sample size is only about three-fourths that of the long-form sample and that the ACS then uses subsampling for the CAPI interviews. For planning purposes, the Census Bureau estimated that the sampling errors (known as the standard errors) of the ACS 5-year period estimates would be about 20 percent greater than the errors of the long-form-sample estimates, but recent work (Starsinic, 2005) suggests that the ACS sampling errors will exceed the long-form-sample errors by about 50 percent.

The ACS design, like the long-form-sample design, oversamples very small governmental jurisdictions (refer back to Table 2-3a). This oversampling reduces the sampling errors of estimates for those units, but it increases the errors for larger areas that are undersampled, as well as somewhat increasing the errors for larger units that include some oversampled and some undersampled areas relative to a design with the same sampling rate for all areas.

The subsampling used for CAPI interviews in the ACS increases sampling error for two reasons: first, the subsampling reduces the final sample

BOX 2-5

Brief Descriptions of Statistical Terms Used in This Report

  • Standard error of an estimate: A commonly used statistic that expresses the imprecision in an estimate that is due to sampling. This imprecision is known as sampling error. It is to be distinguished from nonsampling errors from such sources as misreporting and nonresponse, which are often systematic in nature and result in biased survey estimates (see Box 2-3).

  • Coefficient of variation (CV) or relative standard error: The standard error expressed as a percentage of the estimate. CVs of 10–12 percent or less are often accepted as a reasonable standard of precision for an estimate.

  • 90 percent margin of error (MOE): Plus or minus 1.65 times the standard error of an estimate.

  • 90 percent confidence interval (CI): The 90 percent MOE expressed as a range around the estimate.

Example Calculations

Consider the example of MEDIUM CITY, 5-Year Period ACS Estimate (see Tables 2-7a, 2-7b, and 2-7c). Assume that MEDIUM CITY has a population of 100,000 with an estimated 20,000 school-age children, of whom 3,000 (15 percent) are estimated to be poor. For a 15.0 percent poverty rate for school-age children with a 1.28 percentage point standard error:

  • CV = 8.5 percent (1.28/15.0)

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