of the small sizes of the adjustments, no further iterations are performed. With the high weighted response rates achieved in the ACS, the noninterview adjustments should not lead to a substantial loss of precision.

Mode Bias Noninterview Factor

A significant drawback to the step 3 noninterview adjustments is that they increase the weights of all mail, CATI, and CAPI responding housing units to represent the nonresponding housing units, whereas the nonresponding housing units are virtually all a subset of the subsampled CAPI housing units. The Census Bureau has chosen to spread the adjustments over all responding occupied and temporarily occupied housing units because of the much smaller responding sample size that would have been available had the adjustment been confined to CAPI housing units. That smaller sample size would have severely limited the extent of control that could have been achieved on the census tract, building type, and month of data collection variables used in the step 3 adjustments. Also, restricting the adjustments to CAPI housing units would have concentrated the adjustments on responding housing units that already had larger weights because of the subsampling. Since CAPI responding housing units likely differ in some characteristics from other responding housing units, however, it seems likely that the step 3 noninterview adjustments create some bias in some estimates.

The Census Bureau introduces another adjustment (step 4), the mode bias noninterview factor (MBF), to address the bias concern with the step 3 weighting. The MBF adjustments are generally small, with only 5 percent being 0.96 or less and 5 percent being 1.03 or more, but the combined effects of their use, together with the noninterview adjustment factors in step 3, on the biases and sampling errors of ACS estimates are not transparent.

The MBF procedure, in essence, has three steps. The first step is to develop an adjustment factor for the step 2 weights—mode noninterview factor (NIFM)—similar to the adjustment factors NIF1 and NIF2 under step 3 but applied only to CAPI occupied and temporarily occupied housing units. The second step is to produce some survey estimates based on the step 2 weights with these adjustments and also the corresponding estimates with the step 3 adjustments and to calculate the MBF as a ratio of the two quantities. The third step is to multiply the step 3 weights by the MBF so that the estimates with the resultant weights conform to those produced with the NIFM-adjusted weights.

In view of the smaller sample size when adjusting only the weights of CAPI respondents, the NIFM adjustments take account of only building type and tabulation month, not census tract, within an estimation area.

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