"effective tax rate — The ratio of taxes paid to a given tax base. For individual income taxes, the effective tax rate is typically expressed as the ratio of taxes to adjusted gross income. For corporate income taxes, it is the ratio of taxes to book profits. For some purposes—such as calculating an overall tax rate on all income sources—an effective tax rate is computed on a base that includes the untaxed portion of Social Security benefits, interest on tax-exempt bonds, and similar items. The effective tax rate is a useful measure because the tax code's various exemptions, credits, deductions, and tax rates make actual ratios of taxes to income very different from statutory tax rates."
It's using your adjusted gross income, so it is accounting for deductions etc., but at higher income levels a lot of these deductions get phased out, so it can bump up your actual rate. It, like most tax questions, depend really on the taxpayer's situation (income level, dependents, and other income or losses from passthroughs, like from partnerships they are involved in), but that is essentially the answer.
posted by equalpants at 4:45 PM on April 12, 2007