The Company’s stock-based awards that result in compensation expense consist of stock options and restricted stock units ("RSUs"). As of April 4, 2021, the Company had 15,650,000 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, 2020	8,970	$	44.73
Granted	421	90.50
Exercised	(897)	38.82
Forfeited or expired	(42)	48.68
Outstanding as of April 4, 2021	8,452	$	47.62	7.11	$	312,675
Exercisable as of April 4, 2021	3,824	$	38.18	5.92	$	176,405
Options vested or expected to vest as of April 4, 2021 (1)	7,733	$	46.78	7.00	$	292,344

(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.

The fair values of stock options granted in each period presented were estimated using the following weighted-average assumptions:


	Three-months Ended
	April 4, 2021		March 29, 2020
Risk-free rate	1.3	%	1.6	%
Expected dividend yield	0.27	%	0.43	%
Expected volatility	39	%	37	%
Expected term (in years)	5.7		6.0

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 April 4, 2021 and March 29, 2020 were $33.63 and $18.46, respectively.

The total intrinsic values of stock options exercised for the three-month periods ended April 4, 2021 and March 29, 2020 were $42,327,000 and $14,454,000, respectively. The total fair values of stock options vested for the three-month periods ended April 4, 2021 and March 29, 2020 were $35,600,000 and $36,664,000, respectively.


	Shares (in thousands)	Weighted-Average Grant Date Fair Value
Nonvested as of December 31, 2020	554	$	51.27
Granted	245	89.86
Vested	(2)	50.94
Forfeited or expired	(15)	52.22
Nonvested as of April 4, 2021	782	$	63.33

The weighted-average grant-date fair values of RSUs granted during the three-month periods ended April 4, 2021 and March 29, 2020 were $89.86 and $50.94, respectively. There were 2,000 RSUs that vested during the three-month period ended April 4, 2021 and no RSUs that vested during the three-month period ended March 29, 2020.

The Company segments 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 8% 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 based on updated estimates of employee turnover. This resulted in a decrease to compensation expense of $255,000 in 2021 and an increase to compensation expense of $1,787,000 in 2020.

As of April 4, 2021, total unrecognized compensation expense related to non-vested equity awards, including stock options and RSUs, was $69,821,000, 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 April 4, 2021 were $12,009,000 and $1,813,000, respectively, and for the three-month period ended March 29, 2020, were $14,790,000 and $2,564,000, respectively. No compensation expense was capitalized as of April 4, 2021 or December 31, 2020.

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
	April 4, 2021		March 29, 2020
Cost of revenue	$	248	$	354
Research, development, and engineering	4,003		5,366
Selling, general, and administrative	7,758		9,070
	$	12,009	$	14,790