Employee Benefit Plans

The Company grants options and RSUs under the 2016 Incentive Plan (the "2016 Plan"), under which awards may be granted to all employees. Award vesting periods for this plan are generally four years. As of April 1, 2018, approximately 0.6 million shares were reserved for future grants under the 2016 Plan.

Additionally, the Company sponsors an Employee Stock Purchase Plan (the “ESPP”), pursuant to which eligible employees may contribute up to 10% of compensation, subject to certain income limits, to purchase shares of the Company’s common stock. The terms of the plan include a look-back feature that enables employees to purchase stock semi-annually at a price equal to 85% of the lesser of the fair market value at the beginning of the offering period or the purchase date. The duration of each offering period is generally six-months. As of April 1, 2018, approximately 0.8 million shares were available for issuance under the ESPP.

Option Activity

Stock option activity during the three months ended April 1, 2018 was as follows:



	Number of shares		Weighted Average Exercise Price Per Share
	(In thousands)		(In dollars)
Outstanding as of December 31, 2017	1,879		$	34.08
Granted	348		70.15
Exercised	(73	)	25.56
Cancelled	—		—
Expired	(5	)	20.07
Outstanding as of April 1, 2018	2,149		$	40.25

RSU Activity

RSU activity during the three months ended April 1, 2018 was as follows:



	Number of shares		Weighted Average Grant Date Fair Value Per Share
	(In thousands)		(In dollars)
Outstanding as of December 31, 2017	1,130		$	43.22
Granted	608		70.15
Vested	(109	)	41.09
Cancelled	(12	)	49.67
Outstanding as of April 1, 2018	1,617		$	53.44

Valuation and Expense Information

The Company measures stock-based compensation at the grant date based on the estimated fair value of the award. Estimated compensation cost relating to RSUs is based on the closing fair market value of the Company’s common stock on the date of grant. The fair value of options granted and the purchase rights granted under the ESPP is estimated on the date of grant using a Black-Scholes-Merton option valuation model that uses the assumptions noted in the following table. The estimated expected term of options granted is derived from historical data on employee exercise and post-vesting employment termination behavior. The risk free interest rate of options granted and the purchase rights granted under the ESPP is based on the implied yield currently available on U.S. Treasury securities with a remaining term commensurate with the estimated expected term. Expected volatility of options granted and the purchase rights granted under the ESPP is based on historical volatility over the most recent period commensurate with the estimated expected term.

The table below sets forth the weighted average assumptions used to estimate the fair value of option grants and purchase rights granted under the ESPP during the three months ended April 1, 2018 and April 2, 2017.



	Three Months Ended
	Stock Options			ESPP
	April 1, 2018		April 2, 2017	April 1, 2018		April 2, 2017
Expected life (in years)	4.4		NA	0.5		0.5
Risk-free interest rate	2.32	%	NA	1.81	%	0.66	%
Expected volatility	30.9	%	NA	37.1	%	27.6	%
Dividend yield	—		NA	—		—

The following table sets forth the stock-based compensation expense resulting from stock options, RSUs and the ESPP included in the Company’s unaudited condensed consolidated statements of operations:



	Three Months Ended
	April 1, 2018		April 2, 2017
	(In thousands)
Cost of revenue	$	852	$	436
Research and development	1,575		1,319
Sales and marketing	2,531		1,247
General and administrative	3,192		2,126
Total stock-based compensation	$	8,150	$	5,128

As of April 1, 2018, $13.3 million of unrecognized compensation cost related to stock options is expected to be recognized over a weighted-average period of 2.8 years. $72.8 million of unrecognized compensation cost related to unvested RSUs is expected to be recognized over a weighted-average period of 2.8 years.