Documentation and methodology
Underlying data are from the Congressional Budget Office, Average Federal Taxes by Income Group, “Sources of Income for All Households, by Household Income Category, 1979 to 2007” [Excel spreadsheet]. Cumulative growth is calculated by dividing the average pretax income in the base year (1979) into average pretax income in each subsequent year (1980–2007). The data provide average pretax income for the bottom, second, middle, fourth, and top fifths, and for the top 10, 5, and 1 percents. For the 80th–<90th percentile, average pretax income is calculated by subtracting the aggregate income of the top 10 percent from aggregate income of the top fifth and dividing by the total number of households in the 80th–<90th percentile. Aggregate income is calculated by multiplying the number of households in each income group by average pretax income. The number of households is calculated by subtracting the number of households in the top 10 percent from the number of households in the top fifth. This same procedure is done between the top 10 percent and top 5 percent to calculate average pretax income for the 90th–<95th percentile and between the top 5 percent and top 1 percent to calculate the average pretax income for the 95th–<99th percentile. Note that this publicly available CBO dataset is rounded, and produces slightly different income dollar values than the unpublished, unrounded CBO data underlying tables 2.13 and 2.16. Data are inflated to 2011 dollars using the CPI-U-RS, and then indexed to 1979=0.