sampling, are the most problematic from the perspective of sampling error. Consider a place of 1,500 people and 300 school-age children, of whom 45 children or 15 percent are estimated to be poor. Table 2-7c shows a 90 percent confidence interval of 7 to 23 percent poor school-age children from 5 years of ACS data. Based on the calculations used to derive Table 2-7c, the margin of error of the ACS estimate is 51 percent greater than that from the 2000 long-form sample, which already has a high margin of error, and this increase may be somewhat underestimated. Moreover, the option of combining small governmental units into larger analytical units in order to improve the precision of estimates is less applicable than in the case of combining census tracts or block groups within a larger jurisdiction.

Chapter 3 discusses possible strategies for data users who are interested in very small governmental units to make effective use of the ACS estimates. It will also be imperative to maintain the planned sample sizes for the ACS over time and, furthermore, for the Census Bureau, in cooperation with users, to seek ways to improve the precision of the estimates for small areas (see Section 4-A.5).



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