National Hospital Ambulatory Medical Care Survey
The National Hospital Ambulatory Medical Care Survey (NHAMCS) collects data on the utilization and provision of medical care services provided in hospital emergency and outpatient departments. Data are collected from medical records.
A four-stage probability sample design is used in NHAMCS, involving (a) samples of geographically defined primary sampling units (PSUs), (b) hospitals within PSUs, (c ) clinics within outpatient departments (OPDs), and (d) patient visits within clinics. Emergency departments (EDs) are treated as their own stratum and all service areas are included. In the OPD survey, a clinic is defined as an administrative unit of the OPD in which ambulatory medical care is provided under the supervision of a physician. Clinics where only ancillary services (e.g., radiology, laboratory services) are provided, or other settings in which physician services are not typically provided, are excluded. EDs included in the survey must offer 24 hour physician services. Visits to eligible EDs and OPDs are systematically sampled over a 4-week reporting period such that about 100 ED encounters and about 200 OPD encounters are selected. Hospital staff are asked to complete patient record forms for each sampled visit. Sample data are weighted to produce national estimates.
in any given year, the hospital sample consists of approximately 500 hospitals, of which 80% have EDs and about one-half have eligible OPDs. Typically, about 1,000 clinics are selected from participating hospital OPDs. In 2007, the number of patient record forms completed for EDs was 35,490 and for OPDs was 34,473. The hospital response rate was 94% for EDs and 84% for ODPs.
The NHAMCS patient record form is modified approximately every 2 to 4 years to reflect changes in physician practice characteristics, patterns of care, and technological innovations.
National Center for Health Statistics. Health United States 2009: With Special Feature on Medical Technology. Hyattsville, Maryland. 2010; pp 466-467.