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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.

Centers for Disease Control and Prevention, National Center for Health Statistics
Data Years Available: 
Mode of Collection: 
Hospital staff are asked to complete a Patient Record Form for a sample of patient visits during an assigned reporting period.
Selected Content: 
Data are collected on types of providers seen, reason for visit, diagnoses, drugs ordered, provided, or continued, and selected procedures and tests performed during the visit. Other data collected include patient demographics, method of payment, and selected characteristics of the hospital.
Population Covered: 
The survey is a representative sample of visits to emergency departments and outpatient departments of nonfederal, short-stay, or general hospitals. Telephone contacts are excluded.

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.

Response Rates and Sample Size: 

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.

Interpretation Issues: 

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.