In addition to defining Metropolitan Statistical Areas (MSAs), the Office of Management and Budget defines subareas of some of the largest MSAs, which are called Metropolitan Divisions. While I suppose there may be some demand for data on smaller parts of large metropolitan areas, the Metropolitan Divisions that have been delineated are an inconsistent mix of very different types of areas and probably should not be used for any analytical purposes. (Before 2003, a somewhat different subdivision called Primary Metropolitan Statistical Areas was used, which will be discussed further below.)
Currently, 9 MSAs are subdivided into Metropolitan Divisions. To be a candidate for subdivision, the MSA must have a population of at least 2,500,000. Metropolitan Divisions are combinations of the MSA counties using a tortured definition involving the percentage of workers living in a county who also work in the county, the ratio of the number of workers working in the county and the number living in the county, and the percentage employment interchange between the counties.
The problem is that the Metropolitan Divisions that result from this are very different types of areas. Some are portions of MSAs where two or three metropolitan areas, each with their own large city, have merged into a single MSA. The Dallas and Fort Worth Metropolitan Divisions are the subareas of their MSA, as are the Miami, Fort Lauderdale, and West Palm Beach Metropolitan Divisions. Other Metropolitan Divisions are centered around cities that are distinctly smaller than and subsidiary to the largest city in their MSA. Examples include Newark, New Jersey in the New York MSA and Camden, New Jersey in the Philadelphia MSA. These are also cities that have been included in the larger city’s metropolitan area probably for as long as we have had a concept of metropolitan areas. And finally, some Metropolitan Divisions are so devoid of significant cities that they are named using the county names. Examples, again in New York and Philadelphia, respectively, are the Nassau and Suffolk Counties Metropolitan Division and the Bucks, Montgomery, and Chester Counties Metropolitan Division.
At least this is an improvement over the somewhat analogous subdivision of metropolitan areas prior to the current system being instituted in 2003. That earlier system was more problemmatic in two ways. First, it subdivided more metropolitan areas into subdivisions, 18 in 2000 versus the current 9. But more problemmatic was the nomenclature used. The subdivisions were called Primary Metropolitan Statistical Areas (PMSAs), while the MSAs of which they were a part were renamed Consolidated Metropolitan Statistical Areas (CMSAs). The PMSAs were as heterogeneous a mix of areas as are the Metropolitan Divisions (Dallas, Fort Worth, and Nassau-Suffolk were PMSAs as well). Terming the areas “Primary” was misleading, suggesting that these were somehow the foundational metropolitan areas. This, in turn, has led some to believe it was appropriate to compare the PMSAs to the (non-CMSA) MSAs. This was not correct. The MSAs were delineated first. Then, some MSAs with populations greater than 1,000,000 were subdivided into PMSAs. It was only after subdivision that those MSAs were given the (confusing) name Consolidated MSAs. The CMSAs were not the product of any consolidation of PMSAs. Rather, the PMSAs were the product of the subdivision of the CMSAs. At least the current Metropolitan Division nomenclature gets that right.
A good example illustrating why it would be inappropriate to use Metropolitan Divisions or PMSAs would be the measurement, comparison, and analysis of urban sprawl for entire metropolitan areas, where one might be using MSAs for those areas that have not been subdivided. It would not necessarily be unreasonable to compare measures of sprawl for the Dallas and Fort Worth Metropolitan Divisions with other MSAs. But now consider the Nassau-Suffolk and Bucks-Montgomery-Chester Metropolitan Divisions. These are both portions of the suburb parts of their MSAs. These are the locations of the first two Levittowns, for goodness sake. As suburban areas, one would expect these Metropolitan Divisions to have higher levels of sprawl. But the effect is not only on their levels of sprawl. By taking these (and other) suburban areas out of the New York and Philadelphia MSAs and then computing levels of sprawl for the New York and Philadelphia Metropolitan Divisions, artificially lower levels of sprawl will be observed for those areas. I am quite confident that the level of sprawl for any MSA can be reduced if one excludes some of the sprawling suburban areas from the computation.
One final point. Some may argue that the use of Metropolitan Divisions or PMSAs along with the (non-subdivided) MSAs may be appropriate in some circumstances. (Though an argument that they are better because they are more homogeneous than the non-subdivided MSA or CMSA is rather silly, because MSAs are inherently heterogeneous.) However, if any work uses both the PMSAs or Metropolitan Divisions and the CMSAs or MSAs for the subdivided MSAs within a single study, I think one has to be very suspicious of the motivations.
A note on names
The current metropolitan area definition seems to favor including as many cities as possible in the naming of MSAs and Metropolitan Divisions. I have included only the names of the major cities necessary for identifying the various areas to which I am referring.