several years of 1-year period ACS estimates will be informative regarding trends and the current situation for the city, even though the estimates are less precise (see discussion in Chapter 3).

Documentation of Sampling Error

The Census Bureau commendably is trying to impress upon users the extent of sampling error in the ACS estimates. Originally, for data products issued through mid-2005 from the C2SS and the ACS test surveys for 2001–2004, the Census Bureau published upper and lower 90 percent confidence interval bounds (for example, 13–17 percent for a 15 percent estimate of poor school-age children). In response to users, who are more accustomed to the MOE concept (as reported in the media for public opinion polls), the Census Bureau decided to replace the upper and lower bounds in tables with the 90 percent MOEs for specific estimates (such as ±0.2 percentage points). In addition, the Census Bureau will not publish 1-year or 3-year estimates when their imprecision is deemed to be too great. In these instances, the standard tabulation categories will be combined to the point at which the tabulations meet the Census Bureau’s threshold for a minimally acceptable level of precision. The 5-year period estimates will not be treated in this manner, even for very small areas for which they are highly imprecise, because the 5-year small-area estimates are the building blocks for a wide range of user applications similar to how the long-form-sample data were used (see Section 4-D.2).

In contrast, the sampling error of the long-form-sample estimates was not highlighted, but instead was contained in footnotes and auxiliary documentation. Moreover, margins of error were not provided for specific estimates; instead, users were provided with general formulas for making their own computations of sampling error. As a result, many users have been unaware of the sampling error in the long form-sample estimates they have been using.


The ACS promises to be of great benefit to many users for a wide range of applications for which they previously relied on information from the decennial census long-form sample. The three major benefits of the ACS are its timeliness, frequency, and the improved quality of the responses when compared with the long-form sample. Not only will the ACS information be released within 8–10 months of completion of data collection, compared with 2 years or more for the long-form sample, but it will also be updated every year instead of once a decade. Moreover, there is strong evidence that

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