6–A.11 Original A.C.E.: Summary of Findings

From the extensive evaluations of the original A.C.E. data conducted by Census Bureau staff, we draw several conclusions. First, coverage evaluation using the dual-systems estimation method is a highly complex effort in the census context. Many things have to go very well, and errors need to be very small, particularly as the quantity being estimated—net undercount—is so small relative to the population. Second, as a major data collection, data processing, and estimation operation, the A.C.E. in fact went extremely well. It was conducted in a timely, controlled manner. Evaluation studies of matching error, the targeted extended search, poststratification inconsistency, and treatment of movers in the A.C.E. found generally improved performance over the PES and only small biasing effects on the DSE estimates—at least for national totals and aggregates of poststrata. Percentages of household noninterviews and cases with unresolved residence, match, or enumeration status were small (although evaluation found significant effects of plausible alternative reweighting and imputation methods on the DSE estimates).

Dwarfing all of these generally positive outcomes for the original A.C.E., however, were the findings from the Evaluation Follow-up and Person Duplication Studies of substantial underestimation of duplicate and other census erroneous enumerations and overestimation of P-sample residents, particularly nonmatched cases (see Sections 6-B.1 and 6-B.2). There was no counterpart of the Person Duplication Studies for the 1990 PES.

Finding 6.1: The 2000 Accuracy and Coverage Evaluation (A.C.E.) Program operations were conducted according to clearly specified and carefully controlled procedures and directed by a very able and experienced staff. In many respects, the A.C.E. was an improvement over the 1990 Post-Enumeration Survey, achieving such successes as high response rates to the P-sample survey, low missing data rates, improved quality of matching, low percentage of movers due to more timely interviewing, and substantial reductions in the sampling variance of coverage correction factors for the total population and important population groups. However, inaccurate



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