area level and therefore are subject to appreciable error. The panel views the use of the population controls in the ACS as a critical issue that requires major research.

Concomitantly, the panel views research on methods to improve the postcensal population estimates as a priority area for the Census Bureau. An important component of that research should be to investigate using ACS data more fully than currently in producing national estimates of international migration and particularly for estimating domestic migration. This research is even more important when considering the time series of ACS estimates. That time series will be affected at each census by the differences between the postcensal controls and the actual census counts. Section 3-G discusses this problem in the context of using the ACS estimates; see also Section 6-D.

Recommendation 5-3: As a high priority, the Census Bureau should undertake research to evaluate the effect of the postcensal population controls on ACS estimates and to examine alternative methods of making an adjustment that may be superior to the one currently used (including dispensing with the population controls entirely). The Census Bureau should make users aware in ACS documentation that biases in the ACS estimates caused by errors in the population controls are not reflected in the margins of error reported with the estimates and should conduct research to examine the effects of these errors on ACS estimates.

The Census Bureau should also give priority to research on ways to improve the postcensal population estimates at the county level, including estimates of internal migration and international immigration and the classification of race and ethnicity.

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