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Block Groups
A Block Group is a combination of census blocks and is a subdivision of a census tract or a BNA. It is used to define all areas where block statistics are collected. Block groups are not outlined on census maps, but are defined as that set of blocks sharing the same first digit within the census tract or BNA. For example, Block Group “2” within a particular census tract would be defined as all blocks numbered between 201 and 299. In practice, the numbering would rarely go above 250 and would involve substantially fewer than 50 blocks. Gaps are occasionally left in the numbering. A block is identified by a three-digit code, which is unique within a census tract, or, where tracts do not exist, a block numbering area. Block numbers with suffixes represent blocks, which were split in order to define a smaller unique geography and its associated population (e.g., 101A and 101B). Civilian and military population aboard ships is identified with the suffix Z (e.g., 105Z). Separate summaries are provided for the parts of a block split by a place or MCD boundary.