BOX 2-4

Four Quality Measures Available for the American Community Survey

The Census Bureau currently provides four indicators of nonsampling errors for the nation and states. They can be accessed from the main ACS web site under “Using the Data” (, middle of the page) or by selecting a subject area for which ACS data are desired, clicking on “survey methodology” and then on “quality measures” (

Sample Size

Sample size is critical for estimating the level of sampling error in survey estimates. The ACS web site provides two sample sizes for the year in question: (1) initial sample addresses, or the total number of addresses selected from the MAF to receive a questionnaire and (2) final interviews, or the total number of questionnaires received by mail, CATI, or CAPI. The second measure is smaller than the first—for example, 2.9 million addresses were initially selected for the 2005 ACS, but the number of final interviews was only 1.9 million. The principal reason is because CAPI is used to follow up only a subsample of addresses that lack a mail or CATI response; in addition, some sampled addresses turn out to be nonexistent or nonresidential, and some households do not respond even after follow-up.

Coverage Error

Coverage error occurs in the ACS as in other household surveys. Undercoverage occurs when the sampling frame does not include all addresses and when not all people in sampled addresses are included on the questionnaire; overcoverage occurs when households or individuals are duplicated or otherwise erroneously included. Net coverage is defined relative to decennial census–based population estimates.

The ACS web site provides net coverage rates for men and women for states and the United States and for six race/ethnicity categories for the United States. These rates are the weighted ACS estimate for the year in question for the relevant demo

High Unit Response Rate in the C2SS

The Census Bureau never expected that the mail response would be as high in the ACS as in the census, nor has it been: it was 56 percent in the C2SS compared with 71 percent in the 2000 long-form sample. To save on costs, the Census Bureau specified that only about one-third of ACS mail and telephone nonrespondents would be interviewed in person. Hence, to obtain a final household response rate that can be compared to the long-form-sample rate as a measure of public cooperation, the subsample of households sent for CAPI (in-person) interviewing in the ACS must be weighted to account for the subsampling before they are added to the mail and CATI (telephone) respondents.

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