Note 3 - Valuation and Qualifying Accounts - Activity in the Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Balance at beginning of period	$ 59	$ 39	$ 35
Provision for doubtful accounts	130	191	85
Write-off of uncollectible accounts receivable	(150)	(171)	(81)
Balance at end of year	$ 39	$ 59	$ 39