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2. STOCK-BASED COMPENSATION - Options (Details 1) - Stock Options - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 6 Months Ended
Nov. 30, 2015
Nov. 30, 2014
Nov. 30, 2015
Nov. 30, 2014
Expected term (in years) 4 years 4 years 4 years 4 years
Volatility 86.00% 90.00% 86.00% 91.00%
Expected dividend $ 0 $ 0 $ 0 $ 0
Risk-free interest rates 1.21% 1.11% 1.21% 1.25%
Estimated forfeiture rate 0.25% 0.25% 0.25% 0.25%
Weighted average grant date fair value $ 1.38 $ 1.32 $ 1.38 $ 1.64