component part costs relating to the current, world-wide supply chain problems, an unfavorable shift in product mix from door-locking products to intrusion products (which include the Company’s Starlink radio products which lead to the more profitable recurring service revenues) as well as aggressive promotional pricing of these radios. The increase in gross profit of equipment sales for the three months was due to the increase in net sales of equipment. Gross profit on sales of services for the three months ended September 30, 2021 increased to $8,801,000 or 86.1% of service sales as compared to $6,101,000 or 83.9% of service sales for the same period a year ago. The increase in gross profit on service revenues was due primarily to the 40.5% increase in sales of these services.
Research and development expenses for the three months ended September 30, 2021 increased $42,000 to $1,931,000, or 6.2% of net sales, as compared to $1,889,000, or 8.2% of net sales, for the same period a year ago. The increase was due primarily to increased payroll while the decrease as a percentage of net sales was due primarily to the increase in net sales.
Selling, general and administrative expenses for the three months ended September 30, 2021 increased 19.5% to $7,346,000 from $6,149,000 for the same period a year ago. Selling, general and administrative expenses as a percentage of net sales decreased to 23.7% for the three months ended September 30, 2021 as compared to 26.5% for the same period a year ago. The increase in selling, general and administrative expenses was due primarily to tradeshow and advertising expenses, which were curtailed during the COVID-19 pandemic, as well as increased sales incentives relating to the increase in net sales as discussed above. The decrease in selling, general and administrative expenses as a percentage of net sales was due primarily to the increase in net sales as partially offset by the increase in expenses.
Other income (expense) for the three months ended September 30, 2021 increased $3,927,000 to income of $3,921,000 as compared to expense of $6,000 for the same period a year ago. The change in Other income (expense) was due primarily to the gain from the extinguishment of the Company’s $3,904,000 in PPP loans, which were forgiven by the SBA during the three months ended September 30, 2021.
The Company’s provision for income taxes for the three months ended September 30, 2021 increased by $19,000 to $348,000 as compared to $329,000 for the same period a year ago. The increase in the provision for income taxes for the three months was primarily due to higher taxable income in the U.S, as compared to income in the DR. The Company’s effective rate for income tax was 4.3% and 12% for the three months ended September 30, 2021 and 2020, respectively. The decrease in the Company’s effective rate for the three months ended September 30, 2021 was due primarily to the income recognized as a result of the PPP loan forgiveness being non-taxable.
Net income for the three months ended September 30, 2021 increased by $5,433,000 to $7,752,000 or $0.42 per diluted share as compared to $2,319,000 or $0.13 per diluted share for the same period a year ago. The increase in net income for the three months ended September 30, 2021 was primarily due to the items described above.
Liquidity and Capital Resources
During the three months ended September 30, 2021, the Company utilized a portion of its cash generated from operations ($541,000 of $3,463,000) to purchase property, plant and equipment ($522,000) and marketable securities ($19,000). The Company believes its current working capital, cash flows from operations and its revolving credit agreement will be sufficient to fund the Company’s operations through the next twelve months.
Accounts receivable at September 30, 2021 decreased by $2,738,000 to $25,343,000 as compared to $28,081,000 at June 30, 2021. This decrease is primarily the result of the higher sales volume of equipment during the quarter ended June 30, 2021, which is typically the Company’s highest, as compared to the quarter ended September 30, 2021.
Inventories at September 30, 2021 increased by $1,824,000 from June 30, 2021. This increase is primarily the result of the Company level-loading its production output throughout the year, whereas the Company’s sales are typically highest in the fourth quarter as well as increasing purchases of certain components that have become difficult to source during the world-wide supply chain problems.
Accounts payable and accrued expenses other than accrued income taxes decreased by $454,000 as of September 30, 2021, as compared to June 30, 2021. This decrease was due primarily to the decrease in the accrued refund liability caused by lower equipment sales for the three months ended September 30, 2021, as compared to equipment sales for the three months ended June 30, 2021, which is typically the Company’s highest.