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 subdivisions cover an entire county; census tracts or BNAs must cover an entire county and, concomitantly, so do BGs and blocks.

Compactness of shape is a desirable quality in a statistical entity, particularly for functionally defined ones; thus, it usually makes sense for their peripheries to be approximately equidistant from the centers. Twisted or elongated areas present the possibility that the statistical characteristics of the extremities will differ from those of the center or each other. If there are irregularities of shape, they should reflect geographic peculiarities related to the population, housing units, or establishments the area contains, and there should be a justification in terms of major criteria such as integration or homogeneity. Irregularities in shape result in a distinct handicap for cartographic presentation and may present problems in data analysis as well. Sometimes these irregularities are unavoidable. For example, exclaves (small portions of a legal or administrative area separated from its main part), may exist for various reasons, such as a city including an outlying reservoir or airport within its legal limits. Although the Census Bureau must recognize such legal/administrative boundaries, it generally requires statistical entities to consist of one contiguous piece of territory.

The successful completion of the 1990 Decennial Census of Population and Housing has brought a vast array of new options to the data user community. Perhaps nowhere within the framework of Census Bureau geography is the effect greater than at the small-area unit level (census tracts/BNAs, BGs, and census blocks). One of the most significant developments has been the extension of census block coverage to include the entire United States, as well as Puerto Rico and the Outlying Areas. For the first time, the entire Nation and its territories have data by census tract or BNA, BG, and census block. This has meant a vast expansion in the number of geographic entities in the data products of the Census Bureau, with the resulting increased opportunities for detailed data analysis.

These developments obviously have far-reaching effects. For the first time, the American public has a vast fund of comparable, nationwide information Geographic Overview2-33