percent.7 The 2005 ACS mail response rate declined to 51 percent from 56 percent in the C2SS, but the CATI and CAPI operations more than made up the difference.

2-B.2.b
Good Population Coverage

When weighted to account for sampling and unit nonresponse, the ACS estimates of the population, like those from other household surveys, typically fall short of census counts (or census-based population estimates). More people are missed in surveys than in the census, either because the sampling frame of addresses is less complete or because larger numbers of people are not reported by sampled households. People may also be duplicated or included erroneously in surveys as in the census, but surveys more often miss people, resulting in greater net undercoverage of the population.

The process of controlling the survey weights to the population estimates attempts to compensate for coverage errors, but the controls are available for only a few characteristics (see Section A.7.b above), so that achieving high coverage rates to begin with is important. Before applying controls, the C2SS covered 97 percent of the household population; subsequent supplementary surveys covered 94 percent of the population, and the 2005 ACS covered 95 percent of the household population.8 By comparison, the 2004 CPS ASEC covered only 88 percent of the population age 16 and over (Nelson, 2006:Table B).

2-B.2.c
More Complete Item Response in the C2SS

Imputation rates for questionnaire items—that is, the percentage of item responses for households (for housing questions) or household residents (for person questions) for which an answer had to be imputed from another household’s responses because the item was missing—are a commonly used measure of missing data. By this measure, the C2SS significantly outperformed the 2000 long-form sample. The C2SS had lower imputation rates for household members for 26 of 27 housing items and 48 of 54 population items that were included on both questionnaires (Schneider, 2004: Appendix). For example, 19.3 percent of household residents in the 2000 long-form sample were imputed a response for the question on number of weeks they worked last year, compared with only 9.6 percent of household residents in the C2SS who were imputed a response for the question on number of weeks they worked in the past 12 months.



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