National Hospital Ambulatory Medical Care Survey (NHAMCS)
National Center for Health Statistics
NHAMCS provides national data on the provision and use of medical care services in hospital emergency and outpatient departments and ambulatory surgery centers, using information collected from medical records. 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. Patient data include age, sex, race, and expected source of payment. Data are also collected on selected characteristics of the hospitals included in the survey.
NHAMCS coverage has changed over time but has included visits to emergency departments (EDs) and outpatient departments (OPDs) of nonfederal, short-stay, or general hospitals and ambulatory surgery centers in the 50 states and the District of Columbia. Visits to federal, military, and Veterans Administration hospitals, as well as telephone contacts, are excluded. In 2012 only, hospitals were oversampled in the five most populous states, which permits state-level estimates for these states. Starting in 2018, only the ED component was fielded.
The three-stage probability sample design used for 2018 involves samples of a) geographically defined primary sampling units (PSUs), b) hospitals within PSUs, and c) patient visits within emergency service areas. The first-stage sample of NHAMCS consists of 112 PSUs selected from 1,900 such units that make up the United States. Earlier survey years included a fourth level to sample OPDs and EDs.
These PSUs were stratified by socioeconomic and demographic variables and then selected with a probability proportional to their 1980 population size. Stratification was done within four geographical regions by metropolitan statistical area (MSA) or non-MSA status using 1980 Census of Population data. The NHAMCS PSU sample included with certainty the 26 National Health Interview Survey (NHIS) PSUs with the largest populations. In addition, the NHAMCS sample included one-half of the next 26 largest PSUs, and 1 PSU from each of the 73 PSU strata formed from the remaining PSUs for the NHIS sample. The hospital sampling frame and sample were last updated for the 2017 survey to remove hospitals whose status had changed and were now out of scope for the survey.
Sample data are weighted to produce national estimates. The estimation procedure used in NHAMCS has three basic components: inflation by the reciprocal of the probability of selection, adjustment for nonresponse, and population weighting ratio adjustment.
Sample Size and Response Rate
In any given year when the sample is not supplemented (as done in 2012), the hospital sample consists of about 450 hospitals. In 2018, 490 hospitals were selected, of which 378 were eligible. In 2018, the number of patient record forms (PRFs) completed for EDs was 20,291, and the overall unweighted response rate was 86% for EDs.
Issues Affecting Interpretation
The NHAMCS PRF is modified about every 2–4 years to reflect changes in physician practice characteristics, patterns of care, and technological innovations. Examples of recent changes include an increase in the number of drugs recorded on the PRF and adding checkboxes for specific tests or procedures performed. Differences (some statistically significant) in 2016 and 2017 estimates were noted. These may reflect actual differences, or they may be related to the changes in item format, data collection system, or the increase in census abstraction. Data users are advised to take these factors into account when comparing estimates across years.
- National Center for Health Statistics. 2018 NHAMCS micro-data file documentation. Available from: https://ftp.cdc.gov/pub/health_statistics/nchs/dataset_documentation/NHAMCS/doc18-ed-508.pdf.
For more information, see the National Health Care Surveys website at: https://www.cdc.gov/nchs/dhcs/index.htm, and the Ambulatory Health Care Data website at: https://www.cdc.gov/nchs/ahcd/index.htm.