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Stock-based compensation (Tables)
3 Months Ended
Mar. 31, 2020
Share-based Payment Arrangement [Abstract]  
Schedule of stock based compensation expense
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  
Schedule of stock option and restricted stock unit activity
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  
Schedule of estimated fair value of certain stock-based awards using Black-Scholes-Merton valuation model 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—%