Geographic Variables

All geography below the national level is restricted for continuous NHANES due to disclosure risk; prior to 1999 all geography below the regional level is restricted; thus this information was accessed at the RDC.

Urbanization was defined based on county of residence using the NCHS six-level urban-rural classification scheme for U.S. counties and county-equivalent entities (https://www.cdc.gov/nchs/data_access/urban_rural.htm). In NHANES II, urbanization was classified as urbanized area ≥1 million, urbanized area <1 million, urban place outside of urbanized area, rural areas, central cities, and noncentral cities; we collapsed this variable to categories similar to NHANES III in which urbanization was dichotomized as counties of metropolitan areas ≥1 million or all other areas (<1 million). In NHANES 1999–2016, urbanization was defined as large metropolitan (population ≥1 million), medium and small metropolitan (population <1 million), or nonmetropolitan. Urbanization classification for years 1999–2016 was based on the NCHS urban, rural classification schemes (1990 classification scheme for years 1999–2002, 2006 classification scheme for years 2003–2010, and 2013 classification scheme for 2011–2016) as assigned by the RDC (NCHS 2017). The NCHS metropolitan categories were collapsed for this analysis to increase sample size for subgroups and produce stable estimates based on the NCHS data presentation standards for proportions: metropolitan statistical areas (MSAs) with a population of 1 million or more; MSAs with a population of less than 1 million; and, for more recent survey cycles, areas outside of MSAs.

Geographic region was classified as Northeast, Midwest, South and West for all years. For NHANES 1999–2016, geographic region was categorized based on the 2010 Census Bureau’s regions and is restricted data. Geographic region in NHANES II and III differs from the 2010 Census Bureau definition. Therefore, regional estimates cannot be directly compared across NHANES II, NHANES III, and NHANES 1999–2016.