The following table summarizes the total stock-based compensation expense included in the Company’s statements of operations for the periods presented (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Cost of product sales	$	423	$	743	$	586	$	1,592
Research and development		7,319		7,235		12,149		15,692
Selling, general and administrative		14,263		11,529		22,454		24,571
	$	22,005	$	19,507	$	35,189	$	41,855

The fair value of each employee stock option and each employee stock purchase plan right granted is estimated on the grant date under the fair value method using the Black-Scholes valuation model, which requires the Company to make a number of assumptions including the estimated expected life of the award and related volatility. The fair value of restricted stock units is estimated based on the market price of the Company’s common stock on the date of grant. The estimated fair values of stock options, purchase plan rights, and restricted stock units are then expensed over the vesting period. Performance-based stock awards vest upon the achievement of certain pre-defined company-specific performance-based criteria. Expense related to these performance-based stock awards is generally recognized ratably over the expected performance period once the pre-defined performance-based criteria for vesting becomes probable. During the six months ended June 30, 2021, the Company had a change in estimate related to the achievement of certain performance-based criteria for performance-based stock awards which resulted in a reduction in stock-based compensation expenses by approximately $6.1 million.