The U.S. Office of Management and Budget (OMB) defines a set of core based statistical areas (CBSAs) throughout the country. CBSAs are delineated on the basis of a central urban area or urban clustera contiguous area of relatively high population density. CBSAs are composed of counties and county-equivalents.2 The counties containing the core urban area are known as the central counties of the CBSA. Additional surrounding counties (known as outlying counties) can be included in the CBSA if these counties have strong social and economic ties to the central counties as measured by commuting and employment. Outlying counties are included in the CBSA if the employment interchange measure (total of in commuting and out commuting) is 25% or more. Note that some areas within these outlying counties may actually be rural in nature. CBSAs are subdivided into metropolitan statistical areas (MSAs) and micropolitan statistical areas based on the population of the core urban area. Under certain conditions one or more CBSAs may be grouped together to form a larger statistical entity known as a combined statistical area (CSA). Other names such as Standard Metropolitan Statistical Area (SMSA) and Primary Metropolitan Statistical Area have been used in the past but are now discontinued.3 In New England because of the greater importance of towns over counties similar areas are defined based on town units known as New England City and Town Areas (NECTAs). U.S. census statistics for metropolitan areas are reported based on these definitions. Hierarchy Combined Statistical Area (CSA) Metropolitan Statistical Area (MSA) Metropolitan Division45 Micropolitan Statistical Area (SA) See also United States census statistical areas by state district or territory AL AK AZ AR CA CO CT DE DC FL GA HI ID IL IN IA KS KY LA ME MD MA MI MN MS MO MT NE NV NH NJ NM NY NC ND OH OK OR PA RI SC SD TN TX UT VT VA WA WV WI WY AS MP PR GU VI Geography Human geography Demography United States United States Census Bureau Office of Management and Budget Combined statistical area Core Based Statistical Area Metropolitan Statistical Area Micropolitan Statistical Area United States urban area Demographics of the United States Table of United States Combined Statistical Areas (CSA) Table of United States Core Based Statistical Areas (CBSA) Table of United States Metropolitan Statistical Areas (MSA) Table of United States Micropolitan Statistical Areas (SA) Table of United States primary census statistical areas (PCSA) List of United States urban areas List of United States cities by population List of regions of the United States List of metropolitan areas by population List of metropolitan areas in the Americas by population List of the largest urban areas of the European Union by population Megacity World's largest cities References Nussle Jim (Nov. 20 2008). "Update of Statistical Area Definitions and Guidance on Their Uses". Office of Management and Budget. pp. 1-2. http://www.whitehouse.gov/sites/default/files/omb/assets/omb/bulletins/fy2009/09-01.pdf.  Census Geographic Glossary U.S. Census Bureau "Metropolitan and Micropolitan Statistical Areas". U.S. Census Bureau. http://www.census.gov/population/www/metroareas/metroarea.html. Retrieved 16 February 2010.  "Metropolitan and NECTA Divisions published by CES". U.S. Bureau of Labor Statistics. January 28 2011. http://www.bls.gov/sae/saemd.htm.  "May 2009 Metropolitan and Nonmetropolitan Area Definitions". U.S. Bureau of Labor Statistics. August 16 2010. http://www.bls.gov/oes/current/msadef.htm.  External links US Census Bureau Population Estimates Table 1. Annual Estimates of the Population of Metropolitan and Micropolitan Statistical Areas: April 1 2000 to July 1 2005 U.S. Census 2000 Metropolitan Area Rankings; ranked by population U.S. Census 2000 Metropolitan Area Rankings Definitions of all Metropolitan Statistical Areas Metropolitan Divisions U.S. Census Metropolitan Area Estimates (1990s) U.S. Census Metropolitan and Micropolitan Statistical Area Definitions v d e50 largest metropolitan statistical areas in the United States by population     New York Los Angeles Chicago DallasFort Worth Philadelphia Houston Miami Washington Atlanta Boston Detroit Phoenix San FranciscoOakland RiversideSan Bernardino Seattle MinneapolisSt. Paul San Diego St. Louis TampaSt. Petersburg Baltimore Denver Pittsburgh Portland Cincinnati Sacramento Cleveland Orlando San Antonio Kansas City Las Vegas San Jose Columbus Charlotte Indianapolis Austin Virginia BeachNorfolk Providence Nashville Milwaukee Jacksonville Memphis Louisville Richmond Oklahoma City Hartford New Orleans Birmingham Salt Lake City Raleigh Buffalo