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 and methodological changes is available at https://www.ers.usda.gov/data-products/urban-influence-codes.

The name of this HCUP data element and the version of the categorization have changed over time:

• Starting in the 2024 HCUP databases, the data element name is PL_UIC2023 and the classification of counties is based on the 2020 Census and the OMB 2023 Core Based Statistical Areas (CBSA) definitions. This revision of the UIC makes substantial changes to the number and order of categories compared to prior versions.
• Between 2014 and 2023, the classification of counties is based on the 2010 Census and the OMB 2013 Core Based Statistical Area (CBSA) definitions.
• Between 2013 and 2023, the data element name is PL_UIC.
• Between 2003 and 2012, the data element name is PL_UIC2003 and the 12-category classification of counties is based on the 2000 Census and the OMB 2003 CBSA definitions.
• In 2002, the data element name is PL_UIC and the classification of counties is based on the 1990 Census and the OMB 1993 Metropolitan definitions.
• This information is not included on the HCUP databases prior to 2002.