The U.S. Census Bureau defines 200+ metro areas, in terms of specific counties. These are, loosely speaking, major cities and surrounding areas. Those counties not included in these metros are “non-metro.” A DMA may have more than one metro within it, and some non-metro counties as well.
The Census Bureau’s detailed definition says, in part
Standard definitions of metropolitan areas were first issued in 1949 by the then Bureau of the Budget (predecessor of OMB), under the designation “standard metropolitan area” (SMA). The term was changed to “standard metropolitan statistical area” (SMSA) in 1959, and to “metropolitan statistical area” (MSA) in 1983. The collective term “metropolitan area” (MA) became effective in 1990. MAs include metropolitan statistical areas (MSAs), consolidated metropolitan statistical areas (CMSAs), and primary metropolitan statistical areas (PMSAs).
OMB has been responsible for the official metropolitan areas since they were first defined, except for the period 1977 to 1981, when they were the responsibility of the Office of Federal Statistical Policy and Standards, Department of Commerce. The standards for defining metropolitan areas were modified in 1958, 1971, 1975, 1980, and 1990. OMB announced the adoption of new Standards for Defining Metropolitan and Micropolitan Statistical Areas in the December 27, 2000 Federal Register. OMB will apply the new standards with Census 2000 data and will announce definitions based on these standards in 2003.
Defining MSAs, CMSAs, and PMSAs
The 1990 standards provide that each newly qualifying MSA must include at least:
one city with 50,000 or more inhabitants, or
a Census Bureau-defined urbanized area (of at least 50,000 inhabitants) and a total metropolitan population of at least 100,000 (75,000 in New England).
Under the standards, the county (or counties) that contains the largest city becomes the “central county” (counties), along with any adjacent counties that have at least 50 percent of their population in the urbanized area surrounding the largest city. Additional “outlying counties” are included in the MSA if they meet specified requirements of commuting to the central counties and other selected requirements of metropolitan character (such as population density and percent urban). In New England, the MSAs are defined in terms of cities and towns rather than counties.