weighted average fair values and valuation assumptions used to value performance unit and performance stock grants during the years ended december 31 , 2016 , 2015 and 2014 were as follows: .

2016
2015
2014
Weighted Average Fair Value of Grants
$119.10
$80.64
$119.27
Expected Volatility
32.48%
29.35%
32.18%
Risk-Free Interest Rate
1.15%
1.07%
1.18%

expected volatility is based on the term-matched historical volatility over the simulated term , which is calculated as the time between the grant date and the end of the performance period .
the risk-free interest rate is based on a 3.25 year zero-coupon risk-free interest rate derived from the treasury constant maturities yield curve on the grant date .
at december 31 , 2016 , unrecognized compensation expense related to performance units totaled $ 10 million .
such unrecognized expense will be amortized on a straight-line basis over a weighted average period of 3.0 years .
pension plans .
eog has a defined contribution pension plan in place for most of its employees in the united states .
eog's contributions to the pension plan are based on various percentages of compensation and , in some instances , are based upon the amount of the employees' contributions .
eog's total costs recognized for the plan were $ 34 million , $ 36 million and $ 41 million for 2016 , 2015 and 2014 , respectively .
in addition , eog's trinidadian subsidiary maintains a contributory defined benefit pension plan and a matched savings plan .
eog's united kingdom subsidiary maintains a pension plan which includes a non-contributory defined contribution pension plan and a matched defined contribution savings plan .
these pension plans are available to most employees of the trinidadian and united kingdom subsidiaries .
eog's combined contributions to these plans were $ 1 million , $ 1 million and $ 5 million for 2016 , 2015 and 2014 , respectively .
for the trinidadian defined benefit pension plan , the benefit obligation , fair value of plan assets and accrued benefit cost totaled $ 8 million , $ 7 million and $ 0.3 million , respectively , at december 31 , 2016 , and $ 9 million , $ 7 million and $ 0.2 million , respectively , at december 31 , 2015 .
in connection with the divestiture of substantially all of its canadian assets in the fourth quarter of 2014 , eog has elected to terminate the canadian non-contributory defined benefit pension plan .
postretirement health care .
eog has postretirement medical and dental benefits in place for eligible united states and trinidad employees and their eligible dependents , the costs of which are not material .
8 .
commitments and contingencies letters of credit and guarantees .
at december 31 , 2016 and 2015 , respectively , eog had standby letters of credit and guarantees outstanding totaling approximately $ 226 million and $ 272 million , primarily representing guarantees of payment or performance obligations on behalf of subsidiaries .
as of february 20 , 2017 , there were no demands for payment under these guarantees. .

Question: Analyse this data from a financial earnings document. what is the variation observed in the risk-free interest rate during 2015 and 2016?
Choices:
-0.0107
0.08
0.0222
-0.1885
0.0008
Correct choice index: 4