ESRI Data & Maps 2006 : U.S. Core Based Statistical Areas

U.S. Core Based Statistical Areas represents geographic entities, defined by the United States Office of Management and Budget for use by Federal statistical agencies, based on the concept of a core area with a large population nucleus, plus adjacent communities having a high degree of economic and social integration with that core. A Core-Based Statistical Area consists of a county containing an Incorporated Place or Census Designated Place with a population of at least 10,000 along with any adjacent counties that have at least 25 percent of employed residents of the county who work in the CBSA's core or central county.

CBSAs are categorized as being either Metropolitan or Micropolitan. Each Metropolitan Statistical Area must have at least one urbanized area of 50,000 or more inhabitants. Each Micropolitan Statistical Area must have at least one urban cluster of at least 10,000 but less than 50,000 population.