General and administrative. General and administrative expenses increased $0.8 million, or 8%, to $11.6 million for the nine months ended September 30, 2021 as compared to $10.8 million for the nine months ended September 30, 2020, due to an increase in payroll and benefits expense.
Depreciation and other amortization. Depreciation and other amortization increased $5.5 million, or 27%, to $26.2 million for the nine months ended September 30, 2021 as compared to $20.7 million for the nine months ended September 30, 2020, primarily due to additional capitalized software assets placed into service for digital investments for the AIR MILES Reward Program segment.
Amortization of purchased intangibles. Amortization of purchased intangibles decreased $34.9 million, or 96%, to $1.3 million for the nine months ended September 30, 2021, as compared to $36.2 million for the nine months ended September 30, 2020, due to the fully amortized customer contracts in our BrandLoyalty segment.
Gain on sale of a business. In January 2020, ADS sold Precima, a provider of retail strategy and customer data applications, resulting in a pre-tax gain of $10.9 million.
Interest income, net. Total interest income, net decreased $0.2 million, or 38%, to $0.3 million for the nine months ended September 30, 2021 as compared to $0.5 million for the nine months ended September 30, 2020, due to lower interest rates.
Taxes. Provision for income taxes increased $14.2 million to $31.6 million for the nine months ended September 30, 2021 from $17.4 million for the nine months ended September 30, 2020. The effective tax rate for the nine months ended September 30, 2021 was 35.5% as compared to 22.2% for the prior year. The increase in the effective tax rate for the nine months ended September 30, 2021 as compared to the nine months ended September 30, 2020 was primarily due to an increase in the current year related to additional withholding taxes as well as an unfavorable adjustment related to a settlement of a foreign tax position. Additionally, the effective tax rate for the nine months ending September 30, 2020 included discrete tax benefits related to the expiration of statutes of limitation and the resolution of tax audits in various foreign jurisdictions.
(Income) loss from investment in unconsolidated subsidiary – related party, net of tax. The income from unconsolidated subsidiary – related party was $4.1 million in the nine months ended September 30, 2021 as compared to a loss of $0.2 million in the nine months ended September 30, 2020. Our investment in our unconsolidated subsidiary, Comenity Canada, L.P., was sold to an affiliate of ADS in August 2021 for $4.1 million and we recognized a gain on sale of unconsolidated subsidiary of $4.1 million.
Use of Non-GAAP financial measures
Adjusted EBITDA is a non-GAAP financial measure equal to net income, the most directly comparable financial measure based on accounting principles generally accepted in the United States of America, or GAAP, plus (income) loss from investment in unconsolidated subsidiary – related party, provision for income taxes, interest income, net, depreciation and other amortization, the amortization of purchased intangibles and stock compensation expense. Adjusted EBITDA excludes the gain on the sale of business, strategic transaction costs, which represent costs for professional services associated with strategic initiatives, and restructuring and other charges. These costs, as well as stock compensation expense, were not included in the measurement of segment adjusted EBITDA as the chief operating decision maker did not factor these expenses for purposes of assessing segment performance and decision making with respect to resource allocations.
We use adjusted EBITDA as an integral part of our internal reporting to measure the performance of our reportable segments and to evaluate the performance of our senior management, and we believe it provides useful information to our investors regarding our performance and overall results of operations. Adjusted EBITDA is considered an important indicator of the operational strength of our businesses. Adjusted EBITDA eliminates the uneven effect across all business segments of non-cash depreciation of tangible assets and amortization of intangible assets, including certain intangible assets that were recognized in business combinations. A limitation of this measure, however, is that it does not reflect the periodic costs of certain capitalized tangible and intangible assets used in generating revenues in our