Statements of Cash Flows (USD $)	12 Months Ended
Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2010
OPERATING ACTIVITIES
Net income	$ 11,381,095	$ 13,360,945	$ 11,999,344
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	500,121	411,547	389,273
Stock-based compensation	80,160	76,720	100,842
Excess tax benefits	0	(230,173)	(280,448)
Deferred income taxes	(12,850)	294,384	271,651
Changes in operating assets and liabilities:
Accounts receivable	911,399	625,325	(854,866)
Inventories	114,481	(1,637,430)	541,194
Prepaid expenses and other assets	25,454	(404,012)	(111,987)
Accounts payable and accrued expenses	(187,950)	332,334	491,947
Deferred revenue	0	(20,833)	(83,334)
Net cash provided by operating activities	12,811,910	12,808,807	12,463,616
INVESTING ACTIVITIES
Purchases of fixed assets	(1,480,237)	(732,800)	(305,862)
Purchases of marketable securities	(18,501,362)	(14,742,032)	(13,804,629)
Proceeds from maturities and sales of marketable securities	7,761,980	1,580,068	258,229
Net cash used in investing activities	(12,219,619)	(13,894,764)	(13,852,262)
FINANCING ACTIVITIES
Net proceeds from sale of common stock	36	418,705	622,423
Excess tax benefits	0	230,173	280,448
Net cash provided by financing activities	36	648,878	902,871
Increase (decrease) in cash and cash equivalents	592,327	(437,079)	(485,775)
Cash and cash equivalents at beginning of period	952,209	1,389,288	1,875,063
Cash and cash equivalents at end of period	1,544,536	952,209	1,389,288
Supplemental disclosures of cash flow information:
Cash (refunded) paid during the period for income taxes	$ 5,207,565	$ 6,303,598	$ 5,587,438

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Includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Entity may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For 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. Compensating balance arrangements that do not legally restrict the withdrawal or usage of cash amounts may be reported as Cash and Cash Equivalents, while legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular deposits are not generally reported as cash and cash equivalents. Includes cash and cash equivalents associated with the entity's continuing operations. Excludes cash and cash equivalents associated with the disposal group (and discontinued operation).