13. STOCK-BASED COMPENSATION (Details 1) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
Feb. 29, 2020	Feb. 28, 2019	Feb. 29, 2020	Feb. 28, 2019
Stock Options
Expected term (in years)	5 years	5 years	5 years	5 years
Volatility	72.00%	69.00%	71.00%	72.00%
Risk-free interest rate	1.57%	2.77%	1.85%	2.83%
Weighted average grant date fair value	$ 1.37	$ 1.01	$ 0.98	$ 1.34
Employee Stock Purchase Plan
Expected dividend			$ 0
Estimated forfeiture rate			0.00%
Weighted average grant date fair value			$ 0.80
Employee Stock Purchase Plan \| Minimum
Expected term (in years)			6 months
Volatility			62.00%
Risk-free interest rate			1.56%
Employee Stock Purchase Plan \| Maximum
Expected term (in years)			2 years
Volatility			71.00%
Risk-free interest rate			1.81%

No definition available.

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- Definition

The estimated amount of dividends to be paid to holders of the underlying shares (expected dividends) over the option's term. Dividends are taken into account because payment of dividends to shareholders reduces the fair value of the underlying shares, and option holders generally do not receive dividends.

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- Definition

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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- Definition

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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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