Note 4 - Net Income Per Share - Components of Common Shares Outstanding (Details) - shares	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Weighted average common shares outstanding – basic (in shares)	4,841,010	4,835,564	4,841,010	4,835,289
Dilutive effect of stock options (in shares)	4,622	2,255	4,897	2,004
Shares used in computing net income per share – diluted (in shares)	4,845,632	4,837,819	4,845,907	4,837,293