A Metropolitan Statistical Area (MSA) is a designation the U.S. government uses to refer to a region that, broadly speaking, consists of a city and its suburbs, plus any surrounding communities that are closely linked to the city because of social and/or economical factors.
MSAs are, emphatically, statistical definitions and not administrative subdivisions. Thus, for instance, a reference in a report to “the Detroit MSA” refers to a geographical area of which the official City of Detroit is just a part. No one is actually administratively responsible for the MSA itself.
Defining urban areas has been the responsibility of the Office of Management and Budget (OMB), an element of the White House except in the period 1977 to 1981, during which time the Office of Federal Statistical Policy and Standards, in the Department of Commerce, did the job. The first such definition was issued in 1949 by OMB’s predecessor, the Bureau of the Budget. Since then urban area definitions have been modified in 1958, 1971, 1975, 1980, 1990, and most recently in 2000. The purpose of these definitions has been to give a uniform basis for identifying urbanization in the context of the population census. The designations are widely used in government and industrial reference.
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If many geese are a gaggle, many lions a pride, and many fish a school, one could refer to many designations of statistical areas as “a confusion” of names—because urban statistical designations have a history of being just that—confusing. The categories used before the 2000 redefinition were, in hierarchical order, free-standing MSAs, PMSAs (p from “primary”) which were parts of a larger aggregate, and CMSAs (consolidated metropolitan statistical areas) which held multiple PMSAs. Now it would be nice if we could say, “Forget these old designations!” but we cannot. They still appear in reports issued before 2000. Therefore the analyst must remember the old abbreviations along with the new ones.
The new ones, still in hierarchical order by size, are Micropolitan Statistical Areas (no acronym yet, but in this essay we’ll abbreviate them as MICROs), Metropolitan Statistical Areas (MSAs), and Combined Statistical Areas (CSAs, holding two or more MICROs or MSAs). Further complicating matters, MICROs and MSAs are collectively referred to as Core-Based Statistical Areas or CBSAs. To justify the “confusing” tag, the terminology also includes NECTAs, which are New England City and Town Areas—because in the New England states cities and towns are used as the basis of building up larger aggregates rather than counties—as in the rest of the U.S. and in Puerto Rico.
Units definitions are based on population and population is measured by county except in the New England states where city and town populations are used for designation purposes instead.
Core-Based Statistical Area. A CBSA is one or more counties with an urbanized cluster of at least 10,000 people. The area as a whole is defined by the interaction between the core and the outlying areas. This interaction, measured by commuting, means that at least 25 percent of people in outlying areas are working in the core. The CBSA is a generic definition of MICROs and MSAs, the difference being core population size.
Micropolitan Statistical Areas. A MICRO is simply a small CBSA, i.e., a county or counties with an urbanized core of 10,000 but fewer than 50,000 in population. Outlying areas included are, again, defined by commuting patterns. As of November 2004, according to the Census Bureau, there were 575 MICROs in the U.S. and five in Puerto Rico.
Metropolitan Statistical Areas. An MSA has an urbanized core of minimally 50,000 population and includes outlying areas determined by commuting measures. In 2004, the U.S. had 361 MSAs and Puerto Rico eight.
Combined Statistical Areas. CSAs are two or more adjacent CBSAs in which there is at least a 15 employment interchange (measured by commuting) between cores. If this exchange is 25 percent or higher between a pair of CBSAs, they are combined into a CSA automatically; if the measure is at least 15 percent but below 25, local opinion in both areas is used to decide on combination. The U.S. had 116 CSAs in 2004.
Metropolitan Divisions. Metropolitan divisions are used to further subdivide major metropolitan areas into divisions with minimally 2.5 million core populations.
Thus, for instance, the Boston area is subdivided into four, the Chicago Area into three, Detroit into two, the New York area into five such divisions. All told there were 29 divisions in the 10 largest metro areas: Boston, Chicago, Dallas, Los Angeles, Miami, New York, Philadelphia, San Francisco, Seattle, and Washington, D.C.
Statistical areas are named after the city the OMB defines as the “principal city,” namely the administrative entity which forms the largest urban core. “Atlantic City, NJ MSA” is a typical MSA name. The area includes a single county, Atlantic County, NJ. Under OMB rules, however, additional cities may also qualify for the “principle” designation based on population and employment size measures. The names of up to the top three principal cities are included in the name of the MSA including the state abbreviations in which the cities and component counties are located. An example is “PhiladelphiaCamden-Wilmington, PA-NJ-DE-MD MSA.” The three principle cities shown are in Pennsylvania, New Jersey, and Delaware, but the state of MD is shown as well because Cecil County, MD is part of this metro region. This particular MSA is also divided into three Metropolitan Divisions, namely Camden, NJ, Philadelphia, PA, and Wilmington, DE-MD-NJ. Thus the divisions are named after “principal cities.”
Sometimes, however, division names are based on county rather than on principal city names.