PL_NCHS2006 is a six-category urban-rural classification scheme for U.S. counties developed by the National Center for Health Statistics (NCHS) 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.
The county classifications are based on the Office of Management and Budget (OMB) metropolitan/micropolitan assignments as updated through the 2005 revisions (OMB Bulletin No. 06-01). 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 2000 Census and 2004 Census Bureau population estimates.
Although the six NCHS categories may conceptually be mapped into the four categories of the PL_UR_CAT4 classification used elsewhere in HCUP, the two schemes are not entirely compatible. A few counties are categorized differently in the two schemes because they are based on inconsistent population estimates produced on different dates,
Additional information about the NCHS classification scheme is available on the Internet at http://www.cdc.gov/nchs/r&d/rdc_urbanrural.htm.