Research and development. R&D expense decreased $39,623, or 5.7%, to $650,031 in the three months ended March 31, 2022 from $689,654 in the three months ended March 31, 2021. As a percentage of net sales, R&D expense decreased to 9.5% of net sales in the three months ended March 31, 2022 from 13.5% of net sales in the three months ended March 31, 2021 reflecting increased net sales in the current quarter. The decrease in R&D expense in the quarter was primarily the result of a decrease in payroll and payroll related benefits.
Selling, general and administrative. Selling, general and administrative expense increased by $122,682 to $1,724,800 in the three months ended March 31, 2022 from $1,602,118 in the three months ended March 31, 2021. As a percentage of net sales, selling, general and administrative expenses decreased to 25.2% of net sales in the three months ended March 31, 2022 from 31.3% of net sales in the three months ended March 31, 2021 reflecting increased net sales in the current quarter. The increase in selling, general and administrative expense in the quarter was primarily the result of an increase in professional fees.
Interest income. Interest income increased by $194 to $346 in the three months ended March 31, 2022 from $152 in the three months ended March 31, 2021, mainly a result of increased cash balance in the current year period compared to the same period in the prior year.
Other income. Other income is mainly composed of royalties earned and decreased by $5,816 to $11,555 in the three months ended March 31, 2022 compared to the same period in the prior year.
Income tax expense. The income tax expense for the three months ended March 31, 2022 was $390,110 as compared to an income tax expense of 20,165 for the three months ended March 31, 2021.
The effective tax rate for the three-month period ended March 31, 2022 was 21.4% and differs from the statutory tax rate primarily due to permanent items and state taxes.
The effective tax expense rate for the three-month period ended March 31, 2021 was 3.2% and differs from the statutory tax rate primarily due to net operating loss utilization related to the increase in pretax book income. This loss utilization both decreased the deferred tax asset and the valuation allowance. A full valuation allowance exists on all deferred tax assets. For the three months ended March 31, 2021, the valuation allowance decreased by approximately $140,000.
Net income. The Company reported net income for the three months ended March 31, 2022 of $1,429,570 compared to net income of $608,577 for the three months ended March 31, 2021. On a diluted basis, the net income per share was $0.08 for the three months ended March 31, 2022 compared to net income per share of $0.04 for the three months ended March 31, 2021.
Six Months Ended March 31, 2022 Compared to the Six Months Ended March 31, 2021
Net sales. Net sales were $13,541,598 for the six months ended March 31, 2022 compared to $9,991,497 for the six months ended March 31, 2021, an increase of 35.5%. Product sales increased $3,437,890 and EDC sales increased $112,211 in the six months ended March 31, 2022 compared to the same period in the prior year. This increase in product sales for the six months ended March 31, 2022 primarily resulted from increased shipments of displays for retrofit programs to commercial air transport customers as well as increased shipments to Pilatus under the Company’s PC-24 contract.
Cost of sales. Cost of sales increased $868,584, or 19.2%, to $5,391,267, or 39.8% of net sales, in the six months ended March 31, 2022, compared to $4,522,683 or 45.3% of net sales, in the six months ended March 31, 2021. The increase in cost of sales was primarily the result of an increase in product sales volume for the six months ended March 31, 2022 compared to the six months ended March 31, 2021. The Company’s overall gross margin was 60.2% and 54.7% for the six months ended March 31, 2022 and 2021, respectively. The increase in gross margin percentage for the six months ended March 31, 2022 is attributable to favorable leveraging of fixed costs resulting from the increased sales and production volume, and a favorable product mix.