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 permitted classification of some MCDs as urban. Something more than place/MCD size was needed to define urban area and population.

To improve its measure of urban territory, population, and housing units for the 1950 census, the Census Bureau introduced two new types of geographic units, the UA and the CDP. This step resulted in major changes to the urban and rural classifications. The introduction of CDPs meant that densely settled population centers without legal status were recognized as places, and classified as urban if they had at least 2,500 residents. The identification and delineation of UAs made it possible to include as urban many densely built-up areas that had previously been classified rural because they were not part of any place. By using these new geographic entities—UAs and CDPs—the Census Bureau improved its separation of the urban and rural population. Outside of UAs, the Census Bureau continued to classify a place (whether an incorporated place or a CDP) as urban if its population was at least 2,500.

summarizes important elements of the UA definitional criteria as they have developed through the period 1950 through 1990. Many elements of the criteria have remained largely unchanged. The density criterion of at least 1,000 people per square mile has remained the same since it was adopted for the 1960 census. The provision allowing the inclusion of distinctly urban land uses has been part of the UA criteria since it was adopted for the 1950 census. Measures of density and concentration continue as the basis for including places in the urban fringe zone. With the exception of extended cities, entire places are still, in most instances, the major geographic building blocks of a UA.

In recent years, significant revisions to the UA criteria have included the recognition of extended cities and more liberal criteria for central places and CDPs. Extended cities, identified starting with the 1970 census, better define the extent of urban and rural territory (area), but have very little effect on the urban and rural population and housing unit counts at the Notes and References Urban and Rural Classifications12-3