NHAMCS Scope and Sample Design

The NHAMCS comprises a national probability sample of visits to the emergency and outpatient departments of noninstitutional general and short-stay hospitals, exclusive of Federal, military, and Veterans Administration hospitals in the 50 States and the District of Columbia. The NHAMCS was designed to provide estimates based on the following priority of survey objectives: United States, region, emergency and outpatient departments, and type of ownership. The survey uses a four-stage probability design with samples of primary sampling units (PSUs), hospitals within PSUs, clinics within hospitals, and patient visits within clinics. The survey design is described briefly below, and can be found in more detail within the Public-Use downloadable documentation files.

PSUs are geographic segments composed of counties, groups of counties, county equivalents (such as parishes or independent cities) or towns, townships, and other minor civil divisions (for some PSUs in New England), or a metropolitan statistical area (MSA). MSAs were defined by the U.S. Office of Management and Budget on the basis of the 1980 census. The first stage sample of the NHAMCS consisted of 112 PSUs that comprise a probability subsample of the PSUs used in the 1985-94 National Health Interview Survey (NHIS). The NHAMCS PSU sample included with certainty the 26 NHIS PSUs with the largest populations. In addition, the NHAMCS sample included one-half of the next 26 largest PSUs, and one PSU from each of the 73 PSU strata formed from the remaining PSUs for the NHIS sample.

To preclude hospitals from participating during the same time period each year, the sample was randomly divided into 16 subsets of approximately equal size. Each subset was assigned 1 of 16 4-week reporting periods beginning December 2, 1991. These continue to rotate across each survey year. Therefore, the entire sample does not participate in a given year, and each hospital is inducted approximately once every 15 months.

Within each hospital, either all outpatient clinics and emergency services areas or a sample of such units is selected. Clinics were considered to be eligible for the survey (in-scope) if ambulatory medical care was provided under the supervision of a physician and under the auspices of the hospital. Clinics were required to be organized in the sense that services were offered at established locations and schedules. Clinics where only ancillary services were provided or other settings in which physician services were not typically provided were not included in the survey (out-of-scope). In addition, freestanding clinics were out of scope since they are included in the NAMCS, and ambulatory surgery centers whether in hospitals or freestanding were out of scope since they were to be included in the National Survey of Ambulatory Surgery. A list of in-scope and out-of-scope clinics as well as definitions of other terms related to the survey are available in the complete documentation.

During the visit by a field representative to induct the hospital into the survey, a list of all emergency service areas and outpatient clinics is obtained from the sample hospital. Each outpatient department clinic’s function, specialty, and expected number of visits during the assigned reporting period is also collected. If there are five or fewer clinic sampling units, all are sampled. If a hospital has more than five clinic sampling units, then five units are randomly selected according to a predetermined protocol involving clinic type and size.

Emergency departments are treated as a separate stratum, and all emergency service areas are selected with certainty. In the rare event that a sample hospital has more than five emergency service areas, a sample of five areas is selected with probability proportional to the expected number of visits to each emergency service area.

The basic sampling unit for the NHAMCS is the patient visit or encounter. Only visits made in the United States to EDs and OPDs of non-Federal, short-stay or general hospitals are included. Within emergency service areas or outpatient department clinics, patient visits are systematically selected over a randomly assigned 4-week reporting period. A visit is defined as a direct, personal exchange between a physician, or a staff member operating under a physician’s direction, for the purpose of seeking care and rendering health services. Visits solely for administrative purposes and visits in which no medical care was provided were out of scope.

The target numbers of Patient Record forms to be completed for EDs and OPDs in each hospital are 100 and 200, respectively. In clinics with volumes higher than these desired figures, visits were sampled by a systematic procedure which selected every nth visit after a random start. Visit sampling rates were determined from the expected number  of patients to be seen during the reporting period and the desired number of completed Patient Record forms.