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Why a guideline on rural-urban classification systems?

The DOH Community Health Systems Office has documented differences in health status and related risk and protective factors between residents of rural and urban Washington. Analysts looking at rural health disparities must choose from several classification systems which change over time. Guidelines are useful for promoting consistency, comparability and best practice among statewide analyses that look at rural health. Local public health assessments and measurement of performance standards also benefit from consistent classification systems to compare local health data to areas with similar population and settlement patterns.

These guidelines do not cover the use of rural-urban classification schemes for determining eligibility for state or federal assistance programs. See How Many Agencies does it Take to Define Rural? for a summary of eligibility criteria for rurally-targeted programs. Also, see Washington State Office of Financial Management for references to population density in Washington law.

How rural is Washington State?

In 2008, between 13% and 27% of Washington’s residents lived in areas classified as rural depending on the classification method used. For example, nine Washington counties are classified as rural using the Washington State Office of Financial Management (OFM) designations and as metropolitan using nationally developed systems. This underscores that there is no gold standard for the defining features of rural areas or a single best theoretical basis for classifying rural areas. Some classification systems measure rurality on the basis of population density, others by economic or commuting connections. The choice of the geographic unit (county, census tract, or ZIP code) for analysis introduces further variation.

Regardless of the classification system, Washington is a predominantly urban state and is becoming more urbanized over time. Analysts comparing health indicators in rural and urban areas should be aware that rates may be worse in rural compared to urban areas, but with very few exceptions, the total numbers affected are much higher in urban areas.

What systems are commonly used to classify rural-urban character?

The first rural-urban classification system, developed in 1874 by the US Census Bureau, defined rural as the population of a county living outside of cities or towns with 8,000 or more inhabitants. That population threshold was changed to 2,500 in 1910. The US Census Bureau now defines ‘urban’ as all territory, population, and housing units in Urbanized Areas and Urban Clusters with 2,500 or more persons. This definition is based on a very fine level of geography, the census block group. Urbanized areas and urban clusters are contiguous built-up areas with a population density of more than 1,000 persons per square mile. Currently, an Urbanized Area is a contiguous set of block groups with a population of more than 50,000. An Urban Cluster is a contiguous set of block groups with populations of between 2,500 and 49,999. The remaining areas are Rural Areas. Most rural-urban classification systems are based on these definitions. The US Census Bureau’s most recent revision of these definitions in 2002 resulted in a larger enlargement of Urbanized Areas than would have happened by population increase alone. Figure 1 shows how the boundaries of Urbanized Areas changed in Washington.

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Details about the methodological changes in 2002 and a list of urbanized areas are available by the Census Bureau.

Early systems tended to be binary in nature, classifying counties as urban or rural. These binary systems gave way to more complex county coding systems such as Rural-Urban Continuum Codes (RUCC) that recognize differences in size and the importance of issues like economic influence. With advances in computing power and geographic information systems in the 1990s, sub-county classifications systems, most notably RUCA system, were developed. For more information see Ricketts et al. (1998) and the USDA Economic Research Service: Measuring Rurality Briefing Room.

Table 1: Commonly Used Rural-Urban Classification Systems

* Year published in the Federal Register

County, census tract and ZIP code definitions

Many classification systems (for example, Metropolitan, Micropolitan, and Outside Core-Based Statistical Areas, Figure 2) use county-level geographic aggregation, based on population density and dominant economic commuting pattern.
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Using county-based classification systems is attractive in that county lines tend to be stable over time and many health, social and economic indicators are readily available for counties. However, these county-level systems tend to classify the residents of urban or sub-urban centers in large rural counties as rural. Nationally, 11% of residents of Metropolitan counties, as defined by the OMB, are classified as rural by the US Census Bureau’s block group classifications and 7% of the residents of Non-Metropolitan Counties are classified as urban (Hart et al., 2005).

Compared to county-level classification systems, sub-county classification systems, while often more precise, are more complex. For example, census tract and ZIP code boundaries change more frequently than do county boundaries, adding to the complexity of classification systems based on census tracts or ZIP codes.
 

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