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VOLUME 42 | NUMBER 1 | FEBRUARY 2007


Assigning Ambulatory Patients and Their Physicians to Hospitals: A Method for Obtaining Population-Based Provider Performance Measurements

Objective.
To develop a method for assigning Medicare enrollees and the physicians who serve them to individual hospitals with adequate validity to allow population-based assessments of provider specific costs and quality of care.

Data Sources/Study Setting.
The study populations consist of a 20 percent sample of Medicare fee-for-service enrollees and all physicians submitting claims for Medicare services from 1998 to 2000. Data were obtained from Medicare claims and enrollment files, Medicare's MPIER file, and from the American Hospital Association Annual Survey.

Study Design.
Cross-sectional analysis of the characteristics of hospitals, their extended medical staffs (EMSs) and the utilization patterns of their assigned Medicare enrollees.

Data Collection Methods.
Medicare enrollees were assigned to their predominant ambulatory physician and then to the hospital where that physician provided inpatient services or where a plurality of that physician's patient panel had medical admissions. Each beneficiary was linked to a physician and a hospital regardless of whether the patient was hospitalized creating Ambulatory Provider Specific Cohorts (APSCs).

Principal Findings.
Ninety-six percent of eligible Medicare enrollees who had an index physician visit in 1998 were assigned to a specific provider. Two-thirds of the medical admissions during a 2-year period occurred at the assigned hospital and two-thirds of evaluation and management services were billed by the assigned hospital's EMS. The empirically derived EMS for hospitals had reasonable face and discriminant validity in terms of number and type of physicians practicing at different sized and type hospitals. Estimates of risk-adjusted costs across physician groups in year one are highly predictive of costs in a subsequent year (r=0.87, p<.0001 and weighted ?=0.65, p<.0001).

Conclusions.
Medicare claims data can be used to assign virtually all Medicare enrollees to empirically defined care systems comprised of hospitals and the physicians who practice at these hospitals. Studies of patterns of practice, costs and outcomes of care experienced by these APSCs could complement other methods of monitoring provider performance.

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