Incremental contribution
Incremental contribution is defined as subscription services revenue less costs directly incurred from third parties in connection with the provision of such services to our customers (service direct expense). Incremental contribution increased $1.9 million, or 1%, during the three months ended June 30, 2020 compared to the three months ended June 30, 2019 and $6.3 million, or 2% during the six months ended June 30, 2020 compared to the six months ended June 30, 2019.
The increase is primarily due to the decrease in programming expense resulting from reduced Video RGUs, partially offset by the decrease in subscription services revenue. Programming expense decreased from $92.4 million for the three months ended June 30, 2019 to $87.9 million for the three months ended June 30, 2020 and $187.1 million for the six months ended June 30, 2019 to $177.7 million for the six months ended June 30, 2020.
Selling, general and administrative (SG&A) expenses
Selling, general and administrative expenses decreased $4.6 million, or 10%, and $3.9 million, or 4%, during the three and six months ended June 30, 2020, respectively, compared to the corresponding periods in 2019. The decreases are primarily attributable to decreased compensation, restructuring, and travel and entertainment expenses, partially offset by an increase in marketing expense.
Depreciation and amortization expenses
Depreciation and amortization expenses increased $5.9 million, or 12%, and $12.0 million, or 12%, during the three and six months ended June 30, 2020, respectively, compared to the corresponding periods in 2019. The increases are primarily attributable to assets placed in service in the fourth quarter of 2019 combined with an increase of equipment and vehicle lease activity.
Interest expense
Interest expense decreased $3.6 million, or 10%, and $5.7 million, or 8%, during the three and six months ended June 30, 2020, respectively, compared to the corresponding periods in 2019. The decreases are primarily due to lower interest rates during the three and six months ended June 30, 2020 compared to the corresponding periods in 2019.
Income tax expense
We reported income tax expense of $1.4 million and $3.2 million for the three months ended June 30, 2020 and 2019, respectively, and income tax expense of $2.2 million and $6.2 million for the six months ended June 30, 2020 and 2019, respectively. The Company’s effective federal and state income tax rate during the three and six months ended June 30, 2020 is primarily related to a reduction in income for the six months ended, a decrease in expected tax benefits on stock-based compensation, and an additional valuation allowance related to certain state deferred tax assets.
Use of Incremental Contribution and Adjusted EBITDA
We use certain measures that are not defined by GAAP to evaluate various aspects of our business such as adjusted EBITDA and incremental contribution. These measures should be considered in addition to, not as a substitute for, consolidated net income (loss) and operating income (loss) or any other performance measures derived in accordance with GAAP as measures of operating performance or operating cash flows, or as measures of liquidity. Our use of the terms adjusted EBITDA and incremental contribution may vary from others in our industry. These metrics have important limitations as analytical tools and should not be considered in isolation or as a substitute for analysis of our results as reported under GAAP. These metrics do not identify or allocate any other operating costs and expenses that are components of our income (loss) from operations to specific subscription revenues as we do not measure or record such costs and expenses in a manner that would allow attribution to a specific component of subscription revenue.