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 cities without census tracts, it devised block areas to control the numbering. These block areas, renamed block numbering areas (BNAs) in 1960, consisted of one or more enumeration districts, and sometimes city wards. Their boundaries were major streets, railroads, and other physical features. In 1970, the Census Bureau established its current procedure of numbering BNAs within a county (or statistically equivalent entity) beginning with the number 9501. In 1970 and 1980, there was an increase in the number of jurisdictions and areas receiving data by census block under the BNA program (see, “Census Blocks and Block Groups”). Beginning with the preparations for the 1980 census, the Census Bureau changed the BNA delineation criteria to make BNAs more comparable in size and shape to census tracts.

Over time, the Census Bureau and the census statistical areas committees have developed a set of standards to guide the establishment and revision of census tracts. Although not expressly mandated by any legislation, these practices have evolved through custom and usage, and are now an integral part of the principles, policies, guidelines, and criteria that the Census Bureau uses to create and maintain census tracts. These rules promote census tract consistency nationwide, and also serve to meet local needs for small-area data.

The eligibility criteria for the census tract program has evolved over time in response to user demand and the growth in metropolitan areas (MAs). Initially, only metropolitan counties (or statistically equivalent entities) and nonmetropolitan jurisdictions that met specified conditions could participate in the census tract program. Generally, local committees decided to delineate census tracts in nonmetropolitan counties because (1) the counties were likely to gain metropolitan status as a result of an upcoming census, (2) they had a population of at least 100,000, or (3) they contained a city having at least 40,000 people. In addition, committees could establish census tracts in nonmetropolitan counties adjacent to metropolitan areas 10-4Census Tracts and Block Numbering Areas