level. Indeed, as the Census Bureau is considering, the total controls might sometimes be applied at a lower level, such as individual cities with populations of over 65,000 within estimation areas.

A strategy to reduce the amount of cell collapsing needed is to develop the weights through a raking algorithm that makes the ACS sample conform to each of the marginal distributions of the control variables, not to the joint distribution. The paper written for the panel by Jay Breidt and reproduced in Appendix B examines such alternatives.

On the issue of whether to use the race/ethnicity classification, the poor quality of the estimates in the 2000 comparison raises concerns about their comparability to what would have been obtained had the ACS interviewed the entire population. The population estimates start with the last census values and update them using administrative data. The reporting or recording of race/ethnicity in the census and in administrative data differ from each other and also from the ACS. As a result, the population estimates by race/ethnicity may not serve well as controls for the ACS sample.

The use of population controls for the population census and household surveys has a long history. It is instructive to contrast these uses with the use of population controls in the ACS. Although they appear similar, they are in fact very different. With the long-form sample, the data for the full census controls are collected for the same time and by essentially the same methods. Thus, the controls represent a poststratification adjustment, which improves the precision of the long-form estimates in a standard way. The long-form-sample controls achieve this improvement for small areas; they are applied for weighting areas, which are often as small as a block group or a census tract and never larger than a county (see National Research Council, 2004b:App. H). For household surveys, the controls are the population estimates and so subject to more error than the census counts, but for most household surveys other than the ACS the controls represent the same residence concept as the surveys; and the controls are applied at a high level of aggregation, which reduces the level of error in the population estimates. Generally, the controls for household surveys are applied for the nation as a whole by sex, age, and race/ethnicity groups and sometimes for total population by state (as, for example, in the Current Population Survey; Bureau of Labor Statistics and U.S. Census Bureau, 2002:Ch. 10).

In contrast, the population controls used in the ACS are midyear population estimates based on different residence rules and different sources than the yearly accumulation of ACS monthly samples. (For an illustrative example of the effect of population controls on areas with seasonal populations, see Section 3-C.3.) The ACS population controls should therefore not be treated as if they are poststratification controls, as is the current practice. It cannot be assumed that they necessarily improve the quality of the ACS estimates, particularly since they are applied at the estimation



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