The Medical Outcomes Study (MOS) is a large-scale multi-year survey of patients with prevalent and treatable chronic health conditions, particularly hypertension, heart disease, diabetes, and major depression. The MOS, which began in 1986, includes both cross-sectional and longitudinal components. The cross-sectional component describes the impact of chronic diseases on patient well-being as well as the medical care that patients received. The longitudinal component illustrates changes in health conditions over time and explains outcomes in terms of systems of care, provider specialty, styles of practice, and other factors that influence utilization of health care resources. The investigators employed a variety of assessment instruments, such as self-administered questionnaires for patients and clinicians, face-to-face interviews, telephone interviews, clinical reports, and health examinations. Furthermore, the SF-36 health survey was administered longitudinally, with a sample of 3,588 cases, in order to measure important health and functioning domains. The study samples were drawn from patients receiving health care from 523 solo or group practices in Boston, Chicago, and Los Angeles.

The objectives of the study were assessment and evaluation of health status--defined in the MOS as the combination of: (1) clinical status, (2) functioning, and (3) well-being. In particular, the MOS investigated four prevalent and treatable chronic health conditions--three medical conditions of hypertension, heart disease, and diabetes as well as one psychological condition of major depression or dysthymia. The MOS examined these health conditions and their outcomes longitudinally in terms of health care, provider specialty, style of practice, and other factors that influenced utilization of health care resources. Among these, the study focused on the comparisons of (1) systems of care--HMOs, large multispecialty groups, and solo and small groups; (2) payment types--parepaid vs. fee-for-service; and (3) clinician specialty--specialist vs. generalist care. This large-scale multi-year study marked several methodological advances. First, the MOS measured the three parameters of health status in parallel, initially and longitudinally. Second, the MOS focused on patients' own personal evaluation of their functional status and well-being--an attempt to meet the recent needs of patient-based assessments of medical outcomes. Consequently, the MOS was the first large-scale study in which patients with different medical and psychiatric conditions completed the same self-administered questionnaires. Finally, the MOS is also distinctive in that it used both standardized patient surveys and clinical evaluations as measures of health status. With its large samples drawn from three urban cities in the U.S., the MOS increased standardization of general health surveys across studies and populations.