in areas with seasonal fluctuations in population, the use of July 1 controls derived from the previous census can distort the ACS average estimates of the numbers of various types of people in the area over the entire year (refer back to Table 3-6 for an example; see Section 5-D for further discussion of the population controls).

For many—and perhaps most—areas of the country, the somewhat different representation of population and housing in the ACS 1-year period estimates compared with the decennial long-form sample will not be a significant problem. For some areas, however, the differences may be more pronounced. In fast-growing areas, the restriction of the sample to the January MAF housing stock, even when weighted to represent the housing stock as of July, may cause the ACS estimates to lag the situation on the ground. This could happen, not only for total housing, but also for some housing characteristics if new construction differs markedly from older housing stock. In areas with large seasonal fluctuations in population, as was just noted, the application of census-based July 1 population controls to data that were collected throughout the year may result in estimates of household member characteristics that represent neither a point in time nor an average number.

4-A.5.c
Smaller Initial Sample Size and CAPI Subsampling

Budget constraints limit the size of the sample initially selected for the ACS to 3 million housing units per year, cumulating to 15 million housing units over 5 years. Even if data were collected for the full initial sample, the total 5-year ACS initial sample size is smaller than the 18 million housing units that received the 2000 census long-form questionnaire (16.4 million housing units with usable data were included on the final edited data file). The 5-year ACS initial sample size is smaller yet than the expected sample of about 21.7 million housing units that would result if the average 1-in-6 long-form sampling rate were applied to the 130 million MAF housing unit addresses in 2005. Moreover, the initial ACS sample is reduced by 8 percent in census tracts outside oversampled jurisdictions that are expected to have high mail and telephone response rates.

In addition, unlike the long-form sample design, the ACS design sub-samples housing units that do not respond by mail or telephone for follow-up with CAPI. The CAPI subsampling uses three different rates in order to approximately equalize the precision of estimates for areas with higher and lower mail/CATI response rates. The effect of the CAPI subsampling and the 8 percent reduction in the initial sample in census tracts expected to have high mail and telephone response is to reduce the size of the final sample to about 2.1 million housing units per year nationwide, or about 10.5 million housing units cumulated over 5 years. This reduced sample size



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