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Discussion: Quality of Estimates from ACS for Small Population Groups

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This report discusses the quality and usefulness of estimates from the planned American Community Survey (ACS), for very small populations groups. The ACS is intended to replace the long form survey in the 2010 census. Since the long form is unique as a source of information about small population groups, the top objective of the ACS design has been to provide good information about the smallest groups.

The general premise of the ACS design is that by spreading the “long form” sample across the decade, it is possible to provide updated information for all sizes of population groups. In principle, this should be especially advantageous for small population groups, because there is currently very little information about how these populations change over time. Also, the ACS is expected to have more consistent quality because of the advantages of a continuous operation, which is especially important for small, “hard-to-enumerate groups.

However, concerns have been expressed about the quality of ACS estimates for very small population groups. These concerns are described in Section III, with responses in subsequent sections. We have described the ACS as replacing the long form “snapshot” with a “video”. Using this metaphor, the most widespread concerns are 1) that a “freeze frame” from the video is not as clear as a snapshot, and 2) that if the subject of the picture is small and fast-moving, the video may show a blur.

The response is, continuing the metaphor, that the freeze frame is almost as clear as the snapshot, and provides the advantage of being able to look at a freeze frame at any time. For fast-moving subjects, a video at least tells you that the subject is moving and in what direction, while a snapshot misses the action totally. Small population groups have the potential to change more dramatically than larger groups, so having a “video” is most valuable for small groups.

Sections I through V, along with Appendix 1, present the basic issues. Section VI and Appendix 2 provide some additional details. Appendix 3 points out some more complex statistical issues, which are not mentioned in the body of the report to keep the discussion simpler.

Comments on topics where the report may not have effectively explained the issues are welcome. There may be subsequent revisions of the report based on discussions with the Census Bureau Advisory Committees, or as new information is available from evaluations of the ACS and comparisons to Census 2000.  

Page Last Revised - October 8, 2021
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