Block Groups are statistical divisions of census tracts. They are generally defined to contain between 600 and 3,000 people, and are used to present data and control block numbering. A block group consists of clusters of blocks within the same census tract. A block group usually covers a contiguous area. Each census tract contains at least one block group, and block groups are uniquely numbered within the census tract. Within the standard census geographic hierarchy, block groups never cross state, county, or census tract boundaries but may cross the boundaries of any other geographic entity.

Source of definition: U.S. Census Bureau, Geography Division. Geographic Terms and Concepts – Block Groups

Note on data presented by block groups in IECAM: IECAM does not present data by block groups in its online database. However, IECAM’s demographic theme maps present data by both county and block group (i.e., a much smaller region than county). Viewing themes by block groups allow users to visualize more detailed geographic information related to a theme. For example, users can view a demographic theme map of hispanic or latino population by county to get a gross idea of what areas in the state have a high percentage of this population. Then they can zoom in to a selected county and switch the selected region from county to the much smaller block group region. This will provdie a more detailed view of locations in the county where the higher percentage of hispanic or latino population is located.