All observations are July 1 estimates of each year.
This series is calculated by FRB-St. Louis using data series from the Census Bureau. The FRB-Chicago District includes all of Iowa and most of Illinois, Indiana, Michigan, and Wisconsin.
Source: US. Bureau of the Census
US. Bureau of the Census, Resident Population in FRB-Chicago District [D7POP], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/D7POP/, July 1, 2015.