Data for "Resident Population" are from estimates as of July 1. Data for 1970 and 1980 are annual census.
Source: US. Bureau of the Census
US. Bureau of the Census, Resident Population in Will County, IL [ILWILL7POP], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/ILWILL7POP/, July 5, 2015.