Both States and counties provide complete coverage of all land area and population in the United States at their geographic levels. Almost all other geographic entities included in the Census Bureau’s data tabulations respect the boundaries of States. American Indian reservations, however, may cross State boundaries. Also, some U.S. Postal Service five-digit ZIP Codes extend into a second State. By design, statistical areas respect State lines, with two exceptions: both metropolitan areas (defined by the Office of Management and Budget) and urbanized areas may cross State boundaries.
Two types of statistical entities, the census region and the census division, encompass combinations of adjacent States. Both regions and divisions are convenient geographic units for summarizing census and sample survey data. For instance, the six New England States compose a single division; together with the Middle Atlantic Division, they form the Northeast Region (see Chapter 6, “Statistical Groupings of States and Counties”).
The nationwide geographic framework provided by the county makes it possible to combine counties and statistically equivalent entities into larger statistical units, which may encompass an entire State or selected parts of several States. One of the best known examples of county combinations are metropolitan areas (see Chapter 13, “Metropolitan Areas”).
At lower levels of the geographic hierarchy, most geographic areas respect the boundaries of counties and county equivalents. The county subdivisions, census county divisions, minor civil divisions, and unorganized territories, provide complete coverage of all land area and population within each county and statistically equivalent entity. On the other hand, in most States, incorporated places may cross county lines; census designated places may do so in all States. Some incorporated places, such as Philadelphia and San Francisco, comprise the entire area of a county (the city of New York encompasses five entire counties); for such entities, the city and county usually have a single government. In five instances at the time of the 1990
