The following table summarizes stock-based compensation expense by financial statement line item in the Company’s condensed consolidated statements of operation for each of the three and six months ended September 30, 2021 and 2020:

	For the Three Months Ended September 30,				For the Six Months Ended September 30,
		2021		2020		2021		2020
	(in thousands)
Cost of revenue	$	1,300	$	923	$	2,330	$	1,627
Research and development		2,294		1,705		4,403		3,147
Selling, general and administrative		12,155		8,937		21,624		16,089
	$	15,749	$	11,565	$	28,357	$	20,863

The following table summarizes the stock option activity for the six months ended September 30, 2021:


						Weighted
				Weighted		Average		Aggregate
				Average		Remaining		Intrinsic
	Options			Exercise		Contractual		Value
	(in thousands)			Price		Term (years)		(in thousands)
Outstanding at beginning of period		711		$	141.87	5.46
Granted		62			286.76
Exercised		(152	)		55.26
Cancelled and expired		(9	)		283.42
Outstanding at end of period		612		$	175.86		5.90		95,180
Exercisable at end of period		466		$	149.39	4.98			85,599
Options vested and expected to vest at end of period		612		$	175.86		5.90		95,180

Stock options generally vest and become exercisable annually over three years. The remaining unrecognized stock-based compensation expense for unvested stock option awards as of September 30, 2021, was approximately $10.9 million and the estimated weighted-average period over which this cost is expected to be recognized is 2.0 years.

The aggregate intrinsic value of stock options exercised was $42.8 million for the six months ended September 30, 2021. The total cash received as a result of employee stock option exercises for the six months ended September 30, 2021, was approximately $8.4 million.

The Company estimates the fair value of each stock option granted at the grant date using the Black-Scholes option valuation model.

The weighted average grant-date fair values and weighted average assumptions used in the calculation of fair value of options granted was as follows:

	For the Three Months Ended September 30,						For the Six Months Ended September 30,
		2021			2020			2021			2020
Weighted average grant-date fair value	$	115.99		$	100.51		$	103.48		$	76.10

Valuation assumptions:
Risk-free interest rate		0.77	%		0.30	%		0.79	%		0.31	%
Expected option life (years)		4.14			4.17			4.20			4.22
Expected volatility		44.72	%		43.50	%		44.30	%		42.82	%

The following table summarizes activity of restricted stock units for the six months ended September 30, 2021:

				Weighted
				Average
	Number of			Grant Date
	Shares			Fair Value
	(in thousands)			(per share)
Restricted stock units at beginning of period		301		$	273.57
Granted		156			288.64
Vested		(103	)		304.20
Forfeited		(19	)		270.35
Restricted stock units at end of period		335		$	271.32

Restricted stock units generally vest annually over three years. The remaining unrecognized compensation expense for outstanding restricted stock units, including performance and market-based awards, as of September 30, 2021 was $70.6 million and the estimated weighted-average period over which this cost is expected to be recognized is 2.1 years.

The weighted average grant-date fair value for restricted stock units granted during the six months ended September 30, 2021 was $288.64. The total fair value of restricted stock units vested during the six months ended September 30, 2021 was $31.4 million.

The Company grants performance-based restricted stock units to certain executive officers and employees, which vest upon achievement of prescribed service milestones by the award recipients and the achievement of prescribed performance milestones by the Company. As of September 30, 2021, the Company recognized compensation expense based on the probable outcomes related to the prescribed performance targets on the outstanding awards.

The Company grants market-based restricted stock units to certain executive officers and employees. These restricted stock units vest upon achievement of prescribed service-based milestones, relative TSR goals by the Company and the achievement of prescribed performance milestones, as defined in the respective agreements.

The relative total shareholder return (“TSR”) is based on the Company’s common stock in relation to the TSR of twenty peer companies over a defined period, based on a comparison of average closing stock prices during the 20 trading days prior to the first day of the performance period, reinstated dividends during each performance period and the average closing stock prices during the final 20 trading days of each performance period. The actual number of market-based restricted stock units that may be earned can range from 0% to 200% of the target number of shares. The payout percentage may be further adjusted based on the Company’s performance relative to the constituents of the S&P 500 Index on the first day of the performance period that are still actively trading on the last day of each performance period, as defined in the respective agreements.

The Company used a Monte-Carlo simulation model to estimate the grant-date fair value of the market-based restricted stock units. The fair value related to these awards are recorded as compensation expense over the period from date of grant, based on the probable outcomes related to the prescribed performance targets on the outstanding awards, regardless of the actual TSR outcome reached.

The table below sets forth the assumptions used to value the awards and the estimated grant-date fair value:

	May 2021			May 2020
Risk-free interest rate		0.3	%		0.2	%
Expected volatility		44.8	%		35.5	%
Dividend yield		—			—
Remaining performance period (years)		2.8		1.9 - 2.9
Estimated fair value per share	$	292.40		$347.05 - $349.28
Target performance (number of shares)		25,172			30,881