11. STOCKHOLDERS' EQUITY AND STOCK-BASED COMPENSATION (Details 2) - Employee Stock Purchase Plan - $ / shares	12 Months Ended
May 31, 2021	May 31, 2020	May 31, 2019
Weighted-average grant date fair value	$ 1.03	$ 0.79	$ 1.14
Minimum
Expected term (in years)	6 months	6 months	6 months
Volatility	74.00%	62.00%	48.00%
Risk-free interest rates	0.04%	0.14%	2.33%
Maximum
Expected term (in years)	2 years	2 years	2 years
Volatility	88.00%	77.00%	78.00%
Risk-free interest rates	0.17%	1.81%	2.82%

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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