Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

We have computed the fair value of employee stock options using the following weighted average assumptions:


	2017			2016			2015
Dividend yield		0.7	%		1.3	%		1.4	%
Volatility		39.1	%		34.5	%		28.6	%
Risk-free interest rate		2.2	%		1.2	%		1.8	%
Weighted average expected option term (in years)		4.6			5.5			5.6
Weighted average fair value per share of options granted	$	10.37		$	4.04		$	2.80
Aggregate intrinsic value of options exercised	$	13,086,167		$	2,391,154		$	6,534,800

A summary of option activity as of December 31, 2017 and the year then ended is presented below:


	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Balance outstanding at December 31, 2016 (1)		1,979,501		$	10.25		4.10	$	29,871,569
Granted		219,413		$	32.09
Exercised (2)		(635,503	)	$	8.65			$	13,086,167
Canceled / Expired		(27,129	)	$	11.91

Balance outstanding at December 31, 2017 (3)		1,536,282		$	13.86		4.15	$	27,771,635

Vested and exercisable at December 31, 2017 (4)		403,300		$	8.59		2.83	$	9,376,622
Expected to vest at December 31, 2017		1,132,982		$	15.74		4.62

Total		1,536,282

(1)	The aggregate intrinsic value represents the difference between the exercise price and $25.34, the closing price of our stock on December 31, 2016, for all in-the-money options outstanding.

(2)	The aggregate intrinsic value of shares exercised represents the difference between the exercise price and the closing price of our stock on the date of exercise.

(3)	The aggregate intrinsic value represents the difference between the exercise price and $31.84, the closing price of our stock on December 31, 2017, for all in-the-money options outstanding.

(4)	The aggregate intrinsic value represents the difference between the exercise price and $31.84, the closing price of our stock on December 31, 2017, for all in-the-money options vested and exercisable as of that date.

A summary of our RSU activity is as follows:


				Weighted
				Average
				Grant Date
	Shares			Fair Value
Balance outstanding at December 31, 2016		239,282		$	11.51
Granted		67,519		$	31.59
Vested (1)		(68,992	)	$	10.09
Canceled		(21,059	)	$	12.17

Balance outstanding at December 31, 2017		216,750		$	18.10

(1)	The number of RSUs vested includes the shares that we withheld on behalf of employees to satisfy minimum statutory tax withholding requirements.

A summary of our repurchases is as follows:


	2017		2016
Shares of common stock repurchased		27,291		21,671
Average per share repurchase price	$	28.51	$	14.33
Aggregate purchase price (in thousands)	$	778	$	311

The components of stock-based compensation expense included in the consolidated statements of operations are as follows:


	2017		2016		2015
	(in thousands)
Stock option awards	$	1,612	$	1,116	$	992
Restricted stock units		644		564		432

Total stock-based compensation	$	2,256	$	1,680	$	1,424

Stock-based compensation is included in our statements of operations as follows:


	2017		2016		2015
	(in thousands)
Cost of sales	$	188	$	175	$	165
Sales and marketing		403		373		284
General and administrative		1,484		983		869
Research and development		181		149		106

Total stock-based compensation	$	2,256	$	1,680	$	1,424

The dividend activity for the periods presented is as follows: