In general, for most NCHS restricted-use data files, geographic variables can only be used to merge external data or to categorize geographic areas with similar characteristics. Although smaller levels of geography are available for NCHS surveys, it is inappropriate to make estimates for areas smaller than what the sample design would support.
Geographic estimates at levels lower than the national level are rarely approved.
- Regional estimates will be considered by the NCHS RDC Review Committee for all surveys.
- State estimates, when pooling multiple years, will be considered by the Review Committee for NHIS, SLAITS, Natality and Mortality only.
- County or lower level estimates are never approved for any NCHS survey data.
Geographic variables are removed after the external data is merged to the NCHS data unless there is justification to keep the variables for analytic purposes only as approved in the proposal.
Geographic variable availability by NCHS Survey
- National Health and Nutrition Examination Survey (NHANES)
- National Health Care Surveys (NHCS)
- National Health Interview Survey (NHIS)
- National Immunization Survey (NIS)
- National Survey of Family Growth (NSFG)
- State and Local Area Integrated Telephone Survey (SLAITS)
- Vital Statistics Birth Certificates (Natality)
- Vital Statistics Death Certificates (Mortality)
NCHS developed a six-level urban-rural classification scheme for U.S. counties and county-equivalent entities that can be applied to NCHS surveys without an urban-rural variable. If you are interested in using this urban-rural measure, include state and county in your list of restricted variables and the NCHS urban/rural continuum as an additional NCHS data source. State and county will be removed after the urban/rural codes are merged.