For most items, the 2000 census imputation rates for enumerator returns exceeded those for mailed-back returns, sometimes by large margins. In contrast, for many items, the C2SS imputation rates were lower for interviewer returns than for mailed-back returns, indicating the higher quality of the follow-up effort in the C2SS (National Research Council, 2004b:Table 7.5).

Moreover, there is evidence suggesting that the quality of the ACS data collection improved after 2000. The imputation rates for 20 of 36 housing items and 51 of 57 population items were lower in the 2001–2004 ACS test surveys compared with the C2SS—sometimes substantially lower—and no rate was higher.9 For example, only 18.8 percent of household members age 15 and over had some or all of their income imputed in the 2004 supplementary survey, compared with 23.9 percent in the C2SS and 29.7 percent in the 2000 long-form sample. Item imputation rates remained at low levels in the 2005 ACS.

Greater Quality Control in the C2SS and the ACS

For the C2SS the Census Bureau implemented quality assurance procedures that were not included in the 2000 long-form-sample procedures because of cost and timing constraints. These same procedures are being used in the ACS.

An important operation that contributes to quality for the ACS is the telephone follow-up of mailed-back questionnaires that do not meet standards for completeness of coverage of household members or content. Moreover, telephone and in-person follow-up of nonrespondents is conducted by experienced, highly trained interviewers who are assisted by computerized questionnaires with built-in edit checks and skip patterns. The 2000 census lacked telephone follow-up for mailed-back questionnaires that were missing several items, and the enumerators who conducted the in-person follow-up of households that did not mail back their forms used paper-and-pencil questionnaires. Moreover, the temporary, lightly trained enumerators focused on obtaining the answers to the basic questions and not on the additional long-form-sample questions.

Another quality assurance procedure in the ACS is not to allow proxies for household respondents, such as neighbors or landlords. In 2000, 6.2 percent of long forms were obtained by proxy, and three-fifths of these had to be dropped from the final tabulation file because the data were so incomplete (National Research Council, 2004b:291).

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