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Stock-Based Incentive Compensation Plans
9 Months Ended
Sep. 30, 2021
Share-based Payment Arrangement [Abstract]  
Stock-Based Incentive Compensation Plans Stock-Based Incentive Compensation Plans
Stock Options

The table below summarizes the stock option activity for the nine months ended September 30, 2021:
Number of
Shares
Weighted-
Average
Exercise
Price
Outstanding as of December 31, 20202,405,957 $8.54
Exercised
(317,001)7.34
Canceled and forfeited(25,217)15.78
Outstanding as of September 30, 20212,063,739 10.00
As of September 30, 2021, the unrecognized compensation cost related to stock options was $1.7 million, which is expected to be recognized over a period of approximately 1.2 years.
There were no stock options granted during the three months ended September 30, 2020 and nine months ended September 30, 2021. The table below summarizes the weighted average grant date fair value per share and the assumptions used to estimate the grant date fair value using the Black-Scholes option-pricing model of the stock options granted in the nine months ended September 30, 2020:
        
Nine Months Ended September 30,
2020
Weighted average grant date fair value per share
$8.37
Expected term (years)
5.5to7.0
Expected volatility
46.7%to47.2%
Risk-free interest rate
1.56%to1.64%
Dividend yield
—%

Early Exercise of Unvested Stock Options
Early exercises of stock options under the Company's 2008 Stock Option Plan are subject to a right of repurchase by the Company of any unvested shares. The repurchase rights lapse over the original vesting period of the options. The Company accounts for the cash received in consideration for the early exercised options as a liability included in accrued liabilities, which is then reclassified to stockholders’ equity as the options vest. As of September 30, 2021, the Company had no shares subject to repurchase. As of December 31, 2020, the Company had a total of 5,836 shares of common stock subject to repurchase under the 2008 Stock Option Plan.
Restricted Stock Units
The table below summarizes restricted stock units activity for the nine months ended September 30, 2021:
Number of SharesWeighted Average Grant Date Fair Value
Outstanding as of December 31, 20201,165,295$20.07
Granted1,049,69929.56
Vested(445,913)21.19
Canceled and forfeited(171,872)24.84
Outstanding as of September 30, 20211,597,20925.48

As of September 30, 2021, the unrecognized compensation cost related to the restricted stock units was $34.4 million, which is expected to be recognized over a period of approximately 2.9 years.
Employee Stock Purchase Plan
The Company’s 2018 Employee Stock Purchase Plan (the “ESPP”) allows eligible employees to purchase shares of the Company's common stock through payroll deductions at the price equal to 85% of the lesser of the fair market value of the stock as of the first date or the ending date of each six month offering period. The offering period generally commences in May and November. On March 26, 2020, the Company's Compensation Committee approved the amendment of the terms of future offerings under the ESPP which, among other things, increased the maximum number of shares that may be purchased on any single purchase date, provided for automatic enrollment in a new offering, and provided that the offering which commenced in May 2020 be twelve months in duration and consist of two purchase periods. The current offering period under the ESPP began on May 17, 2021 and the related purchase will occur on November 15, 2021.
The fair value of the ESPP shares is estimated using the Black-Scholes option pricing model, which is being amortized over the requisite service period. The Company issued 104,861 shares and 74,685 shares under the ESPP, representing approximately $1.6 million and $1.0 million in employee contributions, for nine months ended September 30, 2021 and September 30, 2020, respectively. As of September 30, 2021 and December 31, 2020, total accumulated ESPP related employee payroll deductions amounted to $0.7 million and $0.4 million, respectively, which were included within accrued compensation and related expenses in the condensed consolidated balance sheets.

Stock-Based Compensation
The table below presents the detail of stock-based compensation expense amounts included in the condensed consolidated statements of operations:
Three Months Ended
September 30,
Nine Months Ended
September 30,
2021202020212020
(in thousands)
Cost of goods sold
$128 $87 $432 $243 
Sales and marketing
2,153 1,488 6,177 3,976 
Research and development
434 299 1,257 827 
General and administrative
1,526 1,306 4,662 3,711 
$4,241 $3,180 $12,528 $8,757