The Company recorded approximately $2.2 million and $2.1 million in stock based compensation expense for the three months ended March 31, 2020 and 2019, respectively.

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


	Number of Shares	Weighted Average Exercise Price
Options outstanding at December 31, 2019	5,318,759	$	9.97
Options granted	571,316	$	28.36
Less:
Options exercised	504,127	$	7.58
Options canceled or expired	51,210	$	14.61
Options outstanding at March 31, 2020	5,334,738	$	12.14
Exercisable at March 31, 2020	2,535,621	$	8.35

The fair value of each stock option award granted during the three months ended March 31, 2020 was estimated as of the grant date using a trinomial lattice model with the following weighted average assumptions:


			Three Months Ended March 31, 2020
Expected term (in years)			4.0 - 5.5
Risk-free interest rate (%)			0.9%
Expected volatility (%)			39.9% - 44.5%
Dividend yield (%)			—
Weighted average fair value/share at grant date			$8.90

As of March 31, 2020, there was approximately $8.2 million of unrecognized stock based compensation expense related to stock options that will be recognized over a weighted-average period of approximately 1.54 years.

A summary of the restricted stock activity under the Company’s plans for the three months ended March 31, 2020 is as follows:


	Number of Restricted Shares	Weighted Average Grant Date Fair Value
Nonvested at December 31, 2019	335,298	$	15.75
Granted	88,736	$	28.36
Vested	(37,674)	$	19.29
Forfeited	(4,585)	$	19.66
Nonvested at March 31, 2020	381,775	$	18.29

As of March 31, 2020, there was approximately $4.4 million of unrecognized stock based compensation expense related to restricted stock that will be recognized over a weighted-average period of approximately 1.29 years.

The Company offers an ESPP through which eligible employees may purchase shares of our common stock at a discount of 15% of the fair market value of the Company’s common stock.