11. Stock-based Compensation

The following table presents the classification of stock-based compensation for the periods indicated:

	Three Months Ended October 31,				Nine Months Ended October 31,
	2017		2016		2017		2016
	(in thousands)
Stock-based compensation:
Cost of revenue	$	343	$	282	$	978	$	773
Research and development		8,906		7,804		25,532		21,396
Selling, general and administrative		5,419		4,621		15,565		13,305
Total stock-based compensation	$	14,668	$	12,707	$	42,075	$	35,474

As of October 31, 2017, total unrecognized compensation cost related to unvested stock options was $6.5 million and is expected to be recognized over a weighted-average period of 2.05 years. Total unrecognized compensation cost related to unvested restricted stock units was $110.6 million and is expected to be recognized over a weighted-average period of 2.88 years. Total unrecognized compensation cost related to unvested restricted stock awards was $4.0 million and is expected to be recognized over a weighted-average period of 1.25 years.

The following table sets forth the weighted-average assumptions used to estimate the fair value of stock options and employee stock purchase plan awards for the periods indicated:

	Three Months Ended October 31,						Nine Months Ended October 31,
	2017			2016			2017			2016
Stock Options:
Volatility		53	%		39	%		53	%		38	%
Risk-free interest rate		1.78	%		1.27	%		2.02	%		1.51	%
Expected term (years)	6.08				6.08		6.07				5.98
Dividend yield		0	%		0	%		0	%		0	%
Employee stock purchase plan awards:
Volatility		50	%		41	%		45	%		54	%
Risk-free interest rate		1.17	%		0.49	%		1.03	%		0.51	%
Expected term (years)		0.5			0.5			0.5			0.5
Dividend yield		0	%		0	%		0	%		0	%

The Company calculates expected volatility for stock options based on the weighted average of historical volatilities of its own stock price and the share prices of similar companies that are publicly available for a period commensurate with the expected term. The Company calculates expected volatility for ESPP based on its own historical stock price for a period commensurate with the offering period.

The following table summarizes stock option activity for the nine months ended October 31, 2017:

	Options Outstanding
										Weighted-
								Total Intrinsic		Average
						Weighted-		Value Of		Remaining		Aggregate
				Weighted-		Average		Options		Contractual		Intrinsic
				Average		Grant-date		Exercised		Term		Value
	Shares			Exercise Price		Fair Value		(in thousands)		(in years)		(in thousands)
Outstanding at January 31, 2017		1,703,524		$	21.66
Granted		91,300			52.57	$	27.20
Exercised		(135,180	)		12.34			$	5,569
Forfeited		(21,057	)		56.87
Expired		(18,045	)		30.88
Outstanding at October 31, 2017		1,620,542			23.62					4.74		$	55,763
Exercisable at October 31, 2017		1,351,300		$	17.95						4.06	$	53,559

The intrinsic value of options outstanding and options exercisable is calculated based on the difference between the fair market value of the Company’s ordinary shares on the reporting date and the exercise price. The closing price of the Company’s ordinary shares on October 31, 2017 was $56.44, as reported by The NASDAQ Global Market. The intrinsic value of exercised options is calculated based on the difference between the fair market value of the Company’s ordinary shares on the exercise date and the exercise price.

The following table summarizes restricted stock and restricted stock units activities for the period indicated:

				Weighted-
				Average
				Grant-Date
	Shares			Fair Value
Unvested at January 31, 2017		2,175,673		$	56.76
Granted		1,001,785			46.50
Vested		(767,449	)		45.84
Forfeited		(99,863	)		54.66
Unvested at October 31, 2017		2,310,146		$	56.03

As of October 31, 2017, the aggregate intrinsic value of unvested restricted stock and restricted stock units was $130.4 million.