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 level of urban services, modifying building codes, establishing and implementing zoning ordinances, and preparing plans for capital improvements. In the 1940 and 1950 censuses, for cities that had census blocks, each individual block was identified on the ED map, and enumerators were instructed to write the block number on the census questionnaire as they canvassed each housing unit. Census blocks were numbered beginning with one in each census tract, city ward, or block area that carried a one-letter identifier. (The block area was a summary unit used in place of census tracts in block numbered cities that did not have census tracts; it could not extend beyond the city limits.)

Data published for census blocks in 1940 were limited to selected housing statistics collected for 191 cities that had a population of 50,000 or more at the time of the 1930 census. The same criterion applied in 1950 (using the 1940 census counts), after which the Census Bureau published census block data for 209 places. There were separate published reports for each, including a map to accompany the data. In the 1960 census, the Census Bureau published the total population for each block, and reported block statistics for 295 cities with a 1950 population of 50,000 or more. The Census Bureau also expanded the program to include an additional 172 places, in which data users paid the Census Bureau to collect and publish census block data. In total, the Census Bureau published data for over 736,000 census blocks in the 1960 census.

In 1970, for the first time, a large portion of the U.S. population was enumerated by mail rather than by the conventional door-to-door enumeration. As part of the mail census, the Census Bureau introduced many improvements in the base maps it used for the Nation’s major urban centers (the Metropolitan Map Series), and in its methodologies for the collection, tabulation, and dissemination of small-area data. These changes had a direct positive effect on the delineation of census blocks and BGs, allowing the Census Bureau to expand census block coverage to include the expected extent of all 1970 urbanized areas. Census Blocks and Block Groups11-5