Appendix A.4.a). Controversy over the adjustment issue was even more intense for 2000 than had been the case for 1990.

Although the 2000 A.C.E. was broadly similar to the 1990 PES, the two programs differed in a number of respects (see Hogan, 2000b), and Census Bureau staff expected that changes implemented for the A.C.E. would produce superior results. This section briefly summarizes the major differences (see Appendix E for a description of A.C.E. operations).

The A.C.E. universe excluded people living in institutions, college dormitories, and other group quarters; the PES universe included most noninstitutional group quarters. The Census Bureau decided to limit the A.C.E. to the household population because of its experience in the 1990 PES in which rates of unresolved match status were much higher for group quarters residents than for household members in the PES because of much higher rates of short-term mobility for people in group quarters (e.g., college students taking and returning from spring break, shelter residents moving from one shelter to another, migrant worker dormitory residents moving from one farm to another). However, this decision necessitated an unrealistic assumption of perfect coverage of the group quarters population.

The 2000 A.C.E. was twice the sample size of the 1990 PES—300,000 households in about 11,000 block clusters compared with 165,000 households in about 5,000 block clusters. Because of its larger overall sample size, the A.C.E. could produce reliable direct estimates for minorities and other groups with less oversampling than was used in the PES. The A.C.E. design also improved on several other features of the PES sample design. Consequently, the A.C.E. weights varied less than the PES weights, which contributed to reducing the variance of the A.C.E. estimates (see Appendix E.1).