In the United States, a metropolitan statistical area (MSA) is a geographical region with a relatively high population density at its core and close economic ties throughout the area. Such regions are neither legally incorporated as a city or town would be, nor are they legal administrative divisions like counties or separate entities such as states; because of this, the precise definition of any given metropolitan area can vary with the source. The statistical criteria for a standard metropolitan area were defined in 1949 and redefined as metropolitan statistical area in 1983. A typical metropolitan area is centered on a single large city that wields substantial influence over the region (e.g., New York City or Philadelphia). However, some metropolitan areas contain more than one large city with no single municipality holding a substantially dominant position (e.g., Dallas–Fort Worth metroplex, Norfolk-Virginia Beach (Hampton Roads), Riverside–San Bernardino (Inland Empire) or Minneapolis–Saint Paul (Twin Cities)). MSAs are defined by the U.S. Office of Management and Budget (OMB) and used by the Census Bureau and other federal government agencies for statistical purposes.
U.S. Census statistics for metropolitan areas are reported according to the following definitions.
The U.S. Office of Management and Budget defines a set of core based statistical areas (CBSAs) throughout the country. CBSAs are delineated on the basis of a central urban area or urban cluster – in other words: a contiguous area of relatively high population density. CBSAs are composed of counties and county equivalents. The counties containing the core urban area are known as the central counties of the CBSA. A central county is a county in which 50% of its population lives in urban areas of at least 10,000 in population, or where a population of 5,000 are located in a single urban area of at least 10,000 in population where that urban area is split between more than one county. Additional surrounding counties, known as outlying counties, can be included in the CBSA if these counties have strong social and economic ties to the central counties as measured by commuting and employment. Outlying counties are included in the CBSA if 25% of the workers living in the county work in the central county or counties or, conversely, 25% of the employment in the county is held by workers who live in the central county or counties. All counties in a CBSA must be contiguous, and a county can only be included within one CBSA. In New England, towns have precedence over counties, so statistically similar areas are defined in terms of town-based units known as New England city and town areas (NECTAs).
Adjacent CBSAs are merged into a single CBSA when the central county or counties of one CBSA qualify as an outlying county or counties to the other CBSAs. One or more CBSAs may be grouped together or combined to form a larger statistical entity known as a combined statistical area (CSA) when the employment interchange measure (EIM) reaches 15% or more.
As well as MSAs, CBSAs are also subdivided into micropolitan statistical areas (μSAs) for CBSAs built around an urban cluster of at least 10,000 in population but less than 50,000 in population. Previous terms that are no longer used include standard metropolitan statistical area (SMSA) and primary metropolitan statistical area (PMSA).