Note 11 — Stock-Based Compensation

We issue stock-based awards from our equity incentive plans, which are more fully described in Note 9. Stock-based compensation expense was recognized as follows (in thousands):

	Year Ended December 31,
	2015		2014		2013
Cost of goods sold	$	1,142	$	1,161	$	1,297
Research and development		9,207		7,528		7,910
General and administrative		9,320		8,328		8,501
Total stock-based compensation	$	19,669	$	17,017	$	17,708

As of December 31, 2015, total unrecognized compensation costs of $58.5 million related to unvested stock-based compensation arrangements are expected to be recognized as expense over a weighted-average period of 1.9 years.

Black-Scholes Assumptions

The following table lists the Black-Scholes option-pricing model assumptions used to calculate the fair value of employee and director stock options.

	Year Ended December 31,
	2015			2014			2013
Average risk-free interest rate		1.6	%		1.6	%		0.9	%
Dividend yield		0.0	%		0.0	%		0.0	%
Average volatility factor		50.8	%		51.6	%		61.2	%
Average weighted average expected life	5.3 years			5.2 years			5.2 years

The average risk-free interest rate is based on the U.S. treasury yield curve in effect at the time of grant for periods commensurate with the expected life of the stock-based award. We have never paid dividends, nor do we expect to pay dividends in the foreseeable future; therefore, we used a dividend yield of 0.0%. Our estimate of expected volatility is based on the daily historical trading data of our common stock at the time of grant over a historical period commensurate with the expected life of the stock-based award. For the years ended December 31, 2015, 2014, and 2013, we estimated the weighted-average expected life based on the contractual and vesting terms of the stock options, as well as historical cancellation and exercise data.

Stock-based compensation expense resulting from our ESPP was not material in the years ended December 31, 2015, 2014, and 2013.

Summary of Stock Option Activity

The table below presents a summary of stock option activity under our equity incentive plans (in thousands, except for price per share and contractual life information):

	Number of Shares			Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Life (in Years)		Aggregate Intrinsic Value(1)
Outstanding at December 31, 2014		21,995		$	10.50
Options granted		1,746			14.00
Options exercised		(3,937	)		7.89
Options forfeited & canceled		(1,022	)		13.42
Outstanding at December 31, 2015		18,782		$	11.22		4.67	$	105,830

Vested and expected to vest at December 31, 2015		18,360		$	11.14		4.62	$	104,749
Exercisable at December 31, 2015		12,222		$	9.89		3.60	$	85,074

(1)	Aggregate intrinsic value represents the difference between the exercise price of the option and the closing market price of our common stock on December 31, 2015.

The weighted-average grant-date fair value per share of options granted during the years ended December 31, 2015, 2014, and 2013 was $6.55, $6.50, and $4.95, respectively. The total intrinsic value of options exercised during the years ended December 31, 2015, 2014, and 2013 was $20.5 million, $25.9 million, and $4.5 million, respectively. The estimated fair value of options vested during the years ended December 31, 2015, 2014, and 2013 was $16.8 million, $15.2 million, and $14.1 million, respectively.

Summary of RSU Activity

During the year ended December 31, 2015, we granted RSUs to officers, employees and non-employee directors. No RSUs were granted in 2014 or 2013. Certain of the RSUs granted in 2015 vest upon achievement of pre-determined performance milestones, while the rest of the RSUs have a time-based vesting schedule. We expense the grant date fair value of the RSUs expected to vest ratably over the expected service or performance period.

A summary of RSU award activity is as follows (in thousands except for per share amounts):

	Units Issued		Weighted- Average Grant Date Fair Value		Aggregate Intrinsic Value(1)
Balance at December 31, 2014		—
Granted		1,306	$	15.30
Vested		—
Forfeited and canceled		—
Balance at December 31, 2015		1,306			$	22,012

(1)	Aggregate intrinsic value represents the difference between the grant price of the award and the closing market price of our common stock on December 31, 2015.