Documentation

ERS' 2013 Urban Influence Codes divide the 3,143 counties, county equivalents, and independent cities in the United States into 12 groups. Metro counties are divided into two groups according to the population size of the metro area—those in "large" areas have at least 1 million residents and those in "small" areas have fewer than 1 million residents. Nonmetro counties include all counties outside metro areas and are delineated as micropolitan or noncore using OMB’s classification. Nonmetro micropolitan counties are divided into three groups distinguished by metro size and adjacency: adjacent to a large metro area, adjacent to a small metro area, and not adjacent to a metro area. Nonmetro noncore counties are divided into seven groups distinguished by their adjacency to metro or micro areas and whether or not they contain a town of at least 2,500 residents. This documentation is organized in the following sections:

• Background
• Scope
• Methodology
• Data sources
• Revision/release history
• Reference date

Background

An area's geographic context has a significant effect on its development. Economic opportunities accrue to a place by virtue of both its size and its access to larger economies since access to larger economies—centers of information, communication, trade, and finance—enable local economies to connect to national and international marketplaces. These relationships among economies are basic concepts of the central place theory commonly studied in regional economics. Population size, urbanization, and access to larger communities are often crucial elements in research dependent on county-level data sets.

The Urban Influence Codes were developed by ERS to capture these differences in economic opportunities among counties. The Urban Influence Codes are based on the Office of Management and Budget’s delineation of Metropolitan (metro) and Micropolitan (micro) statistical areas; micropolitan areas are further classified by adjacency and noncore nonmetro counties are classified by adjacency and population of the county’s largest town. The result is a finer rural-urban gradation for use by researchers and policy makers.

Scope

The 2013 Urban Influence Codes classify all counties and county equivalents in the United States and the Commonwealth of Puerto Rico. These include 1,167 metro counties as well as 641 micro and 1,335 noncore nonmetro counties. In Puerto Rico, the Urban Influence Codes comprised 69 metro, 4 micro, and 4 noncore municipios.

The Urban Influence Codes were first released in 1997 by Linda Ghelfi and Tim Parker of the Economic Research Service in their article A County-Level Measure of Urban Influence. The codes were updated in 2003 incorporating the new core based delineation released by the OMB in 2003. See descriptions of the 1993 and 2003 versions of the Urban Influence Codes.

Methodology

The 2013 Urban Influence Codes divide the 3,143 counties, county equivalents, and independent cities in the United States into 12 groups. Metro counties are divided into two groups according to the population size of the metro area—those in "large" areas have at least 1 million residents and those in "small" areas have fewer than 1 million residents. Nonmetro counties include all counties outside metro areas and are delineated as micropolitan or noncore using OMB’s classification. Nonmetro micropolitan counties are divided into three groups distinguished by metro size and adjacency: adjacent to a large metro area, adjacent to a small metro area, and not adjacent to a metro area. Nonmetro noncore counties are divided into seven groups distinguished by their adjacency to metro or micro areas and whether or not they contain a town of at least 2,500 residents. A town refers to an incorporated city or town, or a Census Designated Place, which is an entity that has no legal definition. Nonmetro independent cities of Virginia have been combined with their counties of origin (See data file for details).

The 2013 Urban Influence Codes are based on the OMB metropolitan classification announced in February 2013 (see the OMB Metropolitan Area criteria), which in turn are based on population data from the 2010 Census of Population and commuting data from the 2006-2010 American Community Survey (ACS). Nonmetro counties are defined as adjacent if they abut a metro area (or if nonmetro noncore counties abut a micro area) and have at least 2% of employed persons commuting to work in the core of the metro area (or in the micro area). When a nonmetro county meets the adjacency criterion for more than one metro (or micro) area, it is designated as adjacent to the area to which the largest percentage of its workers commuted.

In concept, the 2013 version of the Urban Influence Codes is comparable with the previous version released in 2003. However, OMB’s release of the 2013 Metropolitan Areas used 5-year average commuting data from the 2006-2010 American Community Survey rather than from the decennial Census, since commuting data are no longer collected as part of the decennial census. The codes released in 2003 were based on a point-in-time commuting measure from the 2000 Census of Population.

Comparisons with versions prior to the 2003 release are more problematic. OMB made major changes in its metro area delineation procedures for the 2000 Census. These changes added additional metro areas by no longer requiring that a metro area must have at least 100,000 population if its urbanized area has no place of at least 50,000 people. More importantly, the changes simplified the worker commuting criteria that determine outlying metro counties and added numerous new outlying counties to metro areas while removing the metro status of a smaller number of counties that were previously metro.

The Census Bureau also changed its method for defining rural and urban areas by liberalizing the procedures for delineating urbanized areas of 50,000 or more people, and dropping place boundary requirements in measuring urban or rural population (See What is Rural? for more information). The procedures used in defining urbanized areas were extended down to clusters of 2,500 or more people, based solely on population density per square mile. In this manner, lightly settled sections of municipalities were treated as rural, and densely settled areas adjoining urban cores were treated as urban, regardless of whether they were incorporated or not. Thus "urban clusters" need not include an incorporated or unincorporated place of 2,500 population, and not all incorporated or unincorporated places of 2,500 population constitute urban clusters. It is therefore not possible to redefine Urban Influence Codes for prior censuses in a manner fully consistent with those of 2003.

Data Sources

U.S. Census Bureau, County Adjacency File 2010.

U.S. Census Bureau, County-to-County Commuting Flows: 2006-2010.

U. S. Census Bureau, Summary File 1 (SF 1), Census Place Files-Summary Levels 155 and 160.

Office of Management and Budget (OMB), Metropolitan and Micropolitan Area Delineation Files, February 2013.

Revision/Release History

The Urban Influence Codes were first released in 1996 and are updated every 10 years after the release of the Decennial Census Data. See previous releases for 2003 and 1993, and "A County-Level Measure of Urban Influence" in Rural Development Perspective, Vol. 12 No. 2, February 1997. 

Reference Date

This document was prepared in May 2013.