 139 thousand rsus with internal performance measures and , separately , certain market thresholds .
these awards vested in january 2015 .
the terms of the grants specified that to the extent certain performance goals , comprised of internal measures and , separately , market thresholds were achieved , the rsus would vest ; if performance goals were surpassed , up to 175% ( 175 % ) of the target awards would be distributed ; and if performance goals were not met , the awards would be forfeited .
in january 2015 , an additional 93 thousand rsus were granted and distributed because performance thresholds were exceeded .
in 2015 , 2014 and 2013 , the company granted rsus , both with and without performance conditions , to certain employees under the 2007 plan .
the rsus without performance conditions vest ratably over the three- year service period beginning january 1 of the year of the grant and the rsus with performance conditions vest ratably over the three-year performance period beginning january 1 of the year of the grant ( the 201cperformance period 201d ) .
distribution of the performance shares is contingent upon the achievement of internal performance measures and , separately , certain market thresholds over the performance period .
during 2015 , 2014 and 2013 , the company granted rsus to non-employee directors under the 2007 plan .
the rsus vested on the date of grant ; however , distribution of the shares will be made within 30 days of the earlier of : ( i ) 15 months after grant date , subject to any deferral election by the director ; or ( ii ) the participant 2019s separation from service .
because these rsus vested on the grant date , the total grant date fair value was recorded in operation and maintenance expense included in the expense table above on the grant date .
rsus generally vest over periods ranging from one to three years .
rsus granted with service-only conditions and those with internal performance measures are valued at the market value of the closing price of the company 2019s common stock on the date of grant .
rsus granted with market conditions are valued using a monte carlo model .
expected volatility is based on historical volatilities of traded common stock of the company and comparative companies using daily stock prices over the past three years .
the expected term is three years and the risk-free interest rate is based on the three-year u.s .
treasury rate in effect as of the measurement date .
the following table presents the weighted-average assumptions used in the monte carlo simulation and the weighted-average grant date fair values of rsus granted for the years ended december 31: .

2015
2014
2013
Expected volatility
14.93%
17.78%
19.37%
Risk-free interest rate
1.07%
0.75%
0.40%
Expected life (years)
3.0
3.0
3.0
Grant date fair value per share
$62.10
$45.45
$40.13

the grant date fair value of restricted stock awards that vest ratably and have market and/or performance and service conditions are amortized through expense over the requisite service period using the graded-vesting method .
rsus that have no performance conditions are amortized through expense over the requisite service period using the straight-line method and are included in operations expense in the accompanying consolidated statements of operations .
as of december 31 , 2015 , $ 4 of total unrecognized compensation cost related to the nonvested restricted stock units is expected to be recognized over the weighted-average remaining life of 1.4 years .
the total grant date fair value of rsus vested was $ 12 , $ 11 and $ 9 for the years ended december 31 , 2015 , 2014 and 2013. .

Question: Analyse this data from a financial earnings document. by what percentage did grant date fair value per share increase from 2013 to 2015?
Choices:
15.69286
0.4149
1.82658
0
0.54747
Correct choice index: 4