Additionally, interest expense is recorded for the revolving credit facility, including fees on the unused portion of the facility and the amortization of loan costs.
Income tax provision was $820 thousand during the three months ended September 30, 2024 compared to $2.9 million for the same period in 2023. The effective provision rate was 19.4% for the three months ended September 30, 2024 compared to 21.6% for the comparable period in the prior year. The decrease in the effective tax rate is primarily due to the impact of favorable permanent differences.
NINE MONTHS ENDED SEPTEMBER 30, 2024 COMPARED TO NINE MONTHS ENDED SEPTEMBER 30, 2023
Net sales for the nine months ended September 30, 2024 decreased $124.1 million or 39.7% compared to the same period in 2023. The change in net sales during the nine months ended September 30, 2024 compared to the prior year was primarily due to a 40% decrease in unit sales volumes. Sales continued to be impacted by soft consumer demand, as dealers tightly manage their inventories to minimize floor plan carrying costs until demand improves.
In the nine months ended September 30, 2024, net sales outside of the United States accounted for 6.1% of net sales compared to 6.6% of net sales in the same period of the prior year.
Cost of goods sold for the nine months ended September 30, 2024 was $152.4 million compared to $235.9 million for the comparable period in 2023, a decrease of $83.5 million or 35.4%. Cost of goods sold as a percentage of net sales was 80.7% for the nine months ended September 30, 2024 compared to 75.4% for the same period in the prior year due to lower sales volumes and associated manufacturing cost inefficiencies, coupled with the impact of reinstituting retail incentive programs. Production schedules and labor costs have been adjusted to more closely align with current demand.
Selling, general and administrative expenses for the nine months ended September 30, 2024 were $21.8 million compared to $35.5 million for the comparable period in 2023, a decrease of $13.7 million or 38.6%. This decrease was primarily due to costs that vary with sales and profitability, such as incentive compensation, sales commissions and warranty expense, as well as a decrease in pension expense in comparison to the same period of the prior year. In the nine months ended September 30, 2023, selling, general and administrative expenses also included a non-cash pension settlement charge of $2.3 million. Selling, general and administrative expenses were 11.6% of net sales in the nine months ended September 30, 2024 compared to 11.3% in the same period of the prior year.
Gain on disposition of assets, net for the nine months ended September 30, 2024 was $51 thousand compared to $2.0 million for the comparable period in 2023. In the nine months ended September 30, 2023, gains on disposition of assets included a $1.8 million gain related to a real estate transaction.
Operating income for the nine months ended September 30, 2024 was $14.6 million compared to $43.4 million in the same period in 2023.
Interest income, net for the nine months ended September 30, 2024 increased to $2.4 million from $2.1 million in the same period of the prior year due to higher average cash balances. Marine Products generates interest income primarily from investments of excess cash in money market funds. Additionally, interest expense is recorded for the revolving credit facility, including fees on the unused portion of the facility and the amortization of loan costs.
Income tax provision was $3.4 million during the nine months ended September 30, 2024 compared to $9.2 million for the same period in 2023. The effective provision rate was 19.8 percent for the nine months ended September 30, 2024 compared to 20.2 percent for the nine months ended September 30, 2023. The effective tax rate decreased primarily due to the impact of favorable permanent differences.