Research Methods

How does the Dartmouth Atlas Project get access to its data? Where does the data come from?

The very large claims databases 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 provided for research purposes. Other data sources include the U.S. Census, the American Hospital Association, the American Medical Association, and the National Center for Health Statistics.

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.

Hospital referral regions (HRRs) represent regional health care markets for tertiary medical care. Each HRR contains at least one hospital that performs major cardiovascular procedures and neurosurgery. 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. More information on how HSAs and HRRs were defined is available in our Appendix on the Geography of Health Care in the United States.

What population does the Dartmouth Atlas Project study?

The Medicare population in an area includes those alive, age 65 to 99, and not enrolled in a risk-bearing health maintenance organization (HMO). For physician services, the population is 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 the Medicare A (inpatient) and B (physician services) programs.

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, etc.) 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 are 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 are defined as the sum of full-time employees and half of 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 are 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 are included and 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 is adjusted for patient migration. Since information on the travel patterns of patients is not available, physicians are allocated in proportion to inpatient days in hospitals located in their HSAs. For example, if an HSA had four 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 one full-time equivalent primary care physician to the neighboring HSA.

What is a rate?

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 Medicare 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 reflect events per 100,000 persons.

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 are reported in parentheses to indicate lack of statistical precision; for these rates, the margin of error is greater than 20%, so the estimate is considered statistically unreliable.

How are rates adjusted?

Most rates of utilization and spending are adjusted to the age, sex and race distribution of the national Medicare population using the indirect method. 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. Click here for more information about indirect adjustment.

Measures of the care of the chronically ill population are adjusted for differences in age, sex, race, primary chronic illness, and the presence of more than one chronic conditions using ordinary least squares regression.

Where can I find more information?

Comprehensive information on such topics as files used, rate definitions, code specifications, physician classifications, allocation and adjustment methods, and so on is available in our Research Methods compendium. Information about our data and methods related to the care of chronic illness is available in the Appendix on Methods of our most recent report on the care of chronically ill patients during the last two years of life.


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The Dartmouth Atlas of Health Care is based at The Dartmouth Institute for Health Policy and Clinical Practice and is supported by a coalition of funders led by the Robert Wood Johnson Foundation, including the WellPoint Foundation, the United Health Foundation, the California HealthCare Foundation, and the Charles H. Hood Foundation.