User Notes


For data derived from the ACS, the Community Data Snapshots uses terminology based on the ACS subject definitions.

Margins of Error

The ACS is a sample-based data product. Exercise caution when using data from low-population communities, as the margins of error are often large compared to the estimates. For more details, please refer to the ACS sample size and data quality methodology.

Regional Values

Regional values are estimated by aggregating ACS data for the seven counties that compose the CMAP region. These counties are Cook, DuPage, Kane, Kendall, Lake, McHenry, and Will.

Chicago Community Area (CCA) Values

CCA values are estimated by aggregating ACS data for census tracts and block groups. Data from tracts and block groups located in multiple CCAs is allocated proportionally based on the block-level distribution of population, households or housing units (as appropriate) from the most recent Decennial Census.

Median Values

The Census Bureau encourages users to aggregate small levels of geographies into larger areas to estimate median values for those areas. Median values for the aggregated geographies (CCAs and the CMAP region) are estimated from the grouped frequency distributions reported in the ACS.

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