Stock Plans (Tables)

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



	Three Months Ended June 30,				Six Months Ended June 30,
	2016		2015		2016		2015
Cost of revenue	$	262	$	147	$	497	$	353
Selling, general and administrative	966		657		1,637		1,023
Research and development	231		207		528		474
Total stock-based compensation expense	$	1,459	$	1,011	$	2,662	$	1,850

The fair value of the stock options granted during the three and six months ended June 30, 2015 was estimated using the Black-Scholes valuation model with the following assumptions:



	Three Months Ended		Six Months Ended
	June 30,		June 30,
	2015		2015
Expected dividend yield	—	%	—	%
Expected volatility	60	%	60	%
Risk-free interest rate	1.55	%	1.58	%
Expected term (in years)	5.0		5.0

The following table summarizes the amount of compensation expense recognized for restricted stock units for the three and six months ended June 30, 2016 and 2015 (in thousands):



	Three Months Ended June 30,					Six Months Ended June 30,
RSU Type	2016		2015			2016		2015
Service-based	$	536	$	363		$	1,069	$	575
Performance-based objectives	29		(27		)	41		32
Market-condition	180		34			371		67
	$	745	$	370		$	1,481	$	674

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



	Three and Six Months Ended
	June 30,
	2016
Expected dividend yield	—	%
Expected volatility	62	%
Risk-free interest rate	1.07	%
Expected term (in years)	3.0

The fair value of the ESPP shares for the three and six months ended June 30, 2016 and 2015 was estimated using the Black-Scholes valuation model for a call and a put option with the following weighted-average assumptions:



	Three and Six Months Ended
	June 30,
	2016			2015
Expected dividend yield	—		%	—		%
Expected volatility	60		%	54		%
Risk-free interest rate	0.49		%	0.11		%
Expected term (in years)	0.5			0.5
Fair value per share	$	2.27		$	5.97