census.gov: U.S. Census Bureau: Metropolitan and Micropolitan Statistical Areas Citat: "... Defining Metropolitan and Micropolitan Statistical Areas. The 2000 standards provide that each CBSA must contain at least one urban area of 10,000 or more population. 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...."
