How does the Dartmouth Atlas Project get access to its data? Where does the data come from?
The very large claims databases used in the DAP come from the Centers for Medicare and Medicaid Services (CMS), the federal agency that collects data for every person and provider using Medicare health insurance. Access to this data is made available for research purposes. Other data sources include U.S. Census, the American Hospital Association, American Medical Association, National Center for Health Statistics, and Claritas, Incorporated.

What is an HSA/HRR? How are the populations determined?
Hospital service areas (HSAs) are local health care markets for hospital care. An HSA is a collection of ZIP codes whose residents receive most of their hospitalizations from the hospitals in that area. HSAs were defined by assigning ZIP codes to the hospital area where the greatest proportion of their Medicare residents were hospitalized. Minor adjustments were made to ensure geographic contiguity. Most hospital service areas contain only one hospital. The process resulted in 3,436 HSAs, ranging in total 1996 population from 604 to 3,067,356.

Hospital referral regions (HRRs) represent regional health care markets for tertiary medical care. Each HRR contained at least one hospital that performed major cardiovascular procedures and neurosurgery. In a similar fashion, HRRs were defined by assigning HSAs to the region where the greatest proportion of major cardiovascular procedures were performed, with minor modifications to achieve geographic contiguity, a minimum population size of 120,000, and a high localization index. The process resulted in 306 hospital referral regions which ranged in total 1996 population from 126,329 to 9,288,694. More information on how HSAs and HRRs were defined is available in our Appendix on the Geography of Health Care in the United States.

The Medicare population in an area includes those alive, age 65 to 99, and not enrolled in a risk bearing HMO. For physician services, the population was restricted to a random sample of Medicare enrollees having Medicare Part B physician claims. For Medicare reimbursement rates, the population was restricted to a random sample belonging to both Medicare A (inpatient) and B (physician services) programs.

In which HSA/HRR is my ZIP code located? What other HSAs are in my HRR?
The Geographic Query Finder will allow you to locate ZIP codes by HSA and HRR, HSAs by HRR and state, and HRRs by state. The assignments are based on 2005 ZIP codes.

How are an area's health care resources measured and allocated?
An area's health care resources consist of acute care hospital beds and medical personnel. As some patients seek care outside their area, these resources (beds, physicians, other hospital personnel) were allocated to HSAs in proportion to the area residents' use of hospital services. This allocation procedure 'transfers' resources from one area to another in proportion to how they are used. Areas with high migration will be allocated more resources but the allocated amount will reflect what is actually used in contrast to what exists in an area. For health policy purposes, it is necessary to be aware of this distinction since reduction in utilization in one area may require reduction in capacity of resources in an adjacent area.

Hospital beds and personnel. All short term medical and surgical hospitals, specialty and children's hospitals were included with a few exceptions. Hospital beds included cribs, pediatric and neonatal bassinets, medical/surgical intensive care, and cardiac intensive care beds. Full time equivalent hospital personnel were defined as the sum of full time employees and 1/2 the part time employees, not including medical or dental interns, residents and trainees.

To account for patients who live in one HSA but obtain medical care in another, hospital resources were allocated to HSAs in proportion to the Medicare hospital days provided by hospitals to that HSA. For example, if 60% of total Medicare inpatient days at a hospital were used by residents of the HSA where the hospital was located, then 60% of that hospital's resources would be assigned to its HSA. If 20% of the Medicare patient days provided by that hospital were used by a neighboring HSA, 20% of the hospital's resources would be assigned to that neighboring HSA.

Physician workforce. All physicians working at least 20 hours a week in clinical practice were included and were classified according to their primary self-designated specialty.

Physicians provide services to patients residing both in and outside the HSA where their practices are located, so the physician workforce was adjusted for patient migration. Since information on the travel patterns of patients is not available, physicians were allocated in proportion to inpatient days in hospitals located in their HSAs. For example, if an HSA had 4 primary care physicians and if 25% of the patient days at the local hospital(s) were to residents of a neighboring HSA, then these physicians contributed 1 FTE primary care physician to the neighboring HSA.

What is a rate? How precisely is it measured?
A rate is the number of events or amount of resources divided by the number in the population. For example, if an area with 100,000 enrollees has 810 hip fracture repairs, then the rate of hip fracture repair is 8.1 per 1,000 Medicare enrollees. For rare events, the rate is often re-scaled to events per 100,000 persons.

Because the rates in the atlas are based on the entire Medicare population, they are for the most part precisely determined. Precision denotes the margin of error (or standard error) associated with the estimate, and is expressed as a percent of the estimated rate. Rates in larger areas and rates for more frequent events are measured more precisely. For example, for an estimated event rate of 5 per 1,000 in a median-sized HRR (N=64,000), the precision is ±6%. This means that the true event rate is very likely between 4.7 and 5.3 per 1000.

Why are some rates suppressed?
Rates based on a count of fewer than 11 patients are not displayed for reasons of patient confidentiality. Rates with fewer than 26 expected events were reported in parentheses to indicate lack of statistical precision; for these rates, the margin of error is greater than 20%, so that the estimate was considered unreliable.

How are rates adjusted?
Rates were adjusted to the age, sex and race distribution of the national Medicare population. First, the national event rate for each age-sex-race category was computed. These rates were then applied to the HSA population to produce the expected number of events in the HSA, that is, the number of events that would have occurred in the HSA if its rate was the same as the national event rate. It is one way to standardize for different distributions of risk factors across areas.

Where can I find more information?
Information on such topics as files used, rate definitions, code specifications, physician classifications, allocation and adjustment methods, and so on for the region-based Dartmouth Atlas series is available in the Appendix on Methods of the Dartmouth Atlas of Health Care 1999. Updated code definitions for surgical procedures and modified diagnosis-related groups are also available. Information on the development of the hospital-, region-, and state-specific rates from our most recent Atlas is available in that report's Appendix on Methods.