pre_text: 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: .
post_text: 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. .
table: 2016 2015 2014
table: Weighted Average Fair Value of Grants $119.10 $80.64 $119.27
table: Expected Volatility 32.48% 29.35% 32.18%
table: Risk-Free Interest Rate 1.15% 1.07% 1.18%
question: Analyse this data from a financial earnings document. what is the variation observed in the risk-free interest rate during 2015 and 2016?