pre_text: The Company utilized the Black-Scholes option pricing model to value the stock options. The Company used an expected life as defined under the simplified method, which is using an average of the contractual term and vesting period of the stock options. The risk-free interest rate used for the award is based on the U.S. Treasury yield curve in effect at the time of grant. The Company accounted for forfeitures as they occur. The historical volatility was calculated based upon implied volatility of the Company's historical stock prices. The fair value of 2019 and 2018 stock options was estimated using the Black-Scholes option pricing model with the following weighted-average assumptions: As of December 31, 2019, there was approximately $419,000 of unrecognized stock-based compensation expense related to outstanding 2019 stock options, expected to be recognized over 3.4 and approximately $418,000 of unrecognized stock-based compensation expense related to outstanding 2018 stock options, expected to be recognized over 2.4 years. There was no unrecognized stock-based compensation expense relating stock options granted prior to 2018.
table: 2019 2018
table: Weighted-average fair value of options granted per share $8.78 $6.63
table: Historical volatility 60% 46%
table: Risk-free interest rate 2.10% 2.84%
table: Dividend yield — —
table: Expected life in years 3.6 5.7
question: Analyse this data from a financial earnings document. What is the change in expected life in years used in the Black-Scholes model between 2018 and 2019?