Ambulatory Health Care Data
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.
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 and emergency service areas within hospitals, and patient visits within clinics and emergency service areas. 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.
The sampling frame for the 2009 NHAMCS was constructed from products of Verispan L.L.C., specifically “Healthcare Market Index” and “Hospital Market Profiling Solution.” These products were formerly known as the SMG Hospital Database. The original sample frame was compiled from hospitals listed on the April 1991 SMG Hospital Market Database. Hospitals with an average length of stay for all patients of less than 30 days (short‑stay) or hospitals whose specialty was general (medical or surgical) or children's general were eligible. Excluded were Federal hospitals, hospital units of institutions, and hospitals with less than six beds staffed for patient use. A fixed panel of 600 hospitals was selected for the NHAMCS sample; 550 hospitals had an ED and/or an OPD and 50 hospitals had neither an ED nor an OPD. Beginning in 2009, hospital-based ambulatory sugery centers (ASCs) were included in the survey.
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, all emergency services areas and ASC locations are selected. In addition, either all outpatient clinics or a sample of such units are selected. Clinics are considered to be eligible for the survey (in-scope) if ambulatory medical care is provided under the supervision of a physician and under the auspices of the hospital. Clinics are required to be organized in the sense that services are offered at established locations and schedules. Clinics where only ancillary services are provided or other settings in which physician services are not typically provided are not included in the survey (out-of-scope). In addition, freestanding clinics are out of scope since they are included in the NAMCS. 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, outpatient clinics, and ASC locations 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 and hospital-based ASCs are treated as separate strata, and all emergency service areas and ASC locations are selected with certainty.
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, outpatient department clinics, or hospital-based ASCs, 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 are out of scope.
The target numbers of Patient Record forms to be completed for EDs, OPDs, and ASCs in each hospital are 100, 200, and 100, respectively. In clinics with volumes higher than these desired figures, visits are sampled by a systematic procedure which selects every nth visit after a random start. Visit sampling rates are determined from the expected number of patients to be seen during the reporting period and the desired number of completed Patient Record forms.
Ambulatory and Hospital Care Statistics Branch
National Center for Health Statistics
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Hyattsville, MD 20782-2064
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