(counties), along with any adjacent counties that have at least 50 percent of their population in the urbanized area surrounding the largest city. Additional 'outlying counties' are included in the MSA if they meet specified requirements of commuting to the central counties and other selected requirements of metropolitan character (such as population density and percent urban) (U.S. Bureau of the Cencus, 1996a:937). An urbanized area is "an area consisting of a central place(s) and adjacent urban fringe that together have a minimum residential population of at least 50,000 people and generally an overall population density of at least 1,000 people per square mile of land area" (U.S. Bureau of the Census, 1994:G-54). Unlike metropolitan areas, which must contain entire counties, urbanized areas are defined with reference to density.

Holding 1960 metropolitan-area boundaries constant, the proportion of the U.S. population living in these areas actually fell slightly, from 63 percent in 1960 to 62 percent in 1990.

Metropolitan areas are divided into primary metropolitan statistical areas (PMSAs) and consolidated metropolitan statistical areas (CMSAs). CMSAs consist of two or more contiguous and closely related PMSAs. As of June 1995, there were 271 metropolitan areas, consisting of 253 PMSAs and 18 CMSAs. The 18 CMSAs contained within them a total of 73 PMSAs. The total number of PMSAs was therefore 326.

Increases in the proportion of the population living in high-poverty areas may result from an increasing number of census tracts being classified as high-poverty areas as nonpoor households move out, from an increasing incidence of poverty, or from an increasing propensity of the poor to move into such areas.