Note 4 - Net Income Per Share - Components of Common Shares Outstanding (Details) - shares	3 Months Ended		6 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Basic (in shares)	4,834,709	4,846,010	4,834,872	4,846,010
Dilutive effect of stock options (in shares)	100	1,871	114	3,347
Shares used in computing net income per share – diluted (in shares)	4,834,809	4,847,881	4,834,986	4,849,357