Urban Influence Codes (UIC) emphasizes the relationship of outlying counties to major metropolitan areas. Counties are subdivided into nine categories distinguished by three features: population size in census-defined urbanized areas, adjacency to metropolitan areas, and the size of those adjacent communities. 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 UIC, 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.
UIC were developed at the U.S. Department of Agriculture's Economic Research Service, as a refinement of the Office of Management and Budget (OMB) Metropolitan Statistical Area (MSA) definition. Additional information about the UIC classification scheme is available on the Internet at http://www.ers.usda.gov/topics/rural-economy-population/rural-classifications.aspx#.U4S5mXYRLTo.
The name of this HCUP data element and the version of the categorization have changed over time:
- Starting in the 2014 HCUP databases, the classification of counties is based on the 2010 Census and the OMB 2013 Core Based Statistical Area (CBSA) definitions.
- Starting in the 2013 HCUP databases, the data element is PL_UIC.
- Between 2003 and 2012, the data element name was PL_UIC2003 and the classification of counties was based on the 2000 Census and the OMB 2003 CBSA definitions.
- In 2002, the data element name was PL_UIC and the classification of counties was based on the 1990 Census and the OMB 1993 Metropolitan definitions.
- This information was not included on the HCUP databases prior to 2002.