entire household (type 1). These cases were genuine household enumerations for which the computer was used to supply characteristics of the missing members on the basis of knowing a great deal about the household. Further analysis is needed of type 1 imputations, which contributed to reducing the net undercount of children in 2000: Were most such imputations among households with more than six members? On mailed-back returns rather than enumerator-obtained returns? On returns that lacked a telephone number, which is a reason that the coverage edit and telephone follow-up operation could have missed them? Did the imputations produce a reasonable distribution of households by size?

Most important, can it be established that the reduced size of the questionnaire in 2000 encouraged mail returns from large households that might otherwise have failed to respond or have been poorly enumerated in follow-up? If so, then there is an argument for continuing to use a shorter questionnaire in 2010, even though more people may require type 1 imputations as a consequence. In that regard, a thorough evaluation is needed of the performance of the coverage edit and telephone follow-up operation, which was intended to complete the census records for partially enumerated households. If many of the households in the telephone follow-up workload did not provide telephone numbers or could not be reached despite repeated calls, then there is an argument for considering the benefits and costs of field follow-up of partially enumerated households as a supplement to telephone follow-up in 2010.

Looking at the numbers of people requiring imputation in a household for which occupancy status and size were known, but not characteristics (type 2), they are similar to 1990. There apparently will always be some households that are reported to be occupied and for which a household size can be obtained but not other characteristics. We believe it makes sense to include people for these households in the census by using information for other households in the immediate neighborhood rather than drop the addresses. Imputations for these kinds of households were proportionately greater among renter households than owner households in 2000, which helps explain the narrowing of the difference in net undercount rates between owners and renters from the 1990 levels.

Looking at categories (3)–(5), it is difficult to explain the much larger number of people in 2000 than in 1990 (1.17 million and 54,000) who required imputation when characteristics of the housing unit had to be imputed first.7 It


There were similarly large numbers of housing status imputations in 1970 (the number in 1980 was between the 1990 and 2000 numbers). However, the 1970 case was different in that most of the housing status imputations involved sample-based imputation of occupancy status to housing units that were originally classified as vacant in nonresponse follow-up. A small sample of such vacant units were rechecked in the field when it appeared that vacancy rates were too high; the sample results were used for imputation.

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