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Note 9 - Stock-based Compensation (Tables)
3 Months Ended
Mar. 31, 2019
Notes Tables  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
Dividend yield
   
 
Expected volatility
   
56.1
%
Risk-free interest rate
   
2.52
%
Expected life of options (in years)
   
4.00