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 by the OMB and the FECMA every ten years, culminating in the publication of revised standards before the decennial census.

Most of the changes in the standards have been minor and have not reflected significant deviations from the concepts underlying the standards used for the 1950 census. Several modifications have been made in the rules for determining how large a city must be to have an MA defined. Until 1958, the standards always required a city of at least 50,000, but subsequent changes have relaxed this rule somewhat, permitting the definition of areas around smaller cities when certain specified conditions were met. Changing national conditions also have resulted in standards changes. For example, counties qualifying for inclusion in SMAs defined for the 1950 census were required to have less than 25 percent of their workforce engaged in agriculture. This requirement was dropped in the 1980 standards because it no longer affected many counties.

The availability of new statistical data has affected the development of the standards. For example, although the 1950 standards specified commuting as the main measure of integration between outlying and central counties, there were no national data available on the subject at that time. Most of the commuting data used to define SMAs in the 1950s were derived from surveys by State and local employment agencies, which were not entirely comparable with one another in their coverage and approach. The inclusion of a question about place of work in the 1960 census made available a national set of data on commuting, thus improving the accuracy and consistency of areas defined as metropolitan. Also, the standards now make greater use of commuting data. Some of the most important changes in the standards were announced before the 1980 census (and implemented in 1983). Chief among these were the provision for qualification of MAs on the basis of the Census Bureau’s UAs, and the introduction of PMSAs as components of CMSAs.

The standards for the 1990s went into effect in December 1992 and June 1993, when OMB issued redefinitions of MAs based on commuting and Metropolitan Areas13-5