NAMCS Scope and Sample Design

The basic sampling unit for the NAMCS is the physician-patient encounter or visit. Only visits to the offices of nonfederally employed physicians classified by the American Medical Association or the American Osteopathic Association as "office-based, patient care" are included in the physician universe. Physicians in the specialties of anesthesiology, pathology, and radiology are excluded. Types of contacts not included are those made by telephone, those made outside the physician’s office (for example, house calls), visits made in hospital settings (unless the physician has a private office in a hospital and that office meets the NAMCS definition of "office"), visits made in institutional settings by patients for whom the institution has primary responsibility over time (e.g., nursing homes), and visits to doctors’ offices that are made for administrative purposes only (e.g., to leave a specimen, pay a bill, or pick up insurance forms).

The NAMCS utilizes a multistage probability design that involves probability samples of primary sampling units (PSUs), physician practices within PSUs, and patient visits within practices. The first-stage sample includes 112 PSUs. PSUs are geographic segments composed of counties, groups of counties, county equivalents (such as parishes or independent cities) or towns and townships (for some PSUs in New England) within the 50 States and the District of Columbia.

The second stage consists of a probability sample of practicing physicians selected from the master files maintained by the American Medical Association and the American Osteopathic Association. Within each PSU, all eligible physicians were stratified by 15 groups: general and family practice, osteopathy, internal medicine, pediatrics, general surgery, obstetrics and gynecology, orthopedic surgery, cardiovascular diseases, dermatology, urology, psychiatry, neurology, ophthalmology, otolaryngology, and a residual category of all other specialties.

The final stage is the selection of patient visits within the annual practices of sample physicians. This involves two steps. First, the total physician sample is divided into 52 random subsamples of approximately equal size, and each subsample is randomly assigned to 1 of the 52 weeks in the survey year. Second, a systematic random sample of visits is selected by the physician during the reporting week. The sampling rate varies for this final step from a 100 percent sample for very small practices, to a 20 percent sample for very large practices as determined in a presurvey interview.