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 blocks were introduced for larger places in the 1940 census; even then, because blocks were numbered only in limited areas, EDs continued to be used as a collection and reporting unit in decennial censuses through the 1980 census.

Like the ED, the census block originally served the operational needs of the Census Bureau. As early as the 1920 census, the Census Bureau was instructing its enumerators in cities and built-up areas to do their work block by block:

The Census Bureau found that following a block-by-block sequence was an efficient way to compile data summaries at the ED level, and it gradually extended this enumeration method to rural areas as well as urban. Canvassing individual blocks in a geographic sequence remains the Census Bureau’s standard method of listing and verifying addresses or conducting a traditional door-to-door enumeration (see ).

Demographers, statisticians, and other data users had wanted to obtain census data by block long before the Census Bureau was able to undertake such detailed geographic subdivision. For instance, in 1909, the Tenement House Department of New York city had wanted the Census Bureau to identify more than 49,000 city blocks as part of the data dissemination for the 1910 census. Expression of such needs undoubtedly influenced the Census Bureau’s eventual decision to compile and present information at the block level.

The Census Bureau first published census block data in 1940 as part of the newly created Census of Housing. These block statistics provided a detailed inventory of housing conditions within major cities during an era of Federal financial support for public works projects. City governments needed detailed housing information for purposes such as efficiently upgrading the Census Blocks and Block Groups11-3