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Note 7 - Stock-based Compensation (Tables)
3 Months Ended
Mar. 31, 2022
Notes Tables  
Schedule of Share-Based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
  

Three months ended March 31,

 
  

2022

  

2021

 

Risk-free interest rate

 1.91% -2.41%  0.46%-0.92% 

Dividend yield

  0%    0%  

Expected term (years)

 5.75 -6.00   5.00  

Expected volatility

 45.8% -46.1%  92.6%-93.8%