Generally, accounting standards require companies to provide for income taxes each quarter based on their estimate of the effective tax rate for the full year. The authoritative guidance for accounting for income taxes allows use of the discrete method when it provides a better estimate of income tax expense. Due to the Company’s valuation allowance position, it is the Company’s position that the discrete method provides a more accurate estimate of income tax expense and therefore income tax expense for the current quarter has been presented using the discrete method. As the year progresses, the Company refines its estimate based on the facts and circumstances by each tax jurisdiction. The following table summarizes income tax expense on (loss) income from continuing operations and the effective tax rate for the three-month periods ended March 31, 2018 and 2017:

Income tax expense for the three-month periods ended March 31, 2018 and 2017 was primarily due to minimum state and local taxes.