9. Stock-Based Compensation

A summary of the Company’s outstanding stock option activity for the three months ended March 31, 2021 and 2020 is as follows:

The Company estimates the fair value of each stock option on the grant date using the Black-Scholes-Merton valuation model, which requires several estimates including an estimate of the fair value of the underlying common stock on grant date. The expected volatility was based on an average of the historical volatility of the Company. The expected term was contractual life of option. The risk-free interest rate was based on the U.S. Treasury yield curve in effect at the time of grant for the expected term of the option. The following weighted average assumptions were used in the calculation to estimate the fair value of options granted to employees and non-employees during the three months ended March 31, 2021. During the three months ended March 31, 2021 the company granted stock options with an estimated fair value of approximately $0.6  million.

Of the 368,500 options granted during the three months ended March 31, 2021, 60,000  were to non-executive members of the board of directors. Of the 833,893 options outstanding as of March 31, 2021, 295,000  were awarded to non-executive members of the board of directors.

Unrecognized stock-based compensation  as of March 31, 2021, is as follows (in thousands):