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.

• Metropolitan counties include (PL_NCHS2 = 21)
  • "Central" counties of metro areas of >=1 million population (PL_NCHS = 1)
  • "Fringe" counties of metro areas of >=1 million population (PL_NCHS = 2)
  • Counties in metro areas of 250,000-999,999 population (PL_NCHS = 3)
  • Counties in metro areas of 50,000-249,999 population (PL_NCHS = 4)
• Non-metropolitan counties include (PL_NCHS2 = 22)
  • Micropolitan counties (PL_NCHS = 5)
  • Not metropolitan or micropolitan counties (PL_NCHS = 6)

The county classifications are based on the Office of Management and Budget (OMB) metropolitan/micropolitan assignments. The version of the categorization has changed over time:

• Starting in 2024 HCUP nationwide databases, the classification of counties is specific to the 2023 revision released by NCHS.
• In HCUP nationwide databases in 2023, the classification of counties is specific to the 2014 revision released by NCHS.

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