by how much did the fair value of stock-based performance units outstanding decrease from 2016 to 2018?
marathon oil corporation notes to consolidated financial statements stock-based performance unit awards 2013 during 2018 , 2017 and 2016 we granted 754140 , 563631 and 1205517 stock- based performance unit awards to officers . at december 31 , 2018 , there were 1196176 units outstanding . total stock-based performance unit awards expense was $ 13 million in 2018 , $ 8 million in 2017 and $ 6 million in 2016 . the key assumptions used in the monte carlo simulation to determine the fair value of stock-based performance units granted in 2018 , 2017 and 2016 were: .

|  | 2018 | 2017 | 2016 |
| :--- | :--- | :--- | :--- |
| Valuation date stock price | $14.17 | $14.17 | $14.17 |
| Expected annual dividend yield | 1.4% | 1.4% | 1.4% |
| Expected volatility | 39% | 43% | 52% |
| Risk-free interest rate | 2.5% | 2.6% | 2.4% |
| Fair value of stock-based performance units outstanding | $19.60 | $19.45 | $21.51 |

18 . defined benefit postretirement plans and defined contribution plan we have noncontributory defined benefit pension plans covering substantially all domestic employees , as well as u.k . employees who were hired before april 2010 . certain employees located in e.g. , who are u.s . or u.k . based , also participate in these plans . benefits under these plans are based on plan provisions specific to each plan . for the u.k . pension plan , the principal employer and plan trustees reached a decision to close the plan to future benefit accruals effective december 31 , 2015 . we also have defined benefit plans for other postretirement benefits covering our u.s . employees . health care benefits are provided up to age 65 through comprehensive hospital , surgical and major medical benefit provisions subject to various cost- sharing features . post-age 65 health care benefits are provided to certain u.s . employees on a defined contribution basis . life insurance benefits are provided to certain retiree beneficiaries . these other postretirement benefits are not funded in advance . employees hired after 2016 are not eligible for any postretirement health care or life insurance benefits. .

fair_value_2018 = 19.60
fair_value_2017 = 21.51
percent_change = (fair_value_2018 - fair_value_2017) / fair_value_2017
answer = percent_change * 100