the ACS will provide data with reduced nonsampling error because of such factors as the use of trained interviewers to collect the information from nonrespondents. In tests of the ACS, improvements in quality are evident in more complete response to almost every item compared with the long-form sample. Furthermore, in personal interviews, some items will be more accurately reported because the computer-assisted interviewing can more readily correct respondent misperceptions about what is being asked and resolve inconsistent responses.

A complication for users of switching from the census long-form sample to the ACS is the continuous fielding and processing of the ACS. This design produces estimates that pertain to periods of time—averages over 12, 36, or 60 months—instead of the traditional point-in-time estimates with which users are familiar from the long-form sample and other household surveys. Users will need to work together and with the Census Bureau to develop strategies for application of the ACS information that take account of the survey’s continuous design. In Chapter 3 we outline some of these strategies.

Sampling error or imprecision of the estimates is a problematic aspect of the ACS, although users should remember that many long-form-sample estimates did not meet common standards of precision for small areas, either (see Tables 2-7a, 2-7b, 2-7c, and 2-8). When the data are averaged over 5 years, it appears that the ACS will provide reasonably precise estimates for small population groups, such as poor school-age children, for areas with 50,000 or more people but not for smaller areas. The ACS 1-year estimates for such a small population group will have low precision unless the area has at least 250,000 people. For larger population groups, such as total poor, the ACS 5-year estimates will likely provide reasonably precise estimates for areas of at least 10,000 people, while the ACS 1-year estimates will meet that standard for areas of at least 50,000 people.

ACS estimates for census tracts, which average 4,000 people, and block groups, which average 1,500 people, will be very imprecise. Indeed, they were not precise from the long-form sample for other than large population groups. However, these areas can be combined in various ways by users who want to compare planning districts, wards, or other components of large cities, counties, and other areas.

The bottom line for large geographic areas—such as states, congressional districts, and large metropolitan areas, cities, and counties—is that the ACS estimates will be a great asset to data users. The data will be timely, up to date, of good quality, and reasonably precise. The 5-year data for census tracts and block groups, while not precise in and of themselves, will provide building blocks that should enable detailed analyses of the populations of large geographic areas.

Estimates from the ACS for small governmental units, even with over-



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