Stock Plans (Tables)

For the three months ended March 31, 2019 and 2018, RSU grants were composed of the following:



	Three Months Ended March 31,
	2019			2018
	Shares granted (in thousands)	Average grant date fair value		Shares granted (in thousands)	Average grant date fair value
Service-based	125	$	2.86	921	$	5.77
Performance objectives	—	n/a		78	5.85
Market-condition	—	n/a		335	7.55
Total RSUs granted	125	2.86		1,334	6.22

Stock-based compensation cost, excluding discontinued operations, included in cost of revenue; selling, general and administrative expense; and research and development expense is as follows (in thousands):



	Three Months Ended March 31,
	2019		2018
Cost of revenue	$	277	$	317
Selling, general and administrative	1,479		1,657
Research and development	335		400
Total stock-based compensation expense	$	2,091	$	2,374

The fair value of the market-condition PSUs granted was calculated using a Monte Carlo valuation model with the following assumptions:



	Three Months Ended March 31,
	2018
Expected dividend yield	—	%
Expected volatility	46% - 47%
Risk-free interest rate	2.36% - 2.39%
Expected term (in years)	2.8 - 2.9

The following table summarizes the amount of compensation expense recognized for RSUs for the three months ended March 31, 2019 and 2018 (in thousands):



	Three Months Ended March 31,
RSU Type	2019		2018
Service-based	$	1,153	$	1,084
Performance objectives	83		120
Market-condition	333		298
	$	1,569	$	1,502

The fair value of the ESPP shares was estimated using the Black-Scholes valuation model for a call and a put option with the following weighted-average assumptions:



	Three Months Ended March 31,
	2019			2018
Expected dividend yield	—		%	—		%
Expected volatility	59		%	43		%
Risk-free interest rate	2.51		%	1.39		%
Expected term (in years)	0.50			0.50
Fair value per share	$	0.78		$	1.29