A summary of the status of stock option activity for the three months ended March 31, 2020 is as follows (in thousands, except price data):


	Shares	Weighted-average exercise price per share
Outstanding at December 31, 2019	944	$	30.63
Granted	121	77.16
Exercised	(65)	21.26
Forfeited	(12)	43.34
Outstanding at March 31, 2020	988	$	36.77

A summary of the status of restricted stock unit activity for the three months ended March 31, 2020 is as follows (in thousands, except price data):


	Shares	Weighted-average grant date fair value
Non-vested December 31, 2019	786	$	41.88
Granted	186	77.53
Vested	(60)	18.21
Forfeited	(17)	49.85
Non-vested at March 31, 2020	895	$	50.74

During the three months ended March 31, 2020, the Company issued 27,826 shares of common stock in connection with the Company’s employee stock purchase plan (the “ESPP”).

The expense related to the Company’s stock-based compensation plans included in the accompanying Consolidated Statements of Income was as follows (in thousands):


	Three Months Ended March 31,

	2020		2019
Cost of sales	$	258	$	280
Research and development	642		565
Sales and marketing	1,297		1,119
General and administrative	1,681		1,624
Total stock-based compensation expense	$	3,878	$	3,588

As of March 31, 2020, total unrecognized compensation expense was $35.7 million, which is expected to be recognized over a weighted-average period of approximately 2.8 years.

The estimated fair value of each stock option was determined on the date of grant using the Black-Scholes option valuation model with the following weighted-average assumptions for the option grants.


	Three Months Ended March 31,
	2020			2019
Risk-free interest rate	1.35		%	2.51		%
Expected option life (in years)	5.14			5.68
Volatility rate	39		%	39		%
Dividend rate	0		%	0		%
Weighted-average grant date fair value	$	28.24		$	23.67

The fair value of RSUs is determined based on the closing market price of the Company’s common stock on the grant date. The weighted-average fair value of RSUs granted during the three months ended March 31, 2020 and 2019 was $77.53 and $59.45, respectively.