New metropolitan and micropolitan statistical area definitions were announced by OMB on June 6, 2003, based on application of the 2000 standards with Census 2000 data. Metropolitan and Micropolitan Statistical Areas are collectively referred to as Core-Based Statistical Areas.

• Metropolitan statistical areas have 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.
• Micropolitan statistical areas are a new set of statistical areas that have at least one urban cluster of at least 10,000 but less than 50,000 population, plus adjacent territory that has a high degree of social and economic integration with the core as measured by commuting ties.

Metropolitan and micropolitan statistical areas are defined in terms of whole counties or county equivalents, including the six New England states. As of June 6, 2003, there are 362 metropolitan statistical areas and 560 micropolitan statistical areas in the United States.

The technical paper, The Effects of Using Newly-Defined Metropolitan Area Boundaries When Examining Residential Housing Patterns discusses the effect of the most recent change in metropolitan area definitions on residential housing patterns.

Previous reports using Census 2000 data have employed metropolitan area boundaries defined by the Office of Management (OMB) as of June 30, 1999 and based on 1990 standards. Under the 1990 standards, each metropolitan area included a city of 50,000 or more population or a Census Bureau-defined urbanized area of at least 50,000 population, provided that the component county/counties had a population of at least 100,000. Metropolitan areas consisted of one or more counties, except in the New England States, where the components were cities and towns. Metropolitan areas included metropolitan statistical areas (MSAs) and consolidated metropolitan statistical areas (CMSAs). CMSAs were composed of primary metropolitan statistical areas (PMSAs). There were 258 MSAs, 73 PMSAs and 18 CMSAs in the United States.

Metropolitan statistical areas defined under the 2000 standards with Census 2000 data may not be directly comparable to MSAs and CMSAs defined as of 1999. In addition to the designation of new micropolitan statistical areas, the following kinds of changes may affect the definition of individual metropolitan areas in existence in 1999 and redefined based on Census 2000 data and the 2000 standards:

• A county or counties added;
• A county or counties deleted;
• Two or more areas merged to forma single metropolitan statistical area;
• A metropolitan area split to form multiple metropolitan and micropolitan statistical areas; and
• Reclassification as a micropolitan statistical area.