Behind the Data: Developing a County-Level Measure of Urban Influence

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. Population size, urbanization, and access to larger communities are often crucial elements in county-level research. To advance such research, ERS developed a set of county-level urban influence categories that captures some differences in economic opportunities.

The 2003 Urban Influence Codes divide the 3,141 counties, county equivalents, and independent cities in the United States into 12 groups. Counties are first divided into metropolitan (metro) and nonmetropolitan (nonmetro) categories according to the official classification announced by the Office of Management and Budget (OMB) in June 2003, based on population and commuting data from the 2000 Census of Population. Metro counties are then divided into two groups by the size of the metro area they are in—large and small (see box, “County Definitions”). Nonmetro counties are divided into 10 groups, first by micropolitan (micro) versus noncore status, then by proximity to metro- or micro-areas.

Nonmetro micro counties are divided into three groups by their adjacency to metro areas—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 by their adjacency to metro or micro areas and whether or not they have their “own town” of at least 2,500 residents.

Nonmetro counties are defined as adjacent if they abut a metro area (noncore counties may also abut a micro area) and have at least 2 percent of employed persons commuting to work in the core of the metro area (or in the micro area). When a nonmetro county was adjacent to more than one metro (or micro) area, it was designated as adjacent to the area to which the largest percentage of its workers commuted.

In concept, the 2003 Urban Influence Codes are comparable with those of earlier decades. However, as a result of changes in metro area delineation procedures and in rural and urban area measurement, our new codes are not fully comparable with those of earlier years. Those changes are explained on the ERS website at:

2003 Urban Influence Codes
County typeNumber of counties2000 populationPopulation per sq. mile
Metropolitan counties:
In large area of at least 1 million residents413149,224,067558
In small area of less than 1 million residents67683,355,873132
Nonmetropolitan counties:
Micropolitan counties—
Adjacent to large metro925,147,23355
Adjacent to small metro30114,668,14451
Not adjacent12829,139,82127
Noncore counties—
Adjacent to large metro1232,364,15927
Adjacent to small metro with own town3587,855,59024
Adjacent to small metro with no own town1851,879,2646
Adjacent to micro with own town22013,227,83317
Adjacent to micro with no own town21981,313,1757
Not adjacent to metro or micro with own town11382,247,1895
Not adjacent to metro or micro with no own town174999,5584
1 Micro counties that are not adjacent are often local trade centers. Nonadjacent-noncore counties with towns may be service centers for surrounding smaller counties, especially in less-populated areas of the Great Plains.
2 The micro area that a noncore county is adjacent to may itself be adjacent to a small or large metro area. This hierarchical commuting relationship is not reflected in the coding system.

County Definitions

  • Micropolitan: Areas containing an urban core of at least 10,000 residents
  • Noncore: Counties without an urban core of at least 10,000 residents
  • Large metro: In areas with at least 1 million residents
  • Small metro: In areas with less than 1 million residents