TABLE 5-2 Distribution of the Housing Unit Control Factor 1 Across Counties in the 2004 ACS

Housing Unit Control Factor 1

Number of Counties

Percentage of Counties

Under 0.90















1.10 and over






SOURCE: Based on data provided by the Census Bureau.

tors fall between 0.95 and 1.05 and represent minor adjustments. There are, however, a number of counties for which the factors are substantial, with extremes as low as 0.7 and as high as 1.42. More than half of the factors are 1.0 or greater, consistent with a net undercoverage in the MAF in those counties, but the factors are less than 1.0 for 44 percent of counties, suggesting a net overcoverage in the MAF for those counties, under the assumption that the postcensal estimates are accurate.

Undercoverage in the MAF arises because new housing units added after the January MAF update are not covered and also because of other housing units that are missed. Overcoverage arises if some housing units are listed more than once in the MAF, or if group quarters addresses are misclassified as housing units.1 Both missed housing units and duplicate or misclassified listings can occur in a county. Under the assumption that the postcensal estimates are accurate, the housing unit control factors in Table 5-2 represent the net effects of overcoverage and undercoverage.

As with all weighting adjustments, the housing unit factors are based on certain assumptions that merit review by the Census Bureau. In the case of undercoverage, the factors increase the weights of MAF housing units to represent those not included in the MAF frame under an assumption that the missed housing units are missing at random. This assumption would be false if, for instance, the proportion of vacant units in the postcensal estimates is higher than in the MAF, as it might well be. In the case of overcoverage, the factors decrease the weights of MAF housing units to ad-


Another potential source of coverage problems with the MAF is that some housing units are listed in the wrong county, leading to undercoverage in the county in which they should be listed and overcoverage in the county in which they are listed. However, county misclassification seems likely to be rare.

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