It is likely that differences in response modes characterize other groups as well. For example, non-English-speaking households may be less likely to respond by mail or CATI and more likely to respond by CAPI compared with English-speaking households. There may also be important geographic area differences in response modes.

Overall, the use of mailout-mailback, CATI, and CAPI interviewing results in high housing unit response rates to the ACS. Thus, in the C2SS (see Table 4-1), the overall weighted response rate was 95.4 percent, including 56.2 percent mail response, 7.3 percent CATI response, and 31.9 percent CAPI response (applying weights to CAPI respondents to account for the subsampling). The 2005 ACS overall weighted response rate was even higher (97 percent), although, based on data from January to March 2005, the distribution of responses by data collection mode has changed. Thus, only about 51 percent of the eligible sample in January-March 2005 responded by mail, while 9 percent were interviewed by telephone and 38 percent were CAPI interviews, with 2 percent nonresponse (U.S. Census Bureau, 2006:Figure 7-2).

Differences in response patterns (the mix of the three modes) among population groups and geographic areas—and changes in response patterns over time—may result in different levels and directions of response biases among groups and areas. Whether such effects are important and for which characteristics remains to be established by research.

Recommendation for Mode Effects Research

Recommendation 4-5: The Census Bureau should conduct experimental research on the effects of the different data collection modes used in the ACS—mailout-mailback, CATI, and CAPI—on ACS estimates and, when possible, on response errors for questionnaire items. In addition, the Census Bureau should assess how different patterns of responding by mail, CATI, and CAPI among population groups and geographic areas affect comparisons of ACS estimates and inform data users of consequential differences.

Residence Rules
Two-Month Rule

The decennial census employs a usual place of residence concept; in the 2000 census, this meant that a person was to be counted at the place where he or she lived or stayed most of the time. Most other household surveys also use a similar concept. In contrast, because of its continuous design in which data collection occurs throughout the year, the ACS

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