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Stock-based compensation
3 Months Ended
Mar. 31, 2020
Share-based Payment Arrangement [Abstract]  
Stock-based compensation Stock-based compensation 
The following table summarizes the stock-based compensation expense recognized in the three months ended March 31, 2020 and 2019 (in thousands):

 Three Months Ended
March 31,
 20202019
Cost of goods sold$537  $601  
Research and development758  881  
Sales and marketing530  678  
General and administrative693  1,178  
 $2,518  $3,338  
 Determining Fair Value 
The Company estimated the fair value of certain stock-based awards using a Black-Scholes-Merton valuation model.
Stock Options and Restricted Stock Units (RSUs)
The following table summarizes the Company’s stock option and RSU activity, excluding market-based RSUs, during the three months ended March 31, 2020: 

Stock OptionsRestricted Stock Units
 Number of
Shares
Weighted
Average
Exercise
Price
Number of
Units
Weighted
Average
Grant Date
Fair Value
Balance as of December 31, 20192,598,745  $5.93  3,170,981  $5.80  
Granted
—  —  224,131  6.72  
Exercised/Converted
(52,501) 4.40  (114,188) 5.96  
Cancelled/Forfeited
(23,505) 11.15  (61,675) 6.10  
Balance as of March 31, 20202,522,739  5.91  3,219,249  5.85  
At March 31, 2020, the Company had $0.5 million of unrecognized stock-based compensation expense for stock options, net of estimated forfeitures. At March 31, 2020, the Company had $11.7 million of unrecognized stock-based compensation expense for RSUs, excluding market-based RSUs, net of estimated forfeitures.
Market-based Restricted Stock Units
As of March 31, 2020, the Company has granted 705,000 shares of market-based RSUs to certain employees. These RSUs will vest if the 30-day weighted average closing price of the Company's common stock is equal to or greater than certain price targets per share and the recipients remain in continuous service with the Company through such service period. No market-based RSUs have vested and 93,500 market-based RSUs have been cancelled through March 31, 2020. The weighted average grant-date fair value per share of market-based RSUs granted during 2019 and 2018 was approximately $4.86 and $5.82 per share, respectively. As of March 31, 2020, the Company had $1.1 million of unrecognized stock-based compensation expense for these RSUs, net of estimated forfeitures, which will be recognized over the remaining weighted-average period of 1.30 years. The fair value of market-based RSUs was measured on the grant date using Monte Carlo simulation model with the following assumptions:
Market-based restricted stock unitsAssumptions
Used
Weighted-average volatility66%
Risk-free interest rate2.79%
Expected dividends—%
Stock Appreciation Units (SAU) 
SAUs are liability classified share-based awards. Outstanding SAUs are re-measured each reporting period at fair value until settlement. The Company did not grant any SAUs during the three months ended March 31, 2020 or 2019. As of March 31, 2020 and December 31, 2019, there were 153,404 and 163,471 SAUs outstanding, respectively, and related SAU liabilities were $0.5 million and $0.8 million, respectively.  
Employee Stock Purchase Plan (ESPP)
The Company did not issue any shares under the ESPP during the three months ended March 31, 2020. As of March 31, 2020, there was $0.4 million of unrecognized stock-based compensation expense for employee stock purchase rights that will be recognized over the remaining offering period through November 2020. In June 2019, the Company adopted an Amendment and Restatement of the ESPP to increase the number of shares authorized for issuance under the ESPP by an additional 1,500,000 shares and to eliminate the annual "'evergreen" automatic increase provisions.