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 results have been sufficiently consistent to support a wide range of historical studies for entities both small and large in population and/or area; the same is true for many of the other censuses and sample surveys the Census Bureau conducts, although the historical trail exists for a shorter period of time. As a result, it is desirable to maintain comparability of geographic entities from one census or sample survey to the next.

For legally defined entities, such comparability generally is straightforward. To ensure this capability for statistical entities, where possible and appropriate, both the Census Bureau and the local officials participating in the establishment and review of these entities must pay careful attention to the process of delineating and redelineating each type of entity. The Census Bureau cautions against adjusting the boundaries of some statistical units (particularly census tracts and CCDs) merely to accommodate population growth, or changes and revisions in the street network.

On the other hand, it is appropriate for the boundaries of geographic areas such as UAs and CDPs to change with each decennial census. Just as cities change their boundaries as they expand to provide services to, and gain jurisdiction over, additional territory, the people who live there, and the businesses that operate there, UAs and CDPs are delineated to reflect the settlement pattern at a specific moment in time. For these types of entities, the historical comparison from one census to the next considers the areal extent as well as the numbers and characteristics of the people, homes, and institutions located there.

The Census Bureau uses two basic principles in establishing and revising statistical entities. One recognizes a statistical entity by the similarity of its component parts, or the homogeneity principle. The functional integration principle views a statistical entity as a nucleus with its surrounding zone of influence. The two principles find various applications, depending on the purpose of the inquiry, the type of geographic area under study, and the population size of the specific geographic entity. For instance, one application may focus on the analysis of internal trends, while another Geographic Overview2-27