4. FAIR VALUE MEASUREMENT (Details) - USD ($) $ in Thousands	May 31, 2018	May 31, 2017
Money market funds	$ 7,733	$ 15,516
U.S. Treasury securities	5,983	0
Certificate of deposit	80	50
Assets	13,796	15,566
Level 1
Money market funds	7,733	15,516
U.S. Treasury securities	5,983	0
Certificate of deposit	0	0
Assets	13,716	15,516
Level 2
Money market funds	0	0
U.S. Treasury securities	0	0
Certificate of deposit	80	50
Assets	80	50
Level 3
Money market funds	0	0
U.S. Treasury securities	0	0
Certificate of deposit	0	0
Assets	$ 0	$ 0

A savings certificate entitling the Entity (that is, bearer) to receive interest at an established maturity date, based upon a fixed interest rate. A certificate of deposit may be issued in any denomination. Certificates of deposit are generally issued by commercial banks and, therefore, insured by the FDIC (up to the prescribed limit). Certificates of deposit generally restrict holders from withdrawing funds on demand without the incurrence of penalties. Generally, only certificates of deposit with original maturities of three months or less qualify as cash equivalents. Original maturity means original maturity to the entity holding the investment. As a related example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant