Stock options (Tables)

A summary of stock option activity is as follows:



	Outstanding stock options
(in thousands, except for exercise prices and term)	Number of options		Range of exercise prices	Weighted- average exercise price		Weighted- average contractual term (in years)	Aggregate intrinsic value
Outstanding as of January 31, 2014	6,369		$1.10 - 4.50	$	1.77	6.34	$	14,621
Granted	1,946		$14.00 - 18.93	$	14.29
Exercised	(1,793	)	$0.50 - 2.50	$	1.34
Forfeited	(169	)	$1.25 - 14.00	$	9.97
Outstanding as of October 31, 2014	6,353		$0.10 - 18.93	$	4.79	7.03	$	99,157

A summary regarding non-vested and exercisable stock options is as follows:



	Non-vested stock options				Exercisable stock options
(in thousands, except for exercise prices and term)	Number of shares		Weighted- average grant date fair value		Number of shares		Weighted- average exercise price		Weighted- average contractual term (in years)	Aggregate intrinsic value
Balance as of January 31, 2014	2,812		$	0.26	3,557		$	1.30	5.03	$	9,835
Granted	1,946		$	5.25	—		$	—
Vesting	(2,216	)	$	0.22	2,216		$	1.25
Exercised	—		$	—	(1,793	)	$	1.34
Forfeited/expired	(164	)	$	3.61	(5	)	$	2.25
Balance as of October 31, 2014	2,378		$	4.15	3,975		$	0.80	5.59	$	77,916
Vested and expected to vest as of October 31, 2014					4,772		$	2.01	6.17	$	87,742

The key input assumptions that were utilized in the valuation of the stock options granted during the three and nine months ended October 31, 2014 and 2013 are as follows:



	Three months ended October 31,				Nine months ended October 31,
	2014		2013		2014		2013
Expected dividend yield	—	%	—	%	—	%	—	%
Expected stock price volatility	32.9	%	31.3	%	32.9	%	31.3	%
Risk-free interest rate	1.93	%	0.67	%	1.93% - 2.24%		0.35% - 0.67%
Expected life of options	6.25 years		3 years		6.25 years		3 years