Residence Rules For many purposes, including comparisons with the 2010 census and with the annual population estimates, it is critical to conduct research to understand the implementation of the 2-month residence concept in the ACS and its effects on estimates for geographic areas and population groups. Experiments should be included in the ACS methods panels to determine how respondents interpret the 2-month residence rule in deciding whom to include and not include on the questionnaire and how their responses differ when they are asked to apply the census usual residence rule (see Recommendation 4-6). Such research could identify needed changes to question wording and instructions for reporting residence that would make reporting more consistent with the rules. The Census Bureau plans—and the panel supports—a program of annual methods panels, so that there should be little additional cost of the recommended research.
Estimates of Change A major focus for many data users in using the ACS is to examine estimates of change—from the preceding year, from the last census—for geographic areas and population groups of interest. The ACS provides successive 1-year and (once the necessary data are accumulated) 3-year and 5-year period estimates, but not direct estimates of change. As discussed in Chapters 3 and 6, using period estimates to track trends over time, particularly the 3-year and 5-year estimates, is not straightforward and the interpretation may often be unclear. Users will need specific, detailed guidance on how to work with the period estimates for time-trend analyses if they are not to be frustrated in their use of the ACS.
Comparisons with Other Data Sources It is important that the Census Bureau periodically compare selected ACS estimates with the corresponding estimates from other surveys and administrative records—for example, comparing ACS estimates of income and employment with those from the CPS and the IRS Statistics of Income, or comparing ACS estimates of housing characteristics with those from the American Housing Survey and administrative records. The Census Bureau established a precedent for this kind of work when it performed a large number of aggregate comparisons between estimates from the Census 2000 Supplementary Survey and the 2000 long-form sample; these comparisons helped establish the validity of the ACS (see Section 2-B).
It is often difficult to develop valid comparisons given that data sources differ in details of definitions, data collection operations, and other features. Moreover, analysts cannot assume that a particular comparison source is a gold standard of truth, as all data sets contain errors. Nonetheless, when well executed, aggregate comparisons can document differences in estimates and suggest reasons for differences. In turn, these findings can stimulate further research on which data source—the ACS or another—appears to be