11.6%, 9.7% and 8.8% in Private Passenger Automobile, Commercial Automobile and Homeowners lines, respectively, compared to the same period in 2023.
Net earned premiums for the quarter ended September 30, 2024 increased by $44.3 million, or 20.6%, to $258.7 million from $214.4 million for the comparable 2023 period. Net earned premiums for the nine months ended September 30, 2024 increased by $133.3 million, or 21.9%, to $741.7 million from $608.4 million for the comparable 2023 period.
For the quarter ended September 30, 2024, losses and loss adjustment expenses incurred increased by $23.0 million, or 14.4%, to $182.5 million from $159.5 million for the comparable 2023 period. For the nine months ended September 30, 2024, losses and loss adjustment expenses incurred increased by $53.4 million, or 11.4%, to $523.6 million from $470.2 million for the comparable 2023 period. The increase in losses is driven by our larger policy counts and current market conditions, specifically inflationary impacts on our Private Passenger Automobile book of business.
Loss, expense, and combined ratios calculated for the quarter ended September 30, 2024, were 70.6%, 30.1%, and 100.7%, respectively, compared to 74.4%, 30.4%, and 104.8%, respectively, for the comparable 2023 period. The decrease in loss and expense ratios is due to is driven by the increase in earned premiums. The loss ratio is also favorably impacted by the moderation of loss severity in the Private Passenger Automobile line. Loss, expense, and combined ratios calculated for the nine months ended September 30, 2024 were 70.6%, 30.2%, and 100.8%, respectively, compared to 77.3%, 30.9%, and 108.2%, respectively, for the comparable 2023 period. The decrease in current year loss ratio is also impacted by favorable development of $10.1 million related to the Massachusetts Property Insurance Underwriting Association. The prior year loss ratio was impacted by a severe weather event, totaling $32.1 million of losses.
Total prior year favorable development included in the pre-tax results for the quarter ended September 30, 2024 was $8.6 million compared to $13.5 million for the comparable 2023 period. Total prior year favorable development included pre-tax results for the nine months ended September 30, 2024 was $38.9 million compared to $35.0 million for the comparable 2023 period.
Net investment income for the quarter ended September 30, 2024 decreased by $1.8 million, or 12.8% to $12.2 million from $14.0 million for the comparable 2023 period. Net investment income for the nine months ended September 30, 2024 decreased by $0.6 million, or 1.3%, to $40.9 million from $41.5 million for the comparable 2023 period. The decrease is due to the earned interest from our higher yield bonds and variable rate secured and senior bank loans. Net effective annualized yield on the investment portfolio was 3.4% for the quarter ended September 30, 2024 compared to 4.0% for comparable 2023 period. Net effective annualized yield on the investment portfolio was 3.9% for both the nine months ended September 30, 2024 and 2023. The investment portfolio’s duration on fixed maturities was 3.4 years at September 30, 2024 compared to 3.6 years at December 31, 2023.
Non-GAAP Measures
Management has included certain non-GAAP financial measures in presenting the Company’s results. Management believes that these non-GAAP measures are useful to explain the Company’s results of operations and allow for a more complete understanding of the underlying trends in the Company’s business. These measures should not be viewed as a substitute for those determined in accordance with generally accepted accounting principles (“GAAP”). In addition, our definitions of these items may not be comparable to the definitions used by other companies.
Non-GAAP operating income and non-GAAP operating income per diluted share consist of our GAAP net income adjusted by the net realized gains on investments, change in net unrealized gains on equity securities, credit loss benefit (expense) and taxes related thereto. For the three months ended September 30, 2024, an increase of $10.7 million for the change in unrealized gains on equity securities was recognized within income before income taxes, compared to a decrease of $9.2 million recognized in the comparable 2023 period. For