2010 U.S. Census Mail Return Rates and Demographics by Tract for Cook County. According to the US Census, tracts that fell below the 73% threshold were designated as containing "hard-to-count" (HTC) populations.
The data supplies two primary groups of information: 1) details on the mail return rates for each census tract in Cook County 2) Estimated demographic characteristics for each census tract in Cook County.
For detailed information on the data please go to the following link:
https://datacatalog.cookcountyil.gov/stories/s/hza3-k7kt