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 integration, like homogeneity, may derive from a single factor or from a group of related factors. Various quantitative measures such as statistics on commuting, traffic flow, trading patterns, and communications, often point to the functional cohesiveness of a particular type of geographic area. Because sources of these data generally involve looking at relationships among smaller entities, statistical entities based on functional integration often are more extensive in size than those based on homogeneity. This is particularly the case with MAs, whose purpose is to make it possible to summarize a variety of statistics from many different agencies, groups, and sources for the same geographic area.

In practice, the distinction between these two concepts is somewhat fluid. Frequently, the creation, maintenance, and update of statistical entities involves considerations of both homogeneity and functional integration. Some geographic applications may stress one concept more than the other, while other applications may aim at a balance between them. For example, while the criteria for establishing census tracts emphasize the need to acknowledge the homogeneity that exists on both sides of a major highway at a point in time, they also recognize the importance of major barriers, railroads, freeways, waterways, and topographic features, that impede functional integration, and thus separate one census tract from another.

Sometimes new uses arise for a geographic area that involve criteria different from those applied at the time the area was defined. As an example, the organizing principle for delineating census tracts has been homogeneity at the time of initial delineation. Over several decades, however, the internal characteristics of many individual census tracts have changed, resulting in greater variation in settlement patterns within each census tract. During the same period, however, the boundaries of the census tracts, which had remained stable throughout this process of internal change, became recognized as useful frameworks for making historical comparisons and analyzing trends covering several decades within the original set of areas. The value of census tracts as a stable framework of Geographic Overview2-29