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 Chapter 3

Since the turn of the century, the Bureau of the Census has directed major efforts toward identifying geographic statistical units at the subcounty level that are sufficiently populous to provide statistically significant data. The Census Bureau has developed several programs to provide a geographic framework for the tabulation and analysis of demographic and economic data at a subcounty scale.

The Census Bureau recognizes numerous legally defined geographic entities for data presentation purposes, entities that generally are well known, such as States, counties, cities, and townships, whose governments function to provide services to the people living and working within their borders. These governmental units, however, usually do not provide sufficient geographic coverage to give a comprehensive, detailed picture of the distribution of the population on the landscape, especially in highly populated counties. Moreover, many of these governmental units have frequently changing boundaries, vastly differing population densities, extensive variation in population characteristics, and wide-ranging area sizes. These situations make it difficult for data users to summarize and analyze census statistics.

To meet the need for geographic areas that would effectively supplement and complement the legally established areas, the Census Bureau, in association with data users across the Nation, has devised several types of geographic entities that generally define small, relatively permanent geographic areas for which the Census Bureau can present statistics.

The division of counties, highly populated places, and other entities into small geographic areas based on their statistical significance, rather than on the basis of Census Bureau operational considerations alone, requires Sources of Local Assistance3-1