Selling and Marketing.Selling and marketing expense increased $0.3 million to $2.7 million for the three months ended September 30, 2018 from $2.4 million for the three months ended September 30, 2017. This increase was due to investments made towards our growth strategy including increased sales personnel and marketing strategy costs.
General and Administrative.General and administrative expenses increased by $2.9 million to $13.9 million for the three months ended September 30, 2018 from $11.0 million for the three months ended September 30, 2017. General and administrative costs increased primarily due to increased personnel costs, bad debt expense, legal expenses, acquisitions and acquisition-related costs.
Income Tax Expense.Our effective income tax rate was (12.6%) and (22.2%) for the three months ended September 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.
Nine months Ended September 30, 2018 and 2017
Revenue.Revenue increased by $30.5 million, or 20.6%, to $178.6 million for the nine months ended September 30, 2018 from $148.1 million for the nine months ended September 30, 2017. Revenue increased in all segments. The 2018 acquisitions of Auto-Comm, SAFCON and Intelie contributed revenue of $13.2 million for the nine months ended September 30, 2018. The Systems Integration segment increased $14.8 million, or 94.4%, primarily due to $9.8 million from the acquisition of Auto-Comm and SAFCON and increased activity against a growing backlog of Systems Integration projects. The Apps & IoT segment increased $9.5 million, or 96.8%, due to our focus on growth of the application layer and IoT space including $1.6 million from the acquisition of Intelie, $0.8 million from the acquisition of Auto-Comm and SAFCON and $4.7 million from owning ESS for the full nine months ended September 30, 2018 compared to two months in 2017. The Managed Services segment increased $6.2 million, or 5.0%, due to increased site count coupled with $1.0 Million from the acquisition of Auto-Comm and SAFCON and $2.3 million from owning DTS for the full nine months ended September 30, 2018 compared to two months in 2017.
Cost of Revenue (excluding depreciation and amortization).Cost of revenue (excluding depreciation and amortization) increased by $15.4 million, or 16.1%, to $110.7 million for the nine months ended September 30, 2018 from $95.3 million for the nine months ended September 30, 2017. Cost of revenue (excluding depreciation and amortization) increased in the Systems Integration segment by $9.1 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 Managed Services segment by $3.2 million due to serving an increased site count. Cost of revenue (excluding depreciation and amortization) increased in the Apps & IoT segment by $3.1 million as we invested in our strategy of expanding of the application layer and IoT space including the acquisition of Intelie and ESS.
Depreciation and Amortization.Depreciation and amortization expense increased by $1.9 million to $24.8 million for the nine months ended September 30, 2018 from $22.9 million for the nine months ended September 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 $3.9 million to $9.9 million for the nine months ended September 30, 2018 from $6.0 million for the nine months ended September 30, 2017. This increase was due to investments made towards our growth strategy including increased sales personnel and marketing strategy costs.
General and Administrative.General and administrative expenses increased by $11.7 million to $43.1 million for the nine months ended September 30, 2018 from $31.4 million for the nine months ended September 30, 2017. General and administrative costs increased primarily due to increased personnel costs, bad debt expense, legal expenses, acquisitions and acquisition-related costs.
Income Tax Expense.Our effective income tax rate was 0.1% and (11.5%) for the nine months ended September 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 September 30, 2018, we had working capital, including cash and cash equivalents, of $55.8 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.
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