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Stock-Based Compensation (Tables)
 9 Months Ended
Oct. 01, 2021
Share-based Payment Arrangement [Abstract]  
Schedule of Allocated Employee Stock-Based Compensation Expense
We allocated the stock-based compensation expense for our equity incentive plans and our Employee Stock Purchase Plan (ESPP) as follows (in thousands):
Three Months Ended September 30,Nine Months Ended September 30,
2021202020212020
Research and development$11,487 $18,936 $37,550 $30,134 
Selling, general and administrative22,479 36,719 59,104 55,657 
Total stock-based compensation expense$33,966 $55,655 $96,654 $85,791 
Stock-based compensation for each type of award under our equity incentive plans and ESPP were as follows (in thousands):
Three Months Ended September 30,Nine Months Ended September 30,
2021202020212020
Stock options$4,607 $5,021 $15,203 $15,316 
Restricted stock units13,721 8,816 40,802 25,212 
Performance stock units14,971 41,226 37,616 43,151 
ESPP667 592 3,033 2,112 
Total stock-based compensation expense$33,966 $55,655 $96,654 $85,791 
Schedule of Share-Based Payment Award, Equity Instruments Other Than Options, Valuation Assumptions
A Monte Carlo simulation model was used to determine the grant date fair value of $24.54 for the 2021 PSUs based on the following assumptions:
Fair value of the Company’s common stock on grant date
$21.31 
Expected volatility
49 %
Risk-free interest rate
0.29 %
Dividend yield
— %