Using Summary Files for Research

At present, no equivalent of Summary File 3 or 4 from the 2000 long-form sample exists for the ACS. This lack is a drawback for the research community because summary files permit ready analysis of detailed information across multiple geographic areas, population groups, and subject areas. In contrast, the tables that are available online for the ACS can only be displayed one at a time for a specified type of geographic area within a larger unit—for example, a table of age by sex for one or all towns in a particular county. The detailed tables and single-year and multiyear profiles are also available as spreadsheets through the ACS FTP site, and in that format the data can be manipulated (for example, calculating percentages or adding or subtracting categories), but the spreadsheet contents are limited to a specified geographic area, such as a county or township. An ACS Download Center provides access to up to 50 tables for a geographic summary level, such as all states or all counties. None of these data products are as useful for research purposes as a summary file in the same format as the decennial census summary files.

The Census Bureau recently began work to specify and implement an ACS equivalent of Summary File 3 from the 2000 long-form sample. This is a welcome development, not only for the research community, but also for many other users who require the ability to easily manipulate large amounts of data for multiple areas and population groups. The initial prototype 2005 ACS summary file has just been released and contains all of the detailed tables for every geographic area with 65,000 or more people; eventually, the ACS summary files will be released annually for each year’s 1-year, 3-year, and 5-year period estimates. Users have been invited by the Census Bureau to comment on the prototype summary file.15

Researchers who work with the new product will need to be cognizant of the larger sampling errors of the ACS tables compared with the 2000 long-form-sample tables and develop strategies for effective use of the ACS. Such strategies include combining data for census tracts and block groups into larger areas, collapsing data categories, and combining ACS summary files for nonoverlapping periods. The advantage of the ACS will be that researchers will not need to wait for 10 years to track trends in migration flows and other social, demographic, and economic phenomena.

Using PUMS Files for Research

Many researchers will turn to the ACS 1-year PUMS files for their analyses. The availability of PUMS files year after year will afford much


See http://www.census.gov/acs/www/Special/Alerts/Alert44.htm#News2, ACS Alert 44, December 28, 2006.

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