Census Block Group
A subdivision of a Census tract containing between 600 and 3,000 people, the smallest geographic unit for which the Census Bureau publishes sample (survey) data.
Census block groups sit between Census tracts and Census blocks in the geographic hierarchy. Each block group is identified by a single digit appended to its tract number and contains a cluster of contiguous Census blocks. There are roughly 240,000 block groups in the United States.
Block groups are the smallest unit at which the American Community Survey (ACS) publishes detailed socioeconomic data such as income distribution, educational attainment, commute patterns, and housing characteristics. This makes them invaluable for fine-grained demographic analysis.
In ZIP code analytics, block groups provide a way to estimate demographics for sub-ZIP areas. Since a ZIP code often spans multiple block groups with different characteristics, using block-group data produces more accurate results than ZIP-level averages alone.