IIs in 1990 included only 0.3 million late enumerations. However, the reinstated cases were distributed in roughly the same proportions across major population groups (see Table 6.9), so that they do not explain the narrowing of coverage differences observed in the A.C.E. compared with the PES. In contrast, the 5.8 million whole-person imputations in 2000 accounted for proportionately more of historically less well-counted groups than of better counted groups, whereas the much smaller number of 1.9 million whole-person imputations in 1990 did not show such large differences among groups (see Table 6.9).

If all of the whole-person imputations were accurately assigned to poststrata and represented people who otherwise would have been omitted from the census, then the puzzle of similarly low P-sample match rates and yet lower net undercount estimates in 2000 compared with 1990 would have a ready explanation. The explanation is that these cases would have matched to the P-sample if they had been enumerated instead of imputed, so that the original A.C.E. would have exhibited higher match rates than the PES and lower net undercount estimates. Instead, because imputation was substituted for additional field work, the original A.C.E. had artificially low match rates, but once the IIs were added back to the census count for comparison with the DSE estimates, the original A.C.E. had lower net undercount estimates than the PES.

The question is the accuracy of the whole-person imputations in 2000 in terms of their numbers and imputed characteristics needed for poststratification. Of the 5.8 million whole-person imputations, a large number—2.3 million—were imputed in situations when the household size and characteristics of other members were known (type 1 imputations). Many of these were children in large households who could not be reported because of lack of room on the questionnaire. (The 2000 questionnaire had room for six persons, compared with seven in 1990.) Another large group—also about 2.3 million—were people imputed into households believed to be occupied for which household size, but not other information, was available (type 2 imputations).

These two types of whole-person imputations did not alter the numbers of people who were included in the census overall so long as household size was accurately reported. Moreover, because the imputation process used the characteristics of other household mem-

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