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 may require the comparison of external differences among several of the entities. The homogeneity principle involves combining a group of people, housing units, or business establishments with similar characteristics into a single geographic area. The purpose can be either to provide summary statistics or to ensure the statistical validity of the data collected from only a sample of respondents; in both cases, each component entity should have, insofar as feasible, a similar population, economy, land use, and/or physical environment throughout its extent. Conversely, homogeneity means identifying, as separate entities, those adjacent areas that are different. Of course, demarcation of homogeneous areas frequently involves other criteria; factors such as population size (for example, each component entity must contain approximately equal numbers of people), permanence, and the presence of appropriate boundaries usually come into play.

Sometimes the differences between two areas occur gradually throughout a transition or border zone of several blocks or even miles, rather than changing sharply on either side of a boundary line. In such cases, it is critical that the person or group defining the areas use careful and knowledgeable judgment in selecting a boundary. Sometimes several kinds of source materials may point to the existence of homogeneous characteristics within an area, thereby providing an approach to choosing boundaries. For instance, developing a layout of statistical entities may involve the combined use of past census data, maps, aerial photography, field observations, and information from local sources.

The functional integration principle involves the grouping together, into a single statistical area, the people, housing, or business establishments that share a central nucleus along with the surrounding, functionally related entities, such as a large city and its suburbs. Such areas tend to form a single geographic whole that operates as a cohesive functional entity or system. These geographic areas are integrated through the communication, movement, and interaction of persons, goods, and services. Some examples are CCDs, CDPs, and traffic analysis zones. Metropolitan areas, although defined by OMB and not by the Census Bureau, are another example. Functional 2-28Geographic Overview