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 Chapter 11

Census blocks, the smallest geographic area for which the Bureau of the Census collects and tabulates decennial census data, are formed by streets, roads, railroads, streams and other bodies of water, other visible physical and cultural features, and the legal boundaries shown on Census Bureau maps. Census data for these areas serve as a valuable source for small-area geographic studies. The Census Bureau has responded to public demand for more geographic coverage of census block data by expanding the Census Block Statistics Program each decade since block data first were published in 1940. For the 1990 decennial census, the Census Bureau tabulated data by census block for the entire Nation, as well as Puerto Rico and the Outlying Areas (American Samoa, Guam, the Northern Mariana Islands, Palau, and the Virgin Islands of the United States). In 1990, the Census Bureau tabulated data for 7,020,924 census blocks.

Block groups (BGs) are the next level above census blocks in the geographic hierarchy (see in ). A BG is a combination of census blocks that is a subdivision of a census tract or block numbering area (BNA). (A county or its statistically equivalent entity contains either census tracts or BNAs; it can not contain both.) A BG consists of all census blocks whose numbers begin with the same digit in a given census tract or BNA; for example, BG 3 includes all census blocks numbered in the 300s. The BG is the smallest geographic entity for which the decennial census tabulates and publishes sample data. It has now largely replaced the earlier enumeration district (ED) as a small-area geographic unit for purposes of data presentation.

For the 1990 decennial census, local census statistical areas committees were given the opportunity to delineate BGs in counties with census tracts. State officials were invited to delineate BGs in the remaining counties. The Census Bureau delineated BGs for counties in which local and State officials chose not to participate in the BG program. The total number of BGs delineated for the 1990 decennial census was 229,466. Census Blocks and Block Groups11-1