The Company’s stock-based awards that result in compensation expense consist of stock options and restricted stock units ("RSUs"). As of June 28, 2020, the Company had 16,335,000 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	(1,822)	26.47
Forfeited or expired	(421)	50.28
Outstanding as of June 28, 2020	11,631	$	40.45	6.98	$	212,847
Exercisable as of June 28, 2020	5,331	$	31.59	5.81	$	144,694
Options vested or expected to vest as of June 28, 2020 (1)	10,683	$	39.58	6.85	$	204,795

(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						Six-months Ended
	June 28, 2020		June 30, 2019		June 28, 2020		June 30, 2019
Risk-free rate	1.6	%	2.7	%	1.6		2.7	%
Expected dividend yield	0.43	%	0.39	%	0.43	%	0.39	%
Expected volatility	37	%	37	%	37	%	37	%
Expected term (in years)	6.0		5.2		6.0		5.3

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.

The weighted-average grant-date fair values of stock options granted during the three-month periods ended June 28, 2020 and June 30, 2019 were $19.13 and $18.68, respectively. The weighted-average grant-date fair values of stock options granted during the six-month periods ended June 28, 2020 and June 30, 2019 were $18.52 and $18.59, respectively.

The total intrinsic values of stock options exercised for the three-month periods ended June 28, 2020 and June 30, 2019 were $39,359,000 and $14,220,000, respectively. The total intrinsic values of stock options exercised for the six-month periods ended June 28, 2020 and June 30, 2019 were $53,814,000 and $36,799,000, respectively. The total fair values of stock options vested for the three-month periods ended June 28, 2020 and June 30, 2019 were $1,287,000 and $887,000, respectively. The total fair values of stock options vested for the six-month periods ended June 28, 2020 and June 30, 2019 were $37,951,000 and $30,859,000, respectively.


	Shares (in thousands)	Weighted-Average Grant Date Fair Value
Nonvested as of December 31, 2019	150	$	48.63
Granted	432	51.53
Vested	—	—
Forfeited or expired	(21)	50.54
Nonvested as of June 28, 2020	561	$	50.79

The weighted-average grant-date fair values of RSUs granted during the three-month and six-month periods ended June 28, 2020 were $56.97 and $51.53, respectively. There were no RSUs that vested during the three-month and six-month periods ended June 28, 2020. There were no RSUs granted or vested during the three-month and six-month periods ended June 30, 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 June 28, 2020, total unrecognized compensation expense related to non-vested equity awards, including stock options and RSUs, was $65,128,000, which is expected to be recognized over a weighted-average period of 2.0 years.

The total stock-based compensation expense and the related income tax benefit recognized for the three-month period ended June 28, 2020 were $8,018,000, which includes credits of $1,401,000 relating to grants cancelled as a result of the Company's workforce reduction, and $1,277,000, respectively, and for the three-month period ended June 30, 2019 were $10,967,000 and $1,813,000, respectively. The total stock-based compensation expense and the related income tax benefit recognized for the six-month period ended June 28, 2020 were $22,808,000 and $3,841,000, respectively, and for the six-month period ended June 30, 2019 were $23,248,000 and $4,035,000, respectively. No compensation expense was capitalized as of June 28, 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						Six-months Ended
	June 28, 2020		June 30, 2019		June 28, 2020		June 30, 2019
Cost of revenue	$	363	$	329	$	717	$	780
Research, development, and engineering	2,401		3,550		7,767		8,017
Selling, general, and administrative	5,254		7,088		14,324		14,451
	$	8,018	$	10,967	$	22,808	$	23,248