In the block numbered areas, the Census Bureau devised the block group (BG) as a data tabulation and publication unit equivalent to the ED in non-block numbered areas. Originally referred to as quarter tracts, BGs were subdivisions of a census tract or block numbering area (BNA).
Each BG consisted of a cluster of contiguous census blocks identified by the same first digit of their three-digit block number. For instance, Block Group 1 consisted of Blocks 101 to 199, Block Group 2 of Blocks 201 to 299, and so forth. Each census tract/BNA could contain as many as nine BGs (Block Groups 1 to 9; there was no Block Group 0). The number of BGs in a census tract/BNA usually depended on the number of census blocks existing within the census tract/BNA.
Another advantage of the BG over the ED for data tabulation was the simplicity of its cartographic presentations. Urban area maps were freer from clutter because it was no longer necessary to depict ED numbers and boundaries.
Approximately 1,618,000 census blocks were numbered in and adjacent to UAs, and in areas that contracted for census block data in the 1970 census. The number of contract block areas grew to 966. For example, the State of New York, anticipating its needs for Congressional and State legislative redistricting, contracted to receive block data for all cities and towns (MCDs) with a 1960 population of 2,500 or more.
The Census Bureau published census block data in reports according to standard metropolitan statistical area (SMSA). For each State, it included all census blocks outside of SMSAs in a single Balance of State report. It also included data to the block level for these areas on the Third Count Summary Tape.
