Table:
|  |  | Year Ended |  |
| --- | --- | --- | --- |
|  | January 3, 2020 | December 28, 2018 | December 29, 2017 |
| Weighted average grant-date fair value | $11.89 | $13.85 | $11.53 |
| Expected term (in years) | 4.4 | 4.7 | 4.7 |
| Expected volatility | 24.3% | 26.6% | 29.7% |
| Risk-free interest rate | 2.4% | 2.6% | 1.9% |
| Dividend yield | 2.2% | 2.0% | 2.5% |
Paragraphs:
Stock Options
Stock options are granted with exercise prices equal to the fair market value of Leidos' common stock on the date of grant and for terms not greater than ten years. Stock options have a term of seven years and a vesting period of four years, except for stock options granted to the Company's outside directors, which have a vesting period of the earlier of one year from grant date or the next annual meeting of stockholders following grant date.
The fair value of the Company's stock option awards is estimated on the date of grant using the Black-Scholes- Merton option-pricing model. The fair value of the Company's stock option awards to employees are expensed on a straight-line basis over the vesting period of four years, except for stock options granted to the Company's outside directors, which is recognized over the vesting period of one year or less.
During fiscal 2017, the Company ceased the usage of peer group volatility, as an input into its blended approach to measure expected volatility, and increased the reliance on historical volatility. The revised blended approach includes the Company's weighted average historical and implied volatilities. The Company continued the use of this approach during fiscal 2018 and fiscal 2019.
The risk-free rate is derived using the yield curve of a zero-coupon U.S. Treasury bond with a maturity equal to the expected term of the stock option on the grant date. During fiscal 2017 and fiscal 2018, Leidos utilized the simplified method for the expected term, which represented an appropriate period of time that the options granted were expected to remain outstanding between the weighted-average vesting period and end of the respective contractual term.
Upon re-examining the Company's exercise history, the methodology used to calculate the expected term changed in fiscal 2019. Based on actual historical settlement data, the midpoint scenario is utilized with a one-year grant date filter assumption for outstanding options. The Company uses historical data to estimate forfeitures and was derived in the same manner as in the prior years presented.
The weighted average grant-date fair value and assumptions used to determine fair value of stock options granted for the periods presented were as follows:
Question:
Which model is used to value company's stock option?
Answer: Black-Scholes- Merton option-pricing model