Appendix A: Dartmouth Atlas regions
The Dartmouth Atlas project has divided the United States into 3,436 geographically distinct hospital service areas (HSAs). Medicare patients living in an HSA get the majority of their health care from hospitals within the area. These areas were defined in three steps. First, all acute care hospitals that provided care to Medicare patients in 1992 and 1993 were assigned to the town or city in which they were located, defining the initial list of HSAs. Second, the ZIP codes of patients hospitalized in 1992 and 1993 were recorded to determine the proportion of patients in that ZIP code that used each hospital. Then, each ZIP code was assigned to the hospital where the greatest proportion of residents received care. Finally, a map of the ZIP code boundaries allowed for the areas to be geographically defined. Using the HSAs, hospital referral regions (HRRs) were then defined by determining where patients in each HSA received major cardiovascular surgery and neurosurgery. First, all of the hospitals performing at least 10 major cardiovascular procedures in 1992 and 1993 were identified and located within the pre-defined HSAs, which became the candidate HRRs. Then, each of the 3,436 HSAs was assigned to the candidate HRR where the greatest proportion of their patients went for these surgical services. The HSAs using hospitals in the same candidate HRR were grouped to form 306 HRRs. These are shown below in the map of the United States.
Map A. The Dartmouth Atlas hospital referral regions
