National Ambulatory Medical Care Survey (NAMCS)
National Center for Health Statistics
NAMCS provides national data about the provision and use of medical care services in office-based physician practices in the United States, using information collected from medical records. Data are collected on type of providers seen; reason for visit; diagnoses; drugs ordered, provided, or continued; and selected procedures and tests ordered or performed during the visit. Patient data include age, sex, race, and expected source of payment. Data are also collected on selected characteristics of physician practices, including practice size and location.
NAMCS covers patient visits to the offices of nonfederally employed physicians classified by the American Medical Association (AMA) or American Osteopathic Association (AOA) as “office-based, patient care” physicians in the United States. Physicians in the specialties of anesthesiology, pathology, and radiology, as well as physicians who are principally engaged in teaching, research, or administration, are excluded. Patient visits with physicians engaged in prepaid practices (health maintenance organizations, independent practice organizations, and other prepaid practices) are included in NAMCS, while telephone contacts and nonoffice visits are excluded. In 2006, a separate sample of community health centers (CHCs) was added to NAMCS. The CHC component samples visits to physicians and nonphysician clinicians. Starting with 2014 data, the sample was expanded to include hospital-based physicians. From 2012 through 2015, the NAMCS survey sample design targeted varying numbers of states to allow for state-level estimates in addition to the regular sample. In 2018, a national probability sample was used; it permits regional estimates.
A multistage probability design is employed. In 1989–2011, the first-stage sample consisted of 112 primary sampling units (PSUs) that were selected from about 1,900 such units into which the United States had been divided. In each sample PSU, a sample of practicing nonfederal, office-based physicians was selected from master files maintained by AMA and AOA. The final stage involved systematic random samples of office visits during randomly assigned 7-day reporting periods. Starting with the 2012 survey, the sampling design was changed to a list sample of physicians, instead of an area sample, to ensure adequate representation for state-level estimates. Another major change, which began in 2012, was in the mode of data collection, which changed from in-person interviews with a paper questionnaire to laptop-assisted data collection by U.S. Census Bureau field representatives using automated survey instruments. In 2016 and 2017, two modes of data collection were used: traditional in-person, manual medical record abstraction by U.S. Census Bureau staff and electronic health records (EHR) from the sampled physician. The 2016 file with data collected through the traditional abstraction method has been released. However, the data collected via EHR require additional processing. Therefore, the 2017 file has not yet been released. In 2018, the only mode of data collection was laptop-assisted data collection.
To sample CHC physicians and nonphysician clinicians, a dual-sampling procedure was used. First, the traditional NAMCS sample was selected using the methods described above. Second, information from the Health Resources and Services Administration and the Indian Health Service was used to select a sample of CHCs. Within CHCs, a maximum of three health care providers—which included physicians as well as nonphysician practitioners—were selected. Nonphysician practitioners included physicians, physician assistants, nurse practitioners, or nurse midwives. After selection, CHC providers followed traditional NAMCS methods for selecting patient visits.
In 2008, a supplemental mail survey on EHR systems was conducted in addition to the core NAMCS. Starting in 2012, the survey was changed from a supplement to become the separate National Electronic Health Records Survey (NEHRS).
Sample data are weighted to produce national estimates. The estimation procedure used in NAMCS has four basic components: inflation by the reciprocal of the probability of selection, adjustment for nonresponse, ratio adjustment to fixed totals, and weight smoothing. The weighting procedure changed with the 2018 data set to account for nonresponse bias.
Sample Size and Response Rate
The physician and visit sample sizes have varied over the years. Most recently, the number of eligible physicians was 2,999 in 2018. The number of visits included was 9,953 and the unweighted response rate was 37%.
Issues Affecting Interpretation
The NAMCS patient record form is modified approximately every 2–4 years to reflect changes in physician practice characteristics, patterns of care, and technological innovations. Examples of recent changes include increasing the number of drugs recorded on the patient record form and adding checkboxes for specific tests or procedures performed. Sample sizes vary by survey year. For some years, analysts are advised to combine 2 or more years of data if they wish to examine relatively rare populations or events. The 2012 sampling design change may affect trending 2012 and subsequent data with earlier data. For more information on the new sampling design, see Hing E, et al. Data anomalies between 2016 and 2018 NAMCS data have been noted and may be due to the changes in weighting procedures. A methodology report is planned.
- Hing E, Shimizu IM, Talwalkar A. Nonresponse bias in estimates from the 2012 National Ambulatory Medical Care Survey. Vital Health Stat 2(171). National Center for Health Statistics. 2016. Available from: https://www.cdc.gov/nchs/data/series/sr_02/sr02_171.pdf.
- National Center for Health Statistics. 2018 NAMCS micro-data file documentation. Available from: https://ftp.cdc.gov/pub/Health_Statistics/NCHS/Dataset_Documentation/NAMCS/doc2018-508.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.