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Note 7 - Option Valuation Assumptions (Details)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Employee Stock Option [Member]      
Risk-free interest rate 1.70% 1.90% 1.10%
Volatility 92.70% 99.00% 104.20%
Expected life (years) 6 years 6 years 6 years
Employee Stock Purchase Plan [Member]      
Risk-free interest rate 0.10% 0.10% 0.10%
Volatility 86.70% 53.80% 76.00%
Expected life (years) 180 days 180 days 180 days