STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2019

STOCK-BASED COMPENSATION

Summary of ESPP expense incurred under the 2016 Employee Stock Purchase Plan and stock-based compensation expense incurred

The following table summarizes ESPP expense incurred under the 2016 Employee Stock Purchase Plan and included in our accompanying consolidated statements of operations:

(in thousands)	Years Ended December 31,
	2019		2018		2017
Cost of sales	$	18	$	11	$	6
Research and development		29		16		1
Selling, general, and administrative		100		75		61
	$	147	$	102	$	68

The following table summarizes stock-based compensation expense incurred under the 2008 Plan and included in our consolidated statements of operations:

	Years Ended December 31,
(in thousands)	2019		2018		2017
Cost of sales	$	101	$	87	$	86
Research and development		756		771		677
Selling, general, and administrative		8,213		5,822		5,259
	$	9,070	$	6,680	$	6,022

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions

For 2019, 2018, and 2017, the fair value of each option grant was estimated using the Black-Scholes option-pricing model, using the following assumptions:

	Years Ended December 31,
	2019	2018	2017
Expected option life (years)	5.50 - 6.25	5.48 - 6.25	5.33 - 7.00
Risk-free interest rate	1.91% - 2.58%	2.64% - 2.93%	1.93% - 2.33%
Expected stock price volatility	63.1% - 66.7%	55.1% - 60.6%	50.3% - 57.4%
Dividend yield	—	—	—

Summary of Stock option and restricted stock activity

A summary of stock option activity under the 2008 Plan during the years ended December 31, 2019, 2018, and 2017 is presented below:

				Weighted		Weighted
				Average		Average
		Weighted		Grant-		Remaining
(in thousands, except per share and	Option	Average		date		Term	Aggregate
remaining term data)	Shares	Exercise Price		Fair Value		(years)	Intrinsic Value
Outstanding December 31, 2016	578	$	39.28			8.2	$	12,928
Granted	207		51.66	$	27.04
Exercised	(13)		15.92					542
Forfeited	(5)		53.29
Expired	—		—
Outstanding December 31, 2017	767	$	42.93			7.8	$	16,785
Granted	156		57.60	$	31.76
Exercised	(142)		19.47					5,863
Forfeited	(18)		60.17
Expired	(4)		74.53
Outstanding at December 31, 2018	759	$	49.74			7.6	$	2,221
Granted	160		65.97	$	40.14
Exercised	(130)		41.99					3,335
Forfeited	(31)		56.66
Expired	(1)		54.36
Outstanding at December 31, 2019	757	$	54.21			7.2	$	6,761
Exercisable at December 31, 2019	366	$	50.47			6.2	$	4,469

Non-vested Restricted Stock Shares Activity

A summary of RSA activity under the Plan during the years ended December 31, 2019, 2018, and 2017 is presented below:

		Weighted
		Average Grant		Weighted Average
(in thousands, except per share and		Date Fair		Remaining Term
remaining term data)	Shares	Value		(years)
Unvested at December 31, 2016	63	$	45.72	2.2
Granted	50		49.51
Vested	(28)		44.49
Forfeited	—		—
Unvested at December 31, 2017	85	$	48.34	2.6
Granted	65		58.11
Vested	(33)		47.34
Forfeited	—		—
Unvested at December 31, 2018	117	$	54.04	2.1
Granted	122		66.39
Vested	(42)		54.77
Forfeited	(5)		62.63
Unvested at December 31, 2019	192	$	61.46	2.6