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. 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 the employment interchange measure (total of in- and out-commuting) is 25% or more, although these numbers are estimates and exceptions are made. Some areas within these outlying counties may be rural in nature. As well as MSAs, CBSAs are subdivided into micropolitan statistical areas (μSAs) based on the population of the core urban area. Under certain conditions, one or more CBSAs may be grouped together to form a larger statistical entity known as a combined statistical area (CSA). Previous terms that are no longer used include standard metropolitan statistical area (SMSA) and primary metropolitan statistical area (PMSA). 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).