dress the problem of duplication. This procedure brings the MAF number of housing units in line with the postcensal estimates. However, in other regards it depends on an assumption that the listings of some housing units are randomly duplicated.

If the MAF was complete and not subject to duplicate listings and if the intercensal estimates were accurate, then the housing unit control factors would all be close to 1, simply adjusting the housing unit counts from the January MAF to the midyear intercensal estimates to allow for new and demolished units in the interim. (In this case the ACS housing units could be weighted to a midyear MAF count.) That many of the housing unit control factors are appreciably larger or smaller than 1 raises concerns about the quality of the MAF or the quality of the postcensal estimates or both.

Recent research by the Census Bureau (Reese, 2007) examined differences between the independent housing unit estimates for 2002–2005 and the housing unit addresses on the MAF used for the ACS in these years. The results show an increasing divergence between the two series, with the national MAF count exceeding the housing unit estimate by 2.6 percent in 2002 and rising to 4.0 percent in 2005. These results suggest a failure to completely identify and weed out duplicate, demolished, and nonresidential addresses from the MAF. The differences between the increase in the MAF and the increase in the housing unit estimates varied among counties as a function of county size, with larger differences occurring for counties with larger numbers of housing units.

The Census Bureau plans to conduct more research to gain an understanding of large discrepancies between the MAF counts and the postcensal housing unit estimates. It is, for example, possible, that the quality of the MAF differs between urban and rural areas associated with a differential updating of the frame, using the Delivery Sequence File in urban areas and the Community Address Updating System in rural areas (see Sections 4-A.4a and 4-A.4.b). As discussed in Section 4-A, a prime concern for the ACS is the continuous maintenance of a high-quality MAF for use each year throughout the decade and beyond. Weighting adjustments that attempt to compensate for deficiencies are necessarily an imperfect remedy. If this research identifies deficiencies in the MAF sampling frame, then steps should be taken to correct the frame.

At present the MAF and the postcensal estimates are developed independently. However, in the panel’s view, they should be integrated to the benefit of both. For example, building permit data could be used to improve the MAF, either collected on an individual permit basis within location or simply using the current aggregates that would indicate areas in which special MAF updating is needed. Similarly, the MAF—and the ACS—could provide valuable information in updating the postcensal estimates.

Another issue that needs examination is the level at which the housing

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