We recognize total stock-based compensation expense in our Consolidated Statements of Operations as follows (in thousands):


	Year Ended December 31,
	2020		2019		2018
Cost of goods sold	$	2,825	$	4,294	$	4,629
Research and development	57,116		63,224		56,193
General and administrative	33,295		32,277		27,279
Impairment of assets and other costs for terminated program	1,025		—		—
Total stock-based compensation	$	94,261	$	99,795	$	88,101

The stock-based compensation expense reported in impairment of assets and other costs for terminated program results from executive severance.

As of December 31, 2020, total unrecognized compensation costs of $193.7 million related to unvested stock-based compensation arrangements are expected to be recognized as expense over a weighted-average period of 1.79 years.

Stock-based compensation expense resulting from PSUs and our ESPP was not significant in the years ended December 31, 2020, 2019, and 2018.

The following table lists the Black-Scholes option-pricing model assumptions used to calculate the fair value of employee and director stock options, as well as the resulting grant-date fair value:


	Year Ended December 31,
	2020			2019			2018
Average risk-free interest rate	0.4		%	1.8		%	2.8		%
Dividend yield	0.0		%	0.0		%	0.0		%
Average volatility factor	64.1		%	62.2		%	61.0		%
Weighted-average expected life	5.6 years			5.6 years			5.1 years
Weighted-average grant-date fair value of options granted	$	10.70		$	12.25		$	29.86

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 zero. 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. 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.

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, 2019	14,885	$	25.23
Options granted	1,654	19.15
Options exercised	(1,669)	11.69
Options forfeited & canceled	(1,233)	45.70
Outstanding at December 31, 2020	13,637	$	24.30	3.96	$	23,478

Exercisable at December 31, 2020	9,523	22.99		2.71	$	23,468

(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, 2020.

The intrinsic value of options exercised during the years ended December 31, 2020, 2019 and 2018 totaled $15.9 million, $30.6 million and $303.4 million, respectively.

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


	Units Issued	Weighted- Average Grant Date Fair Value
Balance at December 31, 2019	4,935	$	30.85
Granted	4,686	19.24
Vested and released	(1,661)	34.83
Forfeited and canceled	(866)	29.66
Balance at December 31, 2020	7,094	$	22.46

The fair value of restricted stock that vested in the years ended December 31, 2020, 2019 and 2018 totaled $33.3 million, $32.4 million and $80.4 million, respectively.