operation, with a follow-up operation to recheck units classified as vacant. However, some of these addresses may have been temporary recreational lodging (e.g., fishing camps) for which it was difficult to determine their housing status. In addition, the Census Bureau identified some data processing problems that produced larger-than-expected numbers of addresses for which housing status was not known (type 5 imputations) and that probably contributed to overimputation of households for which occupancy status was not known (type 4 imputations; see Section 4-D.2).
The 2000 census included several operations that were explicitly designed to reduce duplicate enumerations. They included the Primary Selection Algorithm (PSA) and the various operations to unduplicate MAF addresses and associated households. The MAF unduplication operations included a planned operation prior to nonresponse follow-up and the special unplanned operation in summer 2000, which temporarily deleted census records that appeared to duplicate records at another address and then reinstated some of them (see Section 4-E).
The purpose of the PSA was to determine which households and people to include in the census when more than one questionnaire was returned with the same MAF identification number (see Appendix C.5.c). Such duplication could occur, for example, when a respondent mailed back a census form after the cutoff date for determining the nonresponse follow-up workload and the enumerator then obtained a second form from the household. In all, 9 percent of census housing units had two returns and 0.4 percent had three or more returns that were eligible for the PSA operation. In most instances, the PSA discarded duplicate returns; less often, the PSA found additional people to assign to a basic return or identified more than one household at an address (Baumgardner et al., 2001:22–27).
Despite these operations, however, the original A.C.E. identified 1.9 million census duplicates (Anderson and Fienberg, 2001:Table 2). The original A.C.E. also identified 2.7 million “other residence” erroneous enumerations (e.g., the person should have been enumerated in group quarters or at another home), many of which were probably duplicates. On the basis of the Evaluation Follow-Up Study and the