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Stock-Based Compensation (Tables)

9 Months Ended

Sep. 28, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Allocated Employee Stock-Based Compensation Expense

We allocated the stock-based compensation expense for our equity incentive plans and our 2000 Employee Stock Purchase Plan (“ESPP”) as follows (in thousands):

	Three Months Ended September 30,								Nine Months Ended September 30,
	2018				2017				2018				2017
Research and development	$	3,169			$	1,663			$	9,102			$	4,741
Selling, general and administrative	6,573				3,626				19,228				10,288
Total stock-based compensation	$	9,742			$	5,289			$	28,330			$	15,029

Weighted Average Grant-Date Fair Value of Stock Options and ESPP Purchases

We used a Monte Carlo simulation pricing model to value stock options that include market vesting conditions and a Black-Scholes Merton option pricing model to value other stock options and ESPP purchases. The weighted average grant-date fair value per share of stock options and ESPP purchases were as follows:

	Three Months Ended September 30,								Nine Months Ended September 30,
	2018				2017				2018				2017
Stock options	$	8.67			$	11.75			$	9.13			$	10.32
ESPP	$	6.19			$	6.85			$	6.96			$	5.29

Valuation Assumptions Used to Estimate Grant-Date Fair Value

The grant-date fair value of stock option grants and ESPP purchases was estimated using the following assumptions:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2018			2017			2018			2017
Stock options:
Risk-free interest rate	2.91	%		1.70	%		2.83	%		1.68	%
Dividend yield	—	%		—	%		—	%		—	%
Volatility	55	%		58	%		55	%		61	%
Expected life	4.4 years			4.0 years			4.4 years			4.1 years
ESPP:
Risk-free interest rate	2.11	%		1.14	%		1.74	%		0.88	%
Dividend yield	—	%		—	%		—	%		—	%
Volatility	51	%		55	%		52	%		61	%
Expected life	6 months			6 months			6 months			6 months

Stock Option Activity

Activity for stock options during the nine months ended September 30, 2018 was as follows (dollars in thousands, except per share amounts):

	Shares			Weighted Average Exercise Price Per Share				Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
Options outstanding at December 31, 2017	22,208,446			$	6.83
Granted	3,022,113			$	19.52
Exercised	(2,083,457	)		$	5.01
Forfeited	(216,522	)		$	14.40
Expired	(5,548	)		$	18.62
Options outstanding at September 30, 2018	22,925,032			$	8.59			3.9 years		$	226,821
Exercisable at September 30, 2018	15,896,370			$	5.03			3.1 years		$	203,118

RSU Activity

Activity for RSUs during the nine months ended September 30, 2018 was as follows (dollars in thousands, except per share amounts):

	Shares			Weighted Average Grant Date Fair Value Per Share				Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
RSUs outstanding at December 31, 2017	3,762,990			$	17.76
Awarded	2,411,245			$	18.50
Vested and released	(374,992	)		$	8.18
Forfeited	(249,362	)		$	18.64
RSUs outstanding at September 30, 2018	5,549,881			$	18.69			2.1 years		$	98,344

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Tabular disclosure of the allocation of equity-based compensation costs to a given line item on the balance sheet and income statement for the period. This may include the reporting line for the costs and the amount capitalized and expensed.

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