Other Revenues. The increase for the nine months ended September 30, 2019 was primarily related to an increase in prepayment fees as more of the loans in our servicing portfolio paid off during the nine months ended September 30, 2019 than for the same period in 2018.
Expenses
Personnel. For the three months ended September 30, 2019, the increase was primarily the result of a $4.4 million increase in salaries and benefits due to acquisitions and hiring to support our growth, resulting in a 13% increase in the average headcount from 687 in 2018 to 775 in 2019. Commission costs increased $8.6 million due to the increases in origination fees and property sales broker fees noted previously.
For the nine months ended September 30, 2019, the increase was primarily the result of an $12.9 million increase in salaries and benefits due to acquisitions and hiring to support our growth, resulting in a 14% increase in the average headcount from 658 in 2018 to 750 in 2019. Commission costs increased $23.8 million due to the increases in origination fees and property sales broker fees noted previously. Lastly, subjective bonus expense increased $5.5 million due to the aforementioned increase in average headcount and due to the Company’s improved financial performance year over year.
Amortization and Depreciation. The increase for the nine months ended September 30, 2019 was primarily attributable to loan origination activity and the resulting growth in the average MSR balances. Over the past 12 months, we have added $50.2 million of MSRs, net of amortization and write offs due to prepayment.
Interest Expense on Corporate Debt. The increase in the outstanding balance of our long-term debt for the nine months ended September 30, 2019 was the primary driver of the increases in interest expense on corporate debt, partially offset by lower interest rates. We refinanced our long-term debt in the fourth quarter of 2018, increasing the balance $134.6 million while reducing the spread on the interest rate by 75 basis points.
Other Operating Expenses. The increases in other operating expenses for both the three and nine months ended September 30, 2019 primarily stemmed from increased office and travel costs due to the increase in average headcount period over period and additional costs for recruiting to support the growth of our mortgage banker and property sales broker teams in 2019.
Income Tax Expense. For the three months ended September 30, 2019, the increase in income tax expense related primarily to the 17% increase in income from operations and a slight decrease in excess tax benefits recognized year over year.
For the nine months ended September 30, 2019, the increase in income tax expense related to the 17% increase in income from operations and a $2.8 million decrease in realizable excess tax benefits due to significantly fewer exercises of stock options during the nine months ended September 30, 2019 compared to the same period in 2018 and due to lower executive compensation deductions in the first nine months of 2019 relative to the same period in 2018, resulting in a 24.4% effective tax rate for the nine months ended September 30, 2019 compared to 21.7% for the nine months ended September 30, 2018.
We do not expect our annual estimated effective tax rate to differ significantly from the 26.2% rate estimated for the three months ended September 30, 2019 as the vast majority of our equity compensation plans vest in the first quarter. Accordingly, we expect our estimated effective tax rate for the remainder of the year to be somewhere between 26% and 27%.
Non-GAAP Financial Measures
To supplement our financial statements presented in accordance with GAAP, we use adjusted EBITDA, a non-GAAP financial measure. The presentation of adjusted EBITDA is not intended to be considered in isolation or as a substitute for, or superior to, the financial information prepared and presented in accordance with GAAP. When analyzing our operating performance, readers should use adjusted EBITDA in addition to, and not as an alternative for, net income. Adjusted EBITDA represents net income before income taxes, interest expense on our term loan facility, and amortization and depreciation, adjusted for provision (benefit) for credit losses net of write-offs, stock-based incentive compensation charges, and non-cash revenues such as gains attributable to MSRs. Because not all companies use identical calculations, our presentation of adjusted EBITDA may not be comparable to similarly titled measures of other companies. Furthermore, adjusted EBITDA is not intended to be a measure of free cash flow for our management’s discretionary use, as it does not reflect certain cash requirements such as tax and debt service payments. The amounts shown for adjusted EBITDA may also differ from the amounts