Metropolitan Statistical Areas (MSAs) are by far the most commonly used units for reporting data and conducting analysis for urban or metropolitan areas. They are defined on a consistent basis by the Office of Management and Budget and are used throughout the federal government and by many, many others.
In my research project on changes in urban patterns in large urban areas in the U.S. over time, I delineate urban and exurban portions of the areas using census tracts. To do this, I needed a starting-point definition of metropolitan areas to indicate which tracts were within and beyond the area, which tracts belonged to a given area as opposed to a neighboring one, and which urban areas that may have developed separately should now be considered to be part of a single urban area.
MSAs were an obvious choice. They indicate the extent of the metropolitan area, of course. They provide boundaries between adjacent areas, for example, where the New York area stops and the Philadelphia area begins. (Of course, these boundaries are somewhat arbitrary as MSAs have been created using county boundaries, but at least they provide a basis for making the choice.) And finally, MSAs can specify that two or more previously separate urban areas should be considered to be part of a single, unified area, such as Dallas and Fort Worth.
But the OMB definitions provide an alternative unit, Combined Statistical Areas (CSAs). CSAs are combinations of Core-Based Statistical Areas (CBSAs), MSAs and the small Micropolitan Statistical Areas. As with the determination of which counties are to be included in an MSA, the combination of CBSAs to form a CSA is based on commuting interchange. For adding a county to an MSA, a 25 percent commuting threshold must be met. To combine two CBSAs into a CSA, a 15 percent commuting threshold must be met. The commuting considered is slightly different in the two cases, but the idea is the same: Areas will be combined and considered to be part of a single area if there is significant cross-commuting. The major difference between the MSAs and the CSAs is the level of such commuting required.
After much time spent considering the alternative areas, I have chosen to use the CSAs as the units for the delineation of my areas. I based my decision on the reasonableness of the areas created using the MSA and CSA definitions. I will describe my thinking and in doing so suggest that using CSAs rather than MSAs may make sense for other analyses as well. (Note that not all MSAs are combined into CSAs because no adjacent areas meet the commuting threshold. In those cases, I use the MSAs.)
In many of the cases, the areas combined with a large MSA to form a CSA are fairly small areas. This obviously extends the sizes of the areas. But this was not of great importance to my choice of CSAs over MSAs. I will address the combinations that I did see as being significant.
First, the New York CSA includes the Connecticut suburbs of New York. (The New York MSA does not extend into Connecticut.) To me, these suburbs are so obviously a part of the New York area that this one is a no-brainer.
The San Jose MSA is combined with the San Francisco-Oakland MSA. The peninsula from San Jose to San Francisco is an uninterrupted stretch of intense urban development. There are all the stories of Google and other Silicon Valley employees who work in the San Jose MSA living in San Francisco and taking company buses to work (and being resented by other San Francisco residents). And while the San Jose MSA is associated with Silicon Valley, Facebook and the venture capitalists on Sand Hill Road are located in the San Francisco MSA.
The Riverside-San Bernardino-Ontario MSA is east of and adjacent to the Los Angeles MSA. Urban development is continuous from east of San Bernardino to the Pacific Ocean, as anyone flying into LAX from the east will have observed. Rush-hour commuting on the three freeways extending east from Los Angeles make the ties clear.
Moving to somewhat smaller areas, residents of the Raleigh and Durham MSAs have long considered themselves to be part of a single area. The Research Triangle Park is located in the middle, with Chapel Hill in the Durham MSA being the third point of the triangle. The area was a combined single MSA for a number of decades until the tweaking of the definition in 2003. So having them in a single CSA is highly appropriate. Staying within the Carolinas, Greensboro, Winston-Salem, and High Point, North Carolina, and Greenville and Spartanburg, South Carolina MSAs combine into two CSAs.
The two large combinations that were a little more surprising to me were Baltimore with Washington and Providence with Boston. But the urban areas have certainly grown together. And they meet the commuting threshold. So maybe not so unreasonable. Way back in 1967, when I participated in a march from Baltimore to Washington, much of the area between was already developed. But that is another story…