Income taxes	9 Months Ended
Sep. 30, 2018
Income taxes
Income taxes	8. Income taxes Income tax expense of $54 and $167 reflects an effective tax rate of 42.2% and 5.4% for the three months ended September 30, 2018 and 2017, respectively. Income tax expense of $198 and $507 reflects an effective tax rate of 2,200.0% and 2.9% for the nine months ended September 30, 2018 and 2017, respectively. Our effective tax rate differs from the statutory rate primarily due to our valuation allowance for the three and nine months ended September 30, 2018 and 2017 as well as minimum state taxes and foreign tax expense for the three and nine months ended September 30, 2018. We operate within federal, state and international taxing jurisdictions and are subject to audit in these jurisdictions. These audits can involve complex issues which may require an extended period to resolve. We are subject to taxation in the United States and in various states. Our tax years 2015 and forward are subject to examination by the IRS and our tax years 2013 and forward are subject to examination by material state jurisdictions. However, due to prior year loss carryovers, the IRS and state tax authorities may examine any tax years for which the carryovers are used to offset future taxable income. We are currently subject to examination by the IRS for our 2015 tax year. Although the ultimate outcome is unknown, we believe that any adjustments that may result from examination is not likely to have a material adverse effect on our condensed consolidated results of operations, financial position or cash flows.

8. Income taxes

Income tax expense of $54 and $167 reflects an effective tax rate of 42.2% and 5.4% for the three months ended September 30, 2018 and 2017, respectively. Income tax expense of $198 and $507 reflects an effective tax rate of 2,200.0% and 2.9% for the nine months ended September 30, 2018 and 2017, respectively. Our effective tax rate differs from the statutory rate primarily due to our valuation allowance for the three and nine months ended September 30, 2018 and 2017 as well as minimum state taxes and foreign tax expense for the three and nine months ended September 30, 2018.

We operate within federal, state and international taxing jurisdictions and are subject to audit in these jurisdictions. These audits can involve complex issues which may require an extended period to resolve. We are subject to taxation in the United States and in various states. Our tax years 2015 and forward are subject to examination by the IRS and our tax years 2013 and forward are subject to examination by material state jurisdictions. However, due to prior year loss carryovers, the IRS and state tax authorities may examine any tax years for which the carryovers are used to offset future taxable income. We are currently subject to examination by the IRS for our 2015 tax year. Although the ultimate outcome is unknown, we believe that any adjustments that may result from examination is not likely to have a material adverse effect on our condensed consolidated results of operations, financial position or cash flows.