The definition of taxes paid can affect the calculation. Including payroll taxes -- those for Social Security and Medicare -- makes middle-income workers' tax rates appear higher. Counting only income taxes makes their tax rates appear lower in comparison to those for people with more income.
The calculations also can depend on assumptions about who bears the burden of some taxes. Examples are whether the corporate tax is assigned to households based on capital ownership or whether it's assumed that employees shoulder the cost of the employer's side of the payroll tax.
Williams said taxpayers and lawmakers sometimes confuse marginal and average rates. People in the 15 percent income tax bracket don't pay 15 percent of their income in taxes. Personal exemptions, deductions, tax credits and the effect of the 10 percent bracket reduce those taxpayers' average rate.
The bill is S. 2230.