Note 4 - Net Income Per Share - Components of Common Shares Outstanding (Details) - shares	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Weighted average common shares outstanding – basic (in shares)	4,846,010	4,843,032	4,846,010	4,842,524
Dilutive effect of stock options (in shares)	1,871	9,612	3,347	8,548
Shares used in computing net income per share – diluted (in shares)	4,847,881	4,852,644	4,849,357	4,851,072