The ACS has three major benefits compared with the long-form sample:
The first benefit is timeliness: ACS data products are released 8–10 months, instead of 2 years, after data collection.
The second, and an even more important, benefit is frequency: ACS data products are updated every year instead of every 10 years, which will make it possible in many areas to track trends in such important population characteristics as educational attainment, employment, poverty, diversity, and others.
A third benefit is higher quality of the data in terms of completeness of response to the survey items: the much more complete response to the ACS compared with the 2000 long-form sample is achieved by the use of computer-assisted telephone and personal interviewing of households that do not respond by mail. The ACS interviewers are experienced and highly trained in contrast to the lightly trained temporary enumerators that were used for nonresponse follow-up in the 2000 census. In addition, ACS telephone interviewers contact mail respondents to obtain answers to missing items, a step not done in 2000.
A weakness of the ACS compared with the long-form sample is the significantly larger margins of error in ACS estimates, even when cumulated over 5 years. The primary reason is the much smaller sample size of the ACS. Another important reason is the greater variation in the ACS sample weights resulting from the subsampling for field interviewing of households not responding by mail or telephone. Also, the postcensal population and housing estimates used as survey controls are less effective than the full census controls used with the long-form sample: they are subject to unmeasured estimation error, they are applied at a less detailed level than the census controls, and they are not directly related to the ACS in the way that the census controls are related to the long-form sample.
The larger ACS sampling errors are a particular problem for small cities, counties, and other governmental jurisdictions; they also apply to small neighborhoods in large cities, but neighborhoods can often be combined satisfactorily into larger user-defined areas for analysis. For small areas for which 1-year period estimates are not available or sufficiently precise, users must learn to work with 3-year and 5-year period estimates, which are very different from point-in-time estimates.
The census long-form sample was heavily used by federal, state, and local government agencies, researchers, the private sector, the media, and the public. The ACS continuous design will initially challenge many such users in adapting their applications based on the long-form sample to the