What are Census block groups?

Census block groups are the second smallest unit of measure used in the decennial Census. Only the Census block is smaller. A block group is a cluster of census blocks within the same census tract. The average block group contains 39 blocks.

What do Census block groups look like?

Here is an image of Census block groups in Los Angeles County, California. Every census tract contains at least one block group. In 1990, there could be as many as 9 block groups within each census tract. The ideal size was 400 housing units with a minimum of 250 units and a maximum of 550 units per block group. Block groups usually hold 600 to 3,000 people. Their boundaries never cross county or census tract boundaries, but they can cross the boundaries of county subdivisions, urban areas, voting districts, congressional districts, and tribal areas.

How are block groups used?

Block groups are the smallest geographic units for which the census publishes sample data (so SF3 data) as opposed to �??100 percent�?� data (or SF1 data/data for all households). Within one census tract, all census blocks in a block group begin with the same digit. For example: block group 3 could contain blocks 3001, 3002, 3003, �?� 3999, all within one census tract.

In 1990, block groups in counties with census tracts were delineated by local census statistical area committees, and state officials delineated block groups in other counties. The Census Bureau determined the block groups for counties where local and State officials chose not to participate. In 1990, a total of 229,466 BGs were delineated. In 2008, most block groups were delineated by local officials.