The 2003 version of the Rural-Urban Continuum Codes (RUCC) subdivide counties into 9 categories distinguished by population size in census-defined urbanized areas and by adjacency to metropolitan areas. To be adjacent, counties must be contiguous and have at least 2% of the resident labor force commuting to a central metropolitan county.
A county-based system such as RUCC, which attempts to describe the diversity in settlement patterns in a relatively large area by a single number, may not provide an accurate depiction. However, because county boundaries don't change much, every county will be represented by a measure, even after an extended period of time.
RUCC were developed at the U.S. Department of Agriculture's, Economic Research Service, as a refinement of the Office of Management and Budget (OMB) Core-Based Statistical Area (CBSA) definition. They are based on population and commuting information from the 2000 census and the OMB 2003 CBSA definitions. Additional information about the RUCC classification scheme is available on the Internet at http://www.ers.usda.gov/briefing/rurality/RuralUrbCon/.