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Stockholders' Equity (Tables)
 12 Months Ended
Dec. 31, 2019
Stock-Based Compensation Expense The following table presents stock-based compensation expense in the Company’s consolidated statements of comprehensive income:
 
 
For the Years Ended December 31,
 
 
 
2019
 
 
2018
 
 
2017
 
  
(Amounts in thousands)
 
Cost of product revenue
  $
1,368
    $
1,019
    $
704
 
Research and development
   
1,373
     
917
     
481
 
Selling, general and administrative
   
10,106
     
8,256
     
5,562
 
                         
Total stock-based compensation
  $
12,847
    $
10,192
    $
6,747
 
                         
Estimated Weighted Average Assumptions
The fair value of share-based awards granted during the years ended December 31, 2019, 2018 and 2017 were calculated using the following estimated assumptions:
 
For the Years Ended December 31,
 
  
2019
 
  
2018
 
  
2017
 
Expected term (in years)
   
5.5
-
6.5
      
5.5
-
7.5
      
6.1
 
Expected volatility (range)
   
45.14
 
–
 
50.87
%
     
45.14
 
–
 
50.87
%
     
51.48%
 
Risk-free interest rate
   
1.55
 
–
 
2.56
%
     
2.63
 
–
 
2.96
%
     
1.88
 
–
 
1.99
%
 
Expected dividend yield
   
0%
     
0%
     
0%
 
Summary of Option Activity
Information regarding option activity for the year ended December 31, 2019 under the Plans is summarized below:
  
Shares
 
  
Weighted
average
exercise
price
 
  
Weighted-
Average
Remaining
Contractual
Term
(in Years)
 
  
Aggregate
Intrinsic Value
(in Thousands)
 
Options outstanding at
December
31, 2018
   
998,226
    $
27.54
     
     
 
Granted
   
54,996
    $
64.14
     
     
 
Exercised
   
(87,663
 )   $
13.30
     
     
 
Forfeited/expired/cancelled
   
(8,000
 )   $
43.60
     
     
 
                                 
Options outstanding at December 31, 2019
   
957,559
    $
30.81
     
6.69
    $
59,073
 
                                 
Options exercisable at December 31, 2019
   
513,577
    $
24.44
     
5.39
    $
34,954
 
                                 
Vested and expected to vest at December 31, 2019
(1)
    
923,355
     
     
6.63
    $
57,235
 
                                 
 
(1) Represents the number of vested options as of December 31, 2019 plus the number of unvested options expected to vest as of December 31, 2019 based on the unvested outstanding options at December 31, 2019 adjusted for estimated forfeiture rates of
8
% for awards granted to
non-executive
level employees and
3
% for awards granted to executive level employees.
Summary of Restricted Stock Unit Activity
The fair value of stock units is calculated using the closing price of the Company’s common stock on the date of grant.
 
Information regarding stock unit activity
, which includes activity for restricted stock units and performance stock units,
for the
year
ended December 
31
,
2019
under the Plans is summarized below:
  
Shares
 
  
Weighted-
Average
Remaining
Contractual
Term
(in Years)
 
  
Aggregate
Intrinsic
Value
(in Thousands)
 
Unvested at December 31, 2018
   
705,413
     
     
 
Awarded
   
328,812
     
     
 
Vested
   
(251,776
 )    
     
 
Forfeited/expired/cancelled
   
(47,465
 )    
     
 
                         
Unvested at December 31, 2019
   
734,984
     
3.81
    $
67,986
 
                         
Vested and expected to vest at December 31, 2019
(1)
    
675,618
     
3.34
    $
62,495
 
                         
 
(1)
Represents the number of vested stock units as of December 31, 2019 plus the number of unvested stock units expected to vest as of December 31, 2019 based on the unvested outstanding stock units at December 31, 2019 adjusted for estimated forfeiture rates
of
8
%
for awards granted to
non-executive
 
level employees and
 
3
% for awards granted to executive level employees.