Definition
Block groups are geographic subdivisions of census tracts; their primary purpose is to provide a geographic summary unit for census block data. A block group must comprise a reasonably compact and contiguous cluster of census blocks. Each census tract (ED) contains a minimum of one block group and may have a maximum of nine block groups.
Within a census tract (ED), a block group consists of all census blocks whose numbers begin with the same digit. For example in 1990, block group 3 included all census blocks numbered in the 300s. For Census 2000 the Census Bureau will introduce a four-digit block numbering system, and block group 3 will include all census blocks numbered in the 3000s within a census tract.
The Census Bureau will work with local participants to delineate block groups in the United States, Puerto Rico, and the Island Areas (American Samoa, Guam, the Northern Mariana Islands, and the Virgin Islands of the United States). Population size criteria for block groups are:
Population Thresholds
Area(s) Optimum Minimum Maximum
Island Areas (American Samoa, Guam, Northern Mariana islands) 2,500 1,500 8,000
