The compensation cost that has been included in the Unaudited Consolidated Statements of Operations for the Company’s stock-based compensation plans was as follows:

	Three Months Ended June 30,					Six Months Ended June 30,
(in thousands)	2020		2019			2020			2019
Selling, general and administrative expense (benefit)	$	3,546	$	5,911		$	(615	)	$	10,690
Research and development expense		1,432		752			2,755			1,652
Cost of sales		115		238			95			276
Stock-based compensation expense before taxes		5,093		6,901			2,235			12,618
Related income tax expense (benefit)		587		(1,725	)		258			(3,155	)
Stock-based compensation expense, net of taxes	$	5,680	$	5,176		$	2,493		$	9,463

At June 30, 2020, there was $49.8 million of unamortized compensation expense for restricted stock units (“RSUs”) and performance-based restricted stock units (“PRSUs”) to be recognized over a weighted average period of 2.4 years.

The Company issued approximately 13,000 and 177,000 shares of common stock, before net share settlement, upon vesting of RSUs and PRSUs during the three and six months ended June 30, 2020, respectively, and issued approximately 642,000 shares of common stock, before net share settlement, upon vesting of RSUs and PRSUs during the year ended December 31, 2019.

The weighted average assumptions used to estimate the fair value of stock purchase rights under the employee stock purchase plan (“ESPP”) are as follows:

	Three Months Ended June 30,						Six Months Ended June 30,
	2020			2019			2020			2019
ESPP
Volatility		66	%		37	%		47	%		37	%
Expected term (years)		0.5			0.5			0.5			0.5
Risk free interest rate		0.4	%		2.4	%		1.0	%		2.5	%
Expected dividend yield		—	%		—	%		—	%		—	%

Under the terms of the ESPP, the Company’s employees (referred to as “shareowners”) can elect to have up to 15% of their annual compensation, up to a maximum of $21,250 per year, withheld to purchase shares of the Company’s common stock for a purchase price equal to 85% of the lower of the fair market value per share (at closing) of the Company’s common stock on (i) the commencement date of the six-month offering period, or (ii) the respective purchase date.

The Company has not granted any options since 2011. The Company issued approximately 3,000 shares of common stock, before net share settlement, upon the exercise of outstanding stock options during both the three and six months ended June 30, 2020, and issued approximately 33,000 shares of common stock, before net share settlement, upon the exercise of outstanding stock options during the year ended December 31, 2019.