Selling and Marketing.Selling and marketing expense increased $2.1 million to $4.2 million for the three months ended June 30, 2018 from $2.1 million for the three months ended June 30, 2017. This increase was due to investing in our growth strategy including increased sales and marketing personnel costs.
General and Administrative.General and administrative expenses increased by $5.7 million to $15.5 million for the three months ended June 30, 2018 from $9.9 million for the three months ended June 30, 2017. General and administrative costs increased primarily due to increased stock-based compensation, legal expenses, acquisitions and acquisition related costs.
Income Tax Expense.Our effective income tax rate was 17.7% and 2.3% for the three months ended June 30, 2018 and 2017, respectively. Our effective tax rate is affected by factors including changes in valuation allowances, fluctuations in income across jurisdictions with varying tax rates, and changes in income tax reserves, including related penalties and interest.
Six Months Ended June 30, 2018 and 2017
Revenue.Revenue increased by $16.6 million, or 17.1%, to $113.8 million for the six months ended June 30, 2018 from $97.2 million for the six months ended June 30, 2017. Revenue increased in all segments. The Systems Integration segment increased $8.1 million, or 80.1%, primarily due to the acquisition of Auto-Comm and SAFCON and increased activity of Systems Integration projects. The Apps & IoT segment increased $7.1 million, or 145.1%, due to our growth strategy which focuses on growth into the application layer and IoT space including the acquisition of Intelie and ESS. The Managed Services segment increased $1.5 million, or 1.8%, due to increased site count coupled with the acquisition of DTS.
Cost of Revenue (excluding depreciation and amortization).Cost of revenue (excluding depreciation and amortization) increased by $7.0 million, or 11.1%, to $69.9 million for the six months ended June 30, 2018 from $62.9 million for the six months ended June 30, 2017. Cost of revenue (excluding depreciation and amortization) increased in the Systems Integration segment by $4.1 million due to the acquisition of Auto-Comm and SAFCON and increased activity of Systems Integration projects. Cost of revenue (excluding depreciation and amortization) increased in the Apps & IoT segment by $2.8 million as we invested in our strategy of expanding into the application layer and IoT space including the acquisition of Intelie, ESS and Cyphre. Cost of revenue (excluding depreciation and amortization) increased in the Managed Services segment by $0.2 million.
Depreciation and Amortization.Depreciation and amortization expense increased by $1.5 million to $16.3 million for the six months ended June 30, 2018 from $14.9 million for the six months ended June 30, 2017. The increase is primarily attributable to additions to property, plant and equipment and intangibles from acquisitions and capital expenditures.
Selling and Marketing.Selling and marketing expense increased $3.6 million to $7.1 million for the six months ended June 30, 2018 from $3.6 million for the six months ended June 30, 2017. This increase was due to investing in our growth strategy including increased sales and marketing personnel costs.
General and Administrative.General and administrative expenses increased by $8.8 million to $29.2 million for the six months ended June 30, 2018 from $20.4 million for the six months ended June 30, 2017. General and administrative costs increased primarily due to increased stock-based compensation, legal expenses, acquisitions and acquisition related costs.
Income Tax Expense.Our effective income tax rate was 3.2% and (5.3%) for the six months ended June 30, 2018 and 2017, respectively. Our effective tax rate is affected by factors including changes in valuation allowances, fluctuations in income across jurisdictions with varying tax rates, and changes in income tax reserves, including related penalties and interest.
Liquidity and Capital Resources
At June 30, 2018, we had working capital, including cash and cash equivalents, of $40.4 million.
Based on our current expectations, we believe our liquidity and capital resources will be sufficient for the conduct of our business and operations for the foreseeable future. We may also use a portion of our available cash to finance growth through the acquisition of, or investment in, businesses, products, services or technologies complementary to our current business, through mergers, acquisitions, joint ventures or otherwise, or to pay down outstanding debt.
During the next twelve months, we expect our principal sources of liquidity to be cash flows from operating activities, cash and cash equivalents on hand and availability under our credit facility.
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