While the block canvass in 2009, supplemented by a LUCA Program, should improve the MAF not only for the 2010 census, but also for the ACS, the improvements will be made at a point in time rather than continuously over the period leading up to 2010. The consequence is that the ACS sample for years prior to the block canvass will to some extent underrepresent growing areas of the country.

The ACS may also not accurately represent residences in small multi-unit structures (those with 2–9 apartments). Evidence from the 2000 census indicates that the problem of missed or erroneously identified addresses in these types of structures persisted in the 2000 MAF even after the block canvass and LUCA programs. At present there is no research in progress to investigate the problems of addresses in multiunit structures or duplicate addresses, even though research on the accuracy of the 2000 MAF shows that over 2 million duplicate housing unit addresses may not have been weeded out and, at the same time, over 2 million housing unit addresses may have been missed (National Research Council, 2004b:140-141).

Finally, research is needed on questionnaires that are returned by postal carriers because they are “undeliverable as addressed”—about 12 percent of mailed-out ACS questionnaires in 2005 (U.S. Census Bureau, 2006:7-7). These questionnaires are not missed in the ACS sample because they are included in the workload for CAPI follow-up, but they indicate problems in the MAF that need investigation.

Independent housing unit control totals for 1,951 estimation areas (counties and groups of small counties) are used to adjust the weights of ACS housing unit responses with the intent to reduce net coverage error (see Section 5-C for how these controls are developed using the previous census and local information on building permits). The application of the controls will increase (decrease) the housing unit weights in areas for which the unadjusted ACS estimates fall short of (exceed) the controls. While the effectiveness of these controls requires more research, their use may help identify and adjust for possible housing unit coverage errors in the ACS. However, their use will not adjust for coverage errors for specific kinds of housing—for example, the same weight adjustment is made to single-family homes, small multiunit apartments, and large building apartments in an estimation area. Moreover, because the census-based housing unit controls are subject to error, there will likely be inconsistencies between ACS estimates of housing units for 2010 and the 2010 census results.

The MAF in Rural Areas

In rural areas, the CAUS was developed because of the difficulty of using the DSF to identify addresses that should be added to the MAF. Many DSF addresses in rural areas are rural route or post office box number ad-

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