We rely on two main data sources: individual-level travel data and metropolitan-level employment data.
Data on worker commute distance and demographics
For data on workers and their commute lengths, we employ the American Housing Survey (AHS), a panel of housing units (mainly) surveyed every two years by the Census Bureau for the U.S. Department of Housing and Urban Development. We use the seven waves for 1985, 1987, 1989, 1991, 1993, 1995 and 1997. Between 11,000 and 15,000 respondents are included per year. Rich detail is provided on the residents of the housing units, including the reported distance to work (duration is not available for all but one of these years), income, education, marital status, ethnicity, age, gender, and other demographic and economic characteristics. Even greater detail on the condition and characteristics of the housing unit is recorded, though not utilized at this stage of the research.
Eliminating group quarters, mobile homes, units from non-metropolitan areas (as defined by the Census Bureau), households who reported spending less than one percent or more than 150 percent of their income on housing, observations where the number of occupants was not reported, vacant structures, nonurban structures, and observations for which the name of the metropolitan area is not provided leaves 185,085 total observations in the seven waves, representing 42,380 distinct housing units, or about two-thirds of the full sample.
Metropolitan employment and population data
The Bureau of Economic Analysis (BEA) Regional Economic Information System (REIS) provides county-level estimates of employment by industrial sector (one-digit Standard Industrial Classification code level) for the years 1969 to 2000. The REIS includes both regular wage and salary employment and other kinds of employment, such as proprietors' employment and self-employed people.
The source for wage and salary employment is the federal Covered Employment and Wages Program, a joint program with the Bureau of Labor Statistics (BLS) and the State Employment Security Agencies, which administer unemployment insurance programs. The states get the information from reports issued monthly by employers subject to unemployment insurance regulations. The employment figures include both full-time and part-time jobs.
The BEA augments the BLS wage and salary employment data with estimates for self-employment and proprietors' employment that are based partly on IRS income tax reporting. Self-employment and proprietors' employment can be a substantial fraction of total employment.3
In addition to employment by county by one-digit SIC code, the REIS tables include mid-year Census estimates of population by county.
Metropolitan area (MA) definitions
This study uses the official U.S. metropolitan area definitions promulgated by the Office of Management and Budget in 1999 and revised January 28, 2002, which are the same definitions used for Census 2000. The definitions are provided by the Bureau of the Census, and include a classification of counties within metropolitan areas as "central" or "outlying."4
Metropolitan areas must include a city of at least 50,000 inhabitants; alternately, they can include a Census-defined "urbanized area" of 50,000 or more if the county containing the urbanized area has at least 100,000 inhabitants.
The classification of intra-metropolitan-area counties is of particular interest to this study. The Census definitions of central and outlying counties are extended. In brief, central counties are those with at least 50 percent of the population of a designated central city or 50 percent of whose population lives in a Census-defined urbanized area.5 Counties bordering the central county or counties are designated as outlying counties, and included within the metropolitan area, based on commuting patterns and density. For example, if at least half of the residents of the nearby county commute to the central counties to work, and certain threshold density and population requirements are met, the county is designated as outlying and included within the metropolitan area. Peripheral counties whose commuting is at a lower level still qualify as outlying if their density is at a sufficiently high level, if a substantial enough fraction of their population is within the Census-designated urbanized area, or if recent population growth in the area has been high between the previous two decennial censuses.
Metropolitan employment and population variables
Metropolitan employment and population were calculated by summing the BEA county data using the definitions of metropolitan areas described above. The metropolitan employment and population decentralization variables were constructed by dividing the employment in designated central counties by that in all of the counties within the metropolitan area, and representing this number as a percentage in whole numbers. These variables were in turn assigned to respondents based on the match between the AHS designation of metropolitan areas, based on the 1983 OMB/Census definition, and the current (1999) OMB/Census designation. In cases of Consolidated Metropolitan Statistical Areas (CMSAs), which are metropolitan areas made up of more than one Primary Metropolitan Statistical Area (PMSA), the total employment in the CMSA was used and respondents from more than one PMSA were assigned the same decentralization variable values.
Unlike typical measures of decentralization based on distance from a city center, this measure allows non-contiguous or spatially extensive urbanization of sufficient density to be designated as core urbanization. Only employment in counties without large enough agglomerations or sufficiently high density is designated as decentralized or dispersed. The measure is conservative, although somewhat crude due to its reliance on county geography. It can be thought of as a measure of the share of employment in the most dispersed form -- outside of urbanized counties altogether, though adjacent to them.
Matching metropolitan employment data to AHS respondents
The AHS data do not include information about geographical area when the information might enable identification due to small numbers. Therefore part of the total sample has metropolitan area identification codes suppressed. All such respondents are necessarily dropped from the analysis. This likely biases the sample in favor of larger metropolitan areas.
The AHS definitions for New England metropolitan areas are based on aggregations of cities, towns, and some county portions, making them unsuitable for use with county-level BEA REIS employment estimates. In the absence of an alternative method, this required that respondents in 12 New England metropolitan areas were dropped from the analysis.6
Particular hypothesis-related criteria required that other metropolitan areas be dropped from the analysis. First, only individuals living in metropolitan areas with more than one county could be considered under the decentralization hypothesis. Second, only those living in metropolitan areas with clearly identified core/periphery areas corresponding to county lines could be used. Some metropolitan areas including more than one county had no counties defined as outlying.