The Company’s stock-based awards that result in compensation expense consist of stock options and restricted stock units ("RSUs"). As of September 27, 2020, the Company had 16,450,000 shares available for grant under its stock plans. Stock options are granted with an exercise price equal to the market value of the Company’s common stock at the grant date and generally vest over four or five years based upon continuous service and expire ten years from the grant date. RSUs generally vest upon three years of continuous employment or incrementally over such three-year period. Participants are not entitled to dividends on RSUs.


	Shares (in thousands)	Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2019	12,899	$	37.95
Granted	975	51.58
Exercised	(3,408)	26.82
Forfeited or expired	(529)	50.34
Outstanding as of September 27, 2020	9,937	$	42.45	7.03	$	191,245
Exercisable as of September 27, 2020	3,807	$	33.58	5.83	$	107,040
Options vested or expected to vest as of September 27, 2020 (1)	9,125	$	41.73	6.92	$	182,232

(1) In addition to the vested options, the Company expects a portion of the unvested options to vest at some point in the future. Options expected to vest are calculated by applying an estimated forfeiture rate to the unvested options.


	Three-months Ended						Nine-months Ended
	September 27, 2020		September 29, 2019		September 27, 2020		September 29, 2019
Risk-free rate	—	%	2.7	%	1.6	%	2.7	%
Expected dividend yield	—	%	0.39	%	0.43	%	0.39	%
Expected volatility	—	%	37	%	37	%	37	%
Expected term (in years)	—		5.1		6.0		5.2

Generally, the current dividend yield is calculated by annualizing the cash dividend declared by the Company’s Board of Directors and dividing that result by the closing stock price on the grant date.

The expected volatility was based upon a combination of historical volatility of the Company’s common stock over the contractual term of the option and implied volatility for traded options of the Company’s stock.

There were no stock options granted during the three-month period ended September 27, 2020. The weighted-average grant-date fair values of stock options granted during the three-month period ended September 29, 2019 was $18.42. The weighted-average grant-date fair values of stock options granted during the nine-month periods ended September 27, 2020 and September 29, 2019 were $18.52 and $18.58, respectively.

The total intrinsic values of stock options exercised for the three-month periods ended September 27, 2020 and September 29, 2019 were $62,787,000 and $5,746,000, respectively. The total intrinsic values of stock options exercised for the nine-month periods ended September 27, 2020 and September 29, 2019 were $116,600,000 and $42,545,000, respectively. The total fair values of stock options vested for the three-month periods ended September 27, 2020 and September 29, 2019 were $1,331,000 and $1,213,000, respectively. The total fair values of stock options vested for the nine-month periods ended September 27, 2020 and September 29, 2019 were $39,282,000 and $32,072,000, respectively.


	Shares (in thousands)	Weighted-Average Grant Date Fair Value
Nonvested as of December 31, 2019	150	$	48.63
Granted	438	51.73
Vested	—	—
Forfeited or expired	(34)	50.60
Nonvested as of September 27, 2020	554	$	50.96

The weighted-average grant-date fair values of RSUs granted during the three-month periods ended September 27, 2020 and September 29, 2019 were $67.14 and $45.20, respectively. The weighted-average grant-date fair values of RSUs granted during the nine-month periods ended September 27, 2020 and September 29, 2019 were $51.73 and $45.20, respectively. There were no RSUs that vested during the three-month and nine-month periods ended September 27, 2020 and September 29, 2019.

The Company stratifies its employee population into two groups: one consisting of senior management and another consisting of all other employees. The Company currently applies an estimated annual forfeiture rate of 7% to all stock-based awards for senior management and a rate of 12% for all other employees. Each year during the first quarter, the Company revises its forfeiture rate. This resulted in an increase to compensation expense of $1,787,000 in 2020 and a decrease to compensation expense of $499,000 in 2019.

As of September 27, 2020, total unrecognized compensation expense related to non-vested equity awards, including stock options and RSUs, was $56,150,000, which is expected to be recognized over a weighted-average period of 1.8 years.

The total stock-based compensation expense and the related income tax benefit recognized for the three-month period ended September 27, 2020 were $9,268,000 and $1,524,000, respectively, and for the three-month period ended September 29, 2019 were $10,194,000 and $1,804,000, respectively. The total stock-based compensation expense and the related income tax benefit recognized for the nine-month period ended September 27, 2020 were $32,076,000, which includes credits of $1,401,000 relating to grants cancelled as a result of the Company's workforce reduction, and $5,365,000, respectively, and for the nine-month period ended September 29, 2019 were $33,442,000 and $5,839,000, respectively. No compensation expense was capitalized as of September 27, 2020 or December 31, 2019.

The following table presents the stock-based compensation expense by caption for each period presented on the Consolidated Statements of Operations (in thousands):


	Three-months Ended						Nine-months Ended
	September 27, 2020		September 29, 2019		September 27, 2020		September 29, 2019
Cost of revenue	$	324	$	334	$	1,041	$	1,114
Research, development, and engineering	2,815		3,616		10,582		11,633
Selling, general, and administrative	6,129		6,244		20,453		20,695
	$	9,268	$	10,194	$	32,076	$	33,442