Net Sales for the three months ended December 31, 2021 increased by $6,203,000, or 22.8%, to $33,408,000 as compared to $27,205,000 for the same period a year ago. Sales for the six months ended December 31, 2021 increased by $14,081,000, or 28% to 64,459,000 as compared to 50,378,000 for the same period a year ago. The increase in sales for the three months ended December 31, 2021 was due primarily to increased recurring communication service revenues ($2,839,000) and sales of intrusion and access products ($2,532,000) and door-locking products ($832,000). The increase in sales for the six months ended December 31, 2021 was due primarily to increased recurring communication service revenues ($5,788,000) and sales of intrusion and access products ($5,003,000) and door-locking products ($3,290,000).
Overall gross profit for the three months ended December 31, 2021 increased to $11,443,000 or 34.3% of sales as compared to $11,403,000 or 41.9% of sales for the same period a year ago. Gross profit on equipment sales for the three months ended December 31, 2021 decreased to $1,809,000 or 8.1% of equipment sales as compared to $4,417,000 or 23.2% of equipment sales for the same period a year ago.
Overall gross profit for the six months ended December 31, 2021 increased to $24,889,000 or 38.6% of sales as compared to $22,095,000 or 43.9% of sales for the same period a year ago. Gross profit on equipment sales for the six months ended December 31, 2021 decreased to $6,464,000 or 15% of equipment sales as compared to $9,008,000 or 25.8% of equipment sales for the same period a year ago.
The decrease in gross profit on equipment sales and gross profit as a percentage of equipment sales for the three and six months was primarily due to continued inflation of freight and component part costs relating to the current, world-wide supply chain problems, an unfavorable shift in product mix to the Company’s Starlink radio products (products which lead to the more profitable recurring service revenues) as well as more aggressive promotional pricing of these radios and the Company’s other equipment products in order to increase the Company’s market share of these products.
Gross profit on service sales for the three months ended December 31, 2021 increased to $9,634,000 or 87.4% of service sales as compared to $6,986,000 or 85.3% of service sales for the same period a year ago. Gross profit on service sales for the six months ended December 31, 2021 increased to $18,435,000 or 86.7% of service sales as compared to $13,087,000 or 84.6% of service sales for the same period a year ago. The increase in gross profit on service sales was due primarily to the 34.7% and 37.4% increases in sales of these services for the three and six months ended December 31, 2021, respectively, as compared to the same periods a year ago. The increase in gross profit on service sales as a percentage of service sales was due primarily to the continued shift in sales of the company’s fire radio services, which typically have a higher margin than those for intrusion radio services.
Research and development expenses for the three months ended December 31, 2021 increased $94,000 to $1,978,000, or 5.9% of net sales, as compared to $1,884,000, or 6.9% of net sales, for the same period a year ago. Research and development expenses for the six months ended December 31, 2021 increased $136,000 to $3,909,000, or 6.1% of net sales, as compared to $3,773,000, or 7.5% 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 December 31, 2021 increased 40.1% to $8,195,000 from $5,850,000 for the same period a year ago. Selling, general and administrative expenses as a percentage of net sales increased to 24.5% for the three months ended December 31, 2021 as compared to 21.5% for the same period a year ago. Selling, general and administrative expenses for the six months ended December 31, 2021 increased 29.5% to $15,541,000 from $11,999,000 for the same period a year ago. Selling, general and administrative expenses as a percentage of net sales increased to 24.1% for the six months ended December 31, 2021 as compared to 23.82% for the same period a year ago. The increase in selling, general and administrative expenses was due primarily to increased sales incentive compensation relating to the increase in net sales as discussed above, as well as an increase in stock-based compensation expense relating to the granting of stock options as described in Note 9 to the condensed consolidated financial statements.
Other income (expense) for the three months ended December 31, 2021 increased $61,000 to income of $58,000 as compared to expense of $3,000 for the same period a year ago. Other income (expense) for the six months ended December 31, 2021 increased $3,988,000 to income of $3,979,000 as compared to expense of $9,000 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 six months ended December 31, 2021.