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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2019
Stock-Based Compensation [Abstract]  
Classification of Stock-Based Compensation Expense
The following table presents the classification of stock-based compensation expense recognized for the periods below:

  
Three months ended
September 30,
  
Nine months ended
September 30,
 
(in thousands)
 
2019
  
2018
  
2019
  
2018
 
Selling, general and administrative expense
 
$
383
  
$
440
  
$
1,018
  
$
1,446
 
Research and development expense
  
29
   
(3
)
  
95
   
98
 
Total stock-based compensation expense
 
$
412
  
$
437
  
$
1,113
  
$
1,544