The data used in the current study were taken from the National Ambulatory Medical Care Survey (NAMCS): 1999 Summary. The NAMCS is a national probability assessment of visits to office-based physicians in the United States, conducted by the Division of Health Care Statistics of the National Center for Health Statistics, Centers for Disease Control and Prevention. The NAMCS was designed to meet the need for objective, reliable information about the use of ambulatory medical care services in the United States.
The basic sampling unit for the NAMCS is the physician-patient encounter. Only patient visits to the offices of non–federally employed physicians, with visits classified by the American Osteopathic Association and the American Medical Association as office-based, patient care, were included in the 1999 NAMCS. Physicians in the specialties of anesthesiology, pathology, and radiology were excluded. Patient visits not included in the 1999 NAMCS were visits conducted by telephone, visits outside of the physician's office (eg, house calls), and visits in hospital settings (unless the physician had a private office in a hospital that met the NAMCS definition of office). Also excluded were visits that occurred in institutional settings by patients for whom the institution had ongoing responsibility (ie, nursing homes) and visits to physicians' offices made for administrative purposes only (eg, leave a specimen, pay a bill, pick up insurance forms).
In 1999, the NAMCS used a multistage probability design to produce a national sample of office-based physicians. Researchers received 20,760 patient record forms from the 1087 physicians who participated in the NAMCS. To obtain national estimates, each record was assigned an inflation factor called the “patient visit weight.” By aggregating patient visit weights on the 20,760 sample records for 1999, the user obtains the estimated total of 756,733,854 office visits made in the United States in that year. Articles that describe the method and the instrument used to interpret data in the survey have been described elsewhere.
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