J Clin Epidemiol Vol. 49, No. 6, pp. 643-652, 1996 Copyright 0 1996 Elsevier Science Inc. ELSEVIER 0895-4356/96/15.00 PII SO895-4356(96)00008-X Epidemiologic Research in an Integrated Regional Medical Care System: The Marshfield Epidemiologic Study Area Frank DeS tefuno, la* Elaine D. Eaker,’ Steven K. Broste,’ David L. Nordstrom,’ Peggy L. Peissig,’ Robert A. Vie&ant,’ Kate A. Konitzer,’ Robert L. Gruber,’ and Peter M. L&e2 ‘MARSHFIELD MEDICAL RESEARCH FOUNDATION, MARSHFIELD, WISCONSIN 54449, AND ‘MEDICAL COLLEGE OF WISCONSIN, MILWAUKEE, WISCONSIN 53224 ABSTRACT. To capitalize on Marshfield Clinic’s advantages for population-based health research, we devel- oped the Marshfield Epidemiologic Study Area (MESA). Marshfield Cl inic is an integrated system consisting of a large multispecialty clinic and 23 affiliated clinics. Clinic physicians provide virtually all of the medical care, both inpatient and outpatient, for residents of the area. MESA consists of 14 ZIP codes in which over 95% of the 50,000 residents and most significant health events are captured in Marshfield Clinic databases, including all deaths, 94% of hospital discharges, and 92% of medical outpatient visits. MESA exemplifies the research potential of integrated medical care systems and the efforts required to realize that potential. Because it is representative of a defined population and provides an unselected sample of patients, MESA is well suited for epidemiologic research and research elucidating the clinical spectrum and natural history of diseases and the effectiveness of treatment. J CLIN EPIDEMIOL 49;6:643-652, 1996. KEY WORDS. Epidemiologic methods, medical databases. health services research INTRODUCTION Medical care organizations in the United States are increasingly consolidating into integrated systems covering large populations. In- tegrated systems provide all levels of care, both outpatient and inpa- tient, from primary care through highly specialized tertiary care. In such systems, computerized information systems are extensively used in a variety of patient care and administrative functions. The avail- ability of computerized information covering the spectrum of medi- cal care in large populations provides opportunities for many types of health research, including public health surveillance, epidemio- logical studies, and health services research [l-4]. For many years Marshfield Clinic (Marshfield, WI) has been de- veloping an integrated regional health care system that has many of the features being adopted by health care organizations in the United States. The Clinic provides virtually all the health care for the residents of its immediate service area, allowing the possibility of conducting population-based research (i.e., research on a defined population in a defined geographic area). To capitalize fully on the advantages of the Marshfield Clinic system as a resource for popula- tion-based health research, we have developed the Marshfield Epi- demiologic Study Area (MESA). Herein, we present the methods we have used to establish and operate the MESA and some of the lessons we have learned in this effort. Our experience may be useful to others wirh interests in developing and using population-based medical databases for epidemiological studies or for medical out- comes and other health services research. BACKGROUND Marshfield Clinic is a 400-physician m&specialty group medical practice located in a rural region of central Wisconsin. Founded in ‘Address for correspondence: Frank !&Stefano, Marshfield Medical Research Foundation, 1000 North Oak Avenue, Marshfield, Wisconsin 54449. Accepted for puhhcation on 29 August 1995. 1916 as one of the first group practices in the country, the Clinic has grown to include 23 primary care Regional Centers in central and northern Wisconsin. All of the Regional Centers are located in rural communities and most have fewer than 10 physicians on staff. Five of the Regional Centers have between 10 and 40 physi- cians and provide selected specialty services (e.g., cardiology, on- cology, nephrology, orthopedics, vascular surgery) in addition to primary care. In 1971, the Clinic established its own health maintenance organization (HMO), which is the only HMO in the Marshfield area and is one of the largest nonurban HMOs in the United States. The main Clinic facility in Marshfield is contiguous to St. Joseph’s Hospital, which is the only hospital in the Marshfield area. The Clinic and Hospital share a joint unit record for each patient. Information on medical care received by patients in area nursing homes and mental health facilities, which are staffed by Marshfield Clinic physicians, also is included in the medical record. Beginning in 1961, certain elements of the medical record, includ- ing diagnoses, have been stored in computerized databases. The Clinic is the sole provider of medical care in the Marshfield area. Virtually all health conditions requiring medical care among area residents are diagnosed and treated at the Clinic or at St. Jo- seph’s Hospital and the information is recorded in the unified pa- tient record. A regional data network connects the 23 Regional Centers to the Marshfield computer system and information on Re- gional Center patients is available on the Clinic computer. In developing the MESA we had the following objectives: (1) a high level of coverage of a well-defined population, (2) the ability to track individuals prospectively, (3) high levels of ascertainment of health events, and (4) the availability of high-quality diagnostic and treatment information on individuals in the population. DEFINING THE POPULATION The overriding consideration was to define an area in which virtu- ally all the residents receive their medical care through the Marsh-