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 The Census Bureau defines UAs and extended cities for statistical purposes only; that is, for the purpose of presenting its data. Although some other geographic statistical entities, such as census tracts and CDPs, originate from agreement and cooperative action with local governmental officials and other outside groups, the development and implementation of criteria for defining and delineating UAs and extended cities has been largely the responsibility of the Census Bureau from the inception of the concept.

This chapter discusses (1) the Census Bureau’s criteria for classifying areas as urban (with an emphasis on the definition of UAs and extended cities), (2) the procedures used to establish UAs and update their boundaries, and (3) the relationship of UAs to other geographic entities. The Census Bureau applies the same urban and rural definitions to all parts of the United States, Puerto Rico, and the Outlying Areas. Puerto Rico has both UAs and urban places outside of UAs; the Outlying Areas—American Samoa, Guam, the Northern Mariana Islands, Palau, and the Virgin Islands of the United States—have only urban places.

Statistics by urban and rural categories have figured in decennial census publications for over a century. In the censuses of 1880, 1890, and 1900, the Census Office (predecessor of the Census Bureau) published tables based on minimum sizes of 8,000, 4,000, and 2,500 inhabitants; the latter figure was officially adopted for the 1910 census. In the decennial censuses from 1910 through 1940, urban comprised all territory, people, and housing units in incorporated places of 2,500 or more. In addition, some areas—usually minor civil divisions (MCDs) —were classified as urban under special rules relating to population size and density. This definition of urban was not adequate because (1) it excluded many large, densely settled population concentrations merely because they were not part of any incorporated place, and (2) it continued to classify many large, densely built-up areas as rural in spite of the special rules that Notes and References 12-2Urban and Rural Classifications