The United States Office of Management and Budget (OMB) defines metropolitan and micropolitan statistical areas according to published standards that are applied to Census Bureau data. The general concept of a metropolitan or micropolitan statistical area is that of a core area containing a substantial population nucleus, together with adjacent communities having a high degree of economic and social integration with that core. Currently defined metropolitan and micropolitan statistical areas are based on application of 2000 standards [ PDF | Plain text ] (which appeared in the Federal Register on December 27, 2000) to 2000 decennial census data. Current metropolitan and micropolitan statistical area definitions were announced by OMB effective June 6, 2003.
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 term "metropolitan area" (MA) was adopted in 1990 and referred collectively to metropolitan statistical areas (MSAs), consolidated metropolitan statistical areas (CMSAs), and primary metropolitan statistical areas (PMSAs). The term "core based statistical area" (CBSA) became effective in 2000 and refers collectively to metropolitan and micropolitan statistical areas.
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, 1990, and 2000.
Defining Metropolitan and Micropolitan Statistical Areas
The 2000 standards provide that each CBSA must contain at least one urban area of 10,000 or more population. Each metropolitan statistical area must have at least one urbanized area of 50,000 or more inhabitants. Each micropolitan statistical area must have at least one urban cluster of at least 10,000 but less than 50,000 population.
Under the standards, the county (or counties) in which at least 50 percent of the population resides within urban areas of 10,000 or more population, or that contain at least 5,000 people residing within a single urban area of 10,000 or more population, is identified as a "central county" (counties). Additional "outlying counties" are included in the CBSA if they meet specified requirements of commuting to or from the central counties. Counties or equivalent entities form the geographic "building blocks" for metropolitan and micropolitan statistical areas throughout the United States and Puerto Rico.
If specified criteria are met, a metropolitan statistical area containing a single core with a population of 2.5 million or more may be subdivided to form smaller groupings of counties referred to as "metropolitan divisions."
As of June 6, 2000, there are 362 metropolitan statistical areas and 560 micropolitan statistical areas in the United States. In addition, there are 8 metropolitan statistical areas and 5 micropolitan statistical areas in Puerto Rico.
Principal Cities and Metropolitan and Micropolitan Statistical Area Titles
The largest city in each metropolitan or micropolitan statistical area is designated a "principal city." Additional cities qualify if specified requirements are met concerning population size and employment. The title of each metropolitan or micropolitan statistical area consists of the names of up to three of its principal cities and the name of each state into which the metropolitan or micropolitan statistical area extends. Titles of metropolitan divisions also typically are based on principal city names but in certain cases consist of county names.
Defining New England City and Town Areas
In view of the importance of cities and town in New England, the 2000 standards also provide for a set of geographic areas that are defined using cities and towns in the six New England states. The New England city and town areas (NECTAs) are defined using the same criteria as metropolitan and micropolitan statistical areas and are identified as either metropolitan or micropolitan, based, respectively, on the presence of either an urbanized area of 50,000 or more population or an urban cluster of at least 10,000 but less than 50,000 population. If the specified criteria are met, a NECTA containing a single core with a population of at least 2.5 million may be subdivided to form smaller groupings of cities and towns referred to as New England city and town area divisions.
Changes in Definitions over Time
Changes in the definitions of these statistical areas since the 1950 census have consisted chiefly of:
In some instances, formerly separate areas have been merged, components of an area have been transferred from one area to another, or components have been dropped from an area. The large majority of changes have taken place on the basis of decennial census data. However, Census Bureau data serve as the basis for intercensal updates in specified circumstances.
Because of these historical changes in geographic definitions, users must be cautious in comparing data for these statistical areas from different dates. For some purposes, comparisons of data for areas as defined at given dates may be appropriate; for other purposes, it may be preferable to maintain consistent area definitions. Historical metropolitan area definitions are available for 1999, 1993, 1990, 1983, 1981, 1973, 1970, 1963, 1960, and 1950.
