Evaluation of Imputation Procedures for Accuracy of the Population Count

The 2000 census, like previous censuses, included records for people who were wholly imputed because they lacked even basic information. In some cases, every household member lacked basic information, and the household required “whole-household imputation” or “substitution” (the Census Bureau’s term), which was performed by duplicating the record of a nearby household.

Box 4.2, under subheadings “B” and “C,” defines the five types of situations in which whole-person imputation (as distinct from individual item imputation, “A”) may be required in the census (see also Appendix G). Table 4.1 shows the number and percentage of the population of each of these types of imputations for the 1980, 1990, and 2000 censuses. In total, wholly imputed people represented a small percentage of the population in each year: 1.6 percent (3.5 million people), 0.9 percent (2 million people), and 2.2 percent (5.8 million people), respectively, in 1980, 1990, and 2000. The larger numbers of wholly imputed people of all types in 2000 compared with 1990 help explain part of the reduction in net undercount rates between children and adults, renters and owners, and minorities and non-Hispanic whites. If these people had not been imputed into the census, then there would have been a net undercount of about 1.7 percent overall (0.5 percent estimated net overcount minus 2.2 percent whole-person imputations), and differences in net undercount rates among population groups would have been wider (see Section 6-C.1).

The largest percentage of wholly imputed people in 2000 were those in mail return households that did not provide characteristics for all of their members (type 1). These people were genuine household enumerations for which the computer was used to impute their basic characteristics, item by item, on the basis of knowing a great deal about the household. The much larger number of type 1 imputations in 2000 compared with 1990 and 1980 was a direct outcome of the decision in 2000 to reduce the space for reporting household member characteristics on the questionnaire and to follow up households with more members than room to report them by telephone only, with no field follow-up as occurred in 1990. Type 1 imputations were not required for large households enumerated

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