National Center for Health Statistics (NCHS) maintains a six-category urban-rural classification scheme for U.S. counties especially for use in health care research. The classification emphasizes urban distinctions and is unique in differentiating between central and fringe counties of large metropolitan areas. Smaller metropolitan counties are subdivided by population. Non-metropolitan counties are divided simply into micropolitan and non-core categories. PL_NCHS2 combines the six categories into two groups indicating metropolitan versus non-metropolitan counties.

The county classifications are based on the Office of Management and Budget (OMB) metropolitan/micropolitan assignments. These assignments were refined using information from the Rural-Urban Continuum Code (RUCC) and Urban Influence Code (UIC) of the Economic Research Service of the U.S. Department of Agriculture and county characteristics from the Census Bureau population estimates.

Additional information about the NCHS classification scheme is available at https://cdc.gov/nchs/data-analysis-tools/urban-rural.html.