Census Definitions: Metropolitan Statistical Areas Versus Urbanized Areas

There are two different geographies that are often confused with each other. The urbanized area (UZA) and the metropolitan statistical area (MSA) are both measures of urbanity, but they are defined very differently. The UZA is developed by the US Census Bureau based on information collected during each census. The MSA is produced by the Office of Management and Budget (OMB), using information taken from the census and blended with information from other sources, primarily the Department of Labor. The primary purpose of both geographies is to provide statistical information for use by government agencies. A secondary purpose is to serve as the basis for distribution of program funds that use a formula. A wide array of federal, state, and local executive agencies use these urban definitions and delineations to allocate funding and implement regulations.

Urbanized Area

A UZA is a compact area that is entirely urban in character, defined as a contiguous area with more than 50,000 people and with a population density greater than 1,000 persons per square mile. The area that meets the density definition is included in the boundary of the UZA, regardless of political boundaries. The "building block" of a UZA is the census block group, which can be as small as one acre. Because the level of analysis is so small, UZAs are often irregular in shape. Further, UZAs pay no attention to political boundaries. Two UZAs cannot share a border, because such a condition would result in a single contiguous geographic unit. According to the 2000 Census, there are 484 UZAs in the United States, which cover just over 2% of the nation's land area (1). The UZAs currently established in the United States are shown in Figure 1.

Metropolitan Statistical Areas

Metropolitan statistical areas have a more complex definition. Counties serve as the "building blocks" of MSAs. In order to be designated an MSA, the region must have at least one UZA to serve as the core of the MSA (2). The county that contains the UZA is called the "core county" of the MSA. Additional "outlying counties" that have high degrees of economic or social integration with the core are added to the MSA. Outlying counties qualify for inclusion in the MSA if a) more than 25% of the employed residents commute to the core county; or b) more than 25% of the jobs in the outlying county are held by residents of the core county (3). The boundary of an MSA is coterminous with the county boundaries that qualify for inclusion. Therefore, it is common for land area that is rural in appearance and character to be included in the MSA. Two MSAs can share a border. MSAs can become quite large in land area-the Atlanta MSA currently encompasses 28 counties. Most, however, encompass only one county. In New England, the term 'New England City and Town Area' (NECTA) is substituted for MSA because townships are the predominant local government type in those states. Figure 2 shows the 362 existing MSAs in the United States.

The Office of Management and Budget-which is a part of the Executive Office of the President-is responsible for defining, creating, and modifying MSAs. The OMB issues annual updates to the list of MSAs, but does not create new ones until after each Census (4).

MSA and UZA History

It is important to understand the history of how and when each geography was developed. The Seventh Census (1850) was the first to allow regional administrators to delineate and collect data in geographies smaller than the state level. This was the first data collection effort that laid the framework for identifying urbanity, and data were amalgamated for some major municipalities and counties. The Ninth Census in 1870 was the first to identify urbanized areas as distinct units free from political boundaries. Over the next century, collection techniques were repeatedly improved upon, and definitions have changed. Although the Census Bureau has reported "urbanized areas" for over 150 years, it has used the modern definition since the Sixteenth Census in 1940 (5).

The concept of a metropolitan statistical area was first defined by the Office of Management and Budget (OMB) in 1951 and has been modified six times since, most recently in 2003. The MSA took almost two decades to reach its modern definition, and did not gain wide acceptance until the 1980s (6).

In order to visualize the difference between MSAs and UZAs, the map overlays the MSAs in Florida with the UZAs in the state. Each MSA is shown with a different color, but the UZAs are shown in uniform yellow.

REFERENCES

(1) Environmental Protection Agency, "Storm Water Phase II Final Rule: Urbanized Areas: Definitions and Descriptions." Document Number EPA-833-F-00-004, Issued December 2005.

(2) Bureau of the Census. "Metropolitan and Micropolitan Statistical Areas." Accessed July 14^th, 2009. Available from: http://www.census.gov/population/www/metroareas/aboutmetro.html

(3) Federal Register: Volume 65, Number 249 (December 27, 2000) pp. 82228-82230.

(4) Gauthier, Jason, "Measuring America: The Decennial Censuses from 1790 to 2000" United States Census Bureau, September 2002.

(5) Frey, William and Zachary Zimmer. "Chapter 2: Defining the City." From Handbook of Urban Studies, Ronan Paddison, ed. Sage Publications, 2001.

(6) Office of Management and Budget Executive Order No. 10253 (June 11, 1951)