How is a metropolitan area defined?
The U.S. Office of Management and Budget defines metropolitan [statistical] areas as standardized county or equivalent-based areas having at least one urbanized area of 50,000 or more population, plus adjacent territory that has a high degree of social and economic integration with the core, as measured by commuting ties. For more information on how metropolitan areas are defined, see http://www.census.gov/population/www/estimates/metroarea.html.
