edition of the Atlas were retained in the 1999 edition. HSAs were originally defined in three steps using 1993 provider files and 1992-1993 utilization data. First, all acute care hospitals in the 50 states and the District of Columbia were identified from the American Hospital Association Annual Survey of Hospitals and the Medicare Provider of Services files and assigned to a location within a town or city. The list of towns or cities with at least one acute care hospital (N = 3,953) defined the maximum number of possible HSAs. Second, all 1992 and 1993 acute care hospitalizations of the Medicare population were analyzed according to zip code to determine the proportion of residents’ hospital stays that occurred in each of the 3,953 candidate HSAs. Zip codes were initially assigned to the HSA where the greatest proportion (plurality) of residents were hospitalized. Approximately 500 of the candidate HSAs did not qualify as independent HSAs because the plurality of patients resident in those HSAs were hospitalized in other HSAs. The third step required visual examination of the zip codes used to define each HSA. Maps of zip code boundaries were made using files obtained from Geographic Data Technologies, and each HAS’s component zip codes were examined. To achieve contiguity of the component zip codes for each HSA, “island” zip codes were reassigned to the enclosing HSA, and/or HSAs were grouped into larger HSAs. This process resulted in the identification of 3,436 HSAs, ranging in total 1996 population from 604 (Turtle Lake, North Dakota) to 3,067,356 (Houston) in the 1999 edition of the Atlas. Intuitively, one may think of HSAs as representing the geographic level at which “front end” services such as diagnoses are received.

Hospital Service Areas make clear the patterns of use of local hospitals. A significant proportion of care, however, is provided by referral hospitals that serve a larger region. Hospital Referral Regions were defined in the Atlas by documenting where patients were referred for major cardiovascular surgical procedures and for neurosurgery. Each Hospital Service Area was examined to determine where most of its residents went for these services. The result was the aggregation of the 3,436 HSAs into 306 HRRs. Each HRR had at least one city where both major cardiovascular surgical procedures and neurosurgery were performed. Maps were used to make sure that the small number of “orphan” hospital service areas—those surrounded by HSAs allocated to a different HRR—were reassigned, in almost all cases, to ensure geographic contiguity. HRRs were pooled with neighbors if their populations were less than 120,000 or if less than 65 percent of their residents’ hospitalizations occurred within the region. HRR were named for the HSA containing the referral hospital or hospitals used most often by residents of the region. The regions sometimes cross state bound-