Cost of revenues
We had cost of revenues of $3.6 million for the nine months ended September 30, 2023, an increase of $0.5 million, or 17%, compared to $3.1 million in the same period of the prior year. The primary reasons for this change was an increase in payroll cost and increase in inventory writeoff expense. As a percentage of revenues, cost of revenues was 7% in both the nine months ended September 30, 2023 and 2022.
Engineering and product development expense
We had engineering and product development expense of $4.6 million for the nine months ended September 30, 2023, an increase of $1.2 million, or 33%, compared to $3.4 million in the same period of the prior year. The increase was primarily due to personnel expense due to an increased headcount, and increased consulting expenses. As a percentage of revenues, engineering and product development expenses increased to 9% in the nine months ended September 30, 2023, compared to 8% in the prior year period.
Sales and marketing expense
We had sales and marketing expense of $13.6 million for the nine months ended September 30, 2023, an increase of $0.6 million, or 4%, compared to $13.0 million in the same period of the prior year. The increase was primarily due to increased stock based compensation expenses, increased trade show costs and increased travelling expenses. As a percentage of revenues, sales and marketing expense decreased to 26% in the nine months ended September 30, 2023, as compared to 30% in the prior year period.
General and administrative expense
We had general and administrative expense of $11.0 million for the nine months ended September 30, 2023, an increase of $1.2 million, or 13%, compared to $9.8 million in the same period of the prior year. The increase was primarily due to an increase in professional fees, provision for credit losses, professional fees and other expenses. As a percentage of revenues, general and administrative expense decreased to 21% in the nine months of 2023, as compared to 23% in the prior year period.
Other income, net
We had total other income of $1.6 million for the nine months ended September 30, 2023, compared to $0.1 million in the same period of the prior year. The increase was primarily due to higher interest income from investments, partially offset by changes in the fair value of investments.
Income tax provision
We had income tax provision of $4.9 million for the nine months ended September 30, 2023, an increase of $2.3 million or 87%, compared to $2.6 million in the prior year period. The effective tax rate for the nine months ended September 30, 2023 was 23% compared to 19% in the same period of the prior year. The increase was primarily due to lower tax benefits associated with stock-based compensation plans.
Net income
For the foregoing reasons, we had net income of $16.4. million, or $2.44 per basic share and $2.09 per diluted share, for the nine months ended September 30, 2023, an increase of $5.3 million, or 47%, compared to a net income of $11.1 million, or $1.65 per basic share and $1.38 per diluted share, for the same period of the prior year.
Liquidity and Capital Resources
We had cash and cash equivalents and short-term investments of $56.0 million at September 30, 2023 compared to $43.1 million at December 31, 2022, and total current liabilities of $8.6 million at September 30, 2023 compared to $6.9 million at December 31, 2022. As of September 30, 2023, we had working capital of approximately $55.8 million. We believe that our current sources of funds will provide us with adequate liquidity during the period following September 30, 2023, as well as in the long-term.