NOTE 9. STOCK PLANS

Stock-Based Compensation

During the three months ended March 31, 2021, the Company granted options to purchase an aggregate of 92,000 shares of its common stock that are subject to time-based vesting conditions.

The Company valued these stock options using the Black-Scholes Merton option pricing model. The following assumptions were used to compute the grant date fair values of the stock options granted during the three months ended March 31, 2021:

The assumptions used in calculating the fair values of purchase rights granted under the ESPP during the three months ended March 31, 2021 are set forth in the table below:

The Company’s stock-based compensation expense by type of award and by operating expense grouping are presented below:

Equity Award Activity Under Stock Plans

Stock Awards

The Company’s stock award activity for the three months ended March 31, 2021 was as follows:

All stock awards granted during the three months ended March 31, 2021 were fully vested upon grant. As of March 31, 2021, there was no unrecognized compensation cost related to stock awards granted under the Company’s stock plans.

Restricted Stock Units

The Company’s restricted stock unit activity for the three months ended March 31, 2021 was as follows:

As of March 31, 2021, total unrecognized compensation cost related to restricted stock units was $16,840, which is expected to be recognized over a weighted average period of 0.9 year.

Performance-Based Stock Options

The activity during the three months ended March 31, 2021 related to stock options that are subject to performance-based vesting conditions tied to the achievement of stock price goals by the Company was as follows:

During the first quarter of 2021, the Company achieved all of the stock price milestones applicable to substantially all of the performance-based stock options and, as a result, such performance-based stock options vested and all associated unrecognized compensation was accelerated and recognized in full as a one-time expense of $16,268 in the first quarter of 2021. The aggregate intrinsic value of the options exercised during the three months ended March 31, 2021 was $5,815. No options were exercised during the three months ended March 31, 2020. No performance-based stock options were granted during the three months ended March 31, 2021 and 2020, and no performance-based stock options vested during the three months ended March 31, 2020.

Stock Options

The activity during the three months ended March 31, 2021 related to all other stock options was as follows:

The weighted average grant date fair value of stock options granted during the three months ended March 31, 2021 and 2020 was $32.68 and $1.64 per share, respectively. The aggregate intrinsic value of the stock options exercised during the three months ended March 31, 2021 was $7,848. No stock options were exercised during the three months ended March 31, 2020. The total grant date fair value of stock options vested during the three months ended March 31, 2021 and 2020 was $733 and $2,371, respectively. At March 31, 2021, total unrecognized compensation expense related to stock options was $7,765 and is expected to be recognized over a weighted average period of 3.0 years.

The aggregate intrinsic values in the tables above represent the difference between the fair market value of the Company’s common stock and the average option exercise price of in-the-money options, multiplied by the number of such stock options.

Employee Stock Purchase Plan

During the three months ended March 31, 2021, a total of 67,068 shares of common stock were purchased under the Company’s ESPP. As of March 31, 2021, accrued employee contributions for future purchases under the ESPP totaled $119.