Stock-Based Compensation

3 Months Ended

Jun. 30, 2014

Stock-Based Compensation [Abstract]

Stock-based Compensation

7. Stock-Based Compensation

The Company currently has outstanding two types of stock-based compensation awards to its employees, directors and certain consultants: stock options and restricted stock units ("RSUs"). Both stock options and RSUs can be used to acquire shares of the Company's Class A common stock, are exercisable or convertible, as applicable, over a period not to exceed ten years, and are most commonly assigned four-year vesting periods. The Company also has an employee stock purchase plan ("ESPP").

Stock plan activity

The table below summarizes award activity for the three months ended June 30, 2014:

	Stock						Total
	Options			RSUs			Awards
Outstanding at March 31, 2014		6,774			1,476			8,250
Grants		966			927			1,893
Stock option exercises/vesting RSUs		(198	)		(248	)		(446	)
Retired or forfeited		(312	)		(152	)		(464	)
Outstanding at June 30, 2014		7,230			2,003			9,233
Total shares available for future grant at June 30, 2014								5,653

As of June 30, 2014, the total shares available for future grant under the ESPP were 864.

Impact of stock-based compensation

The following table summarizes stock-based compensation expense charged to selling, general and administrative ("SG&A") and research and development ("R&D") expenses for the three months ended June 30, 2014 and 2013:

	Three Months Ended June 30,
	2014		2013
SG&A:
Stock options	$	1,551	$	1,395
RSUs		1,194		768
ESPP		93		127
Total SG&A		2,838		2,290
R&D:
Stock options		214		196
RSUs		179		102
Total R&D		393		298
Total expense	$	3,231	$	2,588

Valuation of stock-based compensation

Stock-based compensation expense related to stock options is calculated based on the fair value of each award on the grant date. In general, the fair value for stock option grants with only a service condition is estimated using the Black-Scholes option pricing model with the following weighted-average assumptions for the three months ended June 30, 2014 and 2013:

	Three Months Ended June 30,
	2014			2013
Expected term (years)		4.63			4.53
Volatility		59.9	%		72.5	%
Risk-free interest rate		1.55	%		0.71	%
Expected dividend yield		-	%		-	%

RSUs are payable in shares of the Company's Class A common stock. The fair value of these stock-based awards is equal to the closing market price of the Company's common stock on the date of grant. The grant-date fair value is recognized on a straight-line basis in compensation expense over the vesting period of these stock-based awards, which is generally four years.

Stock-based compensation expense related to the ESPP is estimated using the Black-Scholes option pricing model with the following assumptions for the three months ended June 30, 2014 and 2013:

	Three Months Ended June 30,
	2014			2013
Expected term (years)		0.5			0.5
Volatility		34.8	%		57.5	%
Risk-free interest rate		0.08	%		0.12	%
Expected dividend yield		-	%		-	%