Research and development. R&D expense increased $0.22 million, or 33.3%, to $0.87 million in the three months ended March 31, 2023 from $0.65 million in the three months ended March 31, 2022. As a percentage of net sales, R&D expense increased to 11.8% of net sales in the three months ended March 31, 2023 from 9.5% of net sales in the three months ended March 31, 2022 reflecting the hiring of engineers, related product development and increased R&D projects.
Selling, general and administrative. Selling, general and administrative expense increased by $0.72 million to $2.45 million in the three months ended March 31, 2023 from $1.72 million in the three months ended March 31, 2022. As a percentage of net sales, selling, general and administrative expenses increased to 33.3% of net sales in the three months ended March 31, 2023 from 25.2% of net sales in the three months ended March 31, 2022. The increase in selling, general and administrative expense in the quarter was primarily due to additions to the sales and business development teams, increased marketing and investor relations activities in an effort to grow the business, as well as one-time non-cash executive stock awards.
Interest income. Interest income increased by $130k to $131k in the three months ended March 31, 2023 from $346 in the three months ended March 31, 2022, mainly as a result of increased cash balance and higher interest rates earned 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 increased by $11.7k to $23.3k in the three months ended March 31, 2023 compared to the same period in the prior year.
Income tax expense. The income tax expense for the three months ended March 31, 2023 was $0.3 million as compared to an income tax expense of $0.4 million for the three months ended March 31, 2022.
The effective tax rate for the three-month period ended March 31, 2023 was 19.6% and differs from the statutory tax rate primarily due to an increased R&D credit, as well as permanent items and state taxes.
Net income. The Company reported net income for the three months ended March 31, 2023 of $1.27 million compared to net income of $1.43 million for the three months ended March 31, 2022. On a diluted basis, the net income per share was $0.07 for the three months ended March 31, 2023 compared to net income per share of $0.08 for the three months ended March 31, 2022.
Six Months Ended March 31, 2023 Compared to the Six Months Ended March 31, 2022
Net sales. Net sales were $13.86 million for the six months ended March 31, 2023 compared to $13.54 million for the six months ended March 31, 2022, an increase of 2.3%. Product sales increased $0.14 million and non-recurring engineering increased by $0.17 million in the six months ended March 31, 2023 compared to the same period in the prior year. This increase in product sales for the six months ended March 31, 2023 primarily resulted from increased sales to our core OEM customers, which include Pilatus, Textron and Boeing. Sales from new auto-throttle installations also increased.
Cost of sales. Cost of sales was consistent at $5.39 million, or 38.9% of net sales, in the six months ended March 31, 2023, compared to $5.39 million or 39.8% of net sales, in the six months ended March 31, 2022. The Company’s overall gross margin was 61.1% and 60.2% for the six months ended March 31, 2023 and 2022, respectively. The increase in gross margin percentage for the six months ended March 31, 2023 is attributable to leverage obtained through increased sales volume, controlled operational spending, and absorption of overhead costs into raw materials inventory.
Research and development. R&D expense increased $0.15 million, or 10.8%, to $1.54 million in the six months ended March 31, 2023 from $1.39 million in the six months ended March 31, 2022. As a percentage of net sales, R&D expense increased to 11.1% of net sales in the six months ended March 31, 2023 from 10.2% of net sales in the six months ended March 31, 2022 reflecting the start of the increased in hiring engineers, working on product development and related programs/internal projects.
Selling, general and administrative. Selling, general and administrative expense increased by $1.18 million to $4.71 million in the six months ended March 31, 2023 from $3.53 in the six months ended March 31, 2022. As a percentage of net sales, selling, general and administrative expenses increased to 34.0% of net sales in the six months ended March 31, 2023 from 26.1% of net sales in the six months ended March 31, 2022. The increase in selling, general and administrative expense in the period was primarily the result of personnel additions in the sales and marketing, business development, investor relations and investor facing activities and non-cash long-term compensation.