While the primary source for these data is the U.S. Census Bureau, this interactive map makes use of historical Census data provided by the Minnesota Population Center's National Historical Geographic Information System. The most recent data come from the American Community Survey's five-year estimates, which collected information from a sample of U.S. households between 2005 and 2009. Earlier data are from decennial Census reports. Until the 1990 Census, educational attainment was tracked by counting years of college completed. In creating this interactive tool, we treated having a bachelor's degree completing four or more years of college as the same thing.
Article Source: Chronicle of Higher Education