graphic group and geographic area before being controlled to population estimates, divided by the corresponding census-based population estimate.

The population and housing unit estimates used to adjust the ACS estimates for coverage errors (see Sections 5-C and 5-D) pertain to only a few characteristics (age, sex, race, ethnicity, and total housing), are only applied for large counties and groups of small counties (estimation areas), and themselves contain errors. To the extent that the controls are flawed and that noncovered or duplicated people differ from correctly covered people, then estimates from the ACS may be biased.

Unit Nonresponse

Unit nonresponse relates to the number of final interviews from a survey. To the extent that responding and nonresponding units differ, estimates from a survey may be biased. The ACS web site provides unit response rates and unit nonresponse rates by type (refusal, unable to locate, no one home, temporarily absent, language problem, other, and insufficient data from an interview to be included in the data set). The numerator for unit response rates is the number of mail, CATI, and CAPI responses for the year in question, weighted to account for different initial sampling and CAPI subsampling rates. The denominator is a similarly weighted estimate of the number of cases eligible to be interviewed. The intent in estimating the denominator is to exclude that fraction of the sample of addresses that turn out to be nonexistent, nonresidential, or otherwise ineligible for inclusion in the ACS.

Item Nonresponse

Item nonresponse occurs when interviews are complete enough to include in the estimation but answers are missing (or invalid) for one or more questions. Item nonresponse rates indicate the potential for measurement error due to differences between the values imputed for missing responses and the actual values. The ACS web site provides nonresponse rates for individual tabulated items. The numerator for each rate is the number of allocated responses (imputations that use reported information from other persons or housing units); the denominator is the total number of responses, including valid responses, allocations, and assignments (assignments use other information for the same person to fill in or correct a response).

Using a weighted response rate, the C2SS compares favorably with the 2000 long-form-sample rate. The 2000 final edited long-form-sample data file included 93 percent of sampled households; the remaining 7 percent were dropped because the information collected for them was too scant (National Research Council, 2004b:Table 7.7). By comparison, the C2SS weighted household response rate was 95 percent. The weighted household response rates in the 2001–2003 ACS test surveys were higher than the C2SS rate, averaging 97 percent. The 2004 rate was lower (93 percent) because of a funding reduction that necessitated dropping telephone and personal follow-up operations for January 2004. The 2005 weighted household response rate (the first year under full implementation) was 97

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