Overall, the 2000 A.C.E. showed similar, but less pronounced, patterns of net undercount than the 1990 PES. Given that P-sample match rates and E-sample erroneous enumeration rates were similar between the A.C.E. and the 1990 PES, the key question at this time is why the A.C.E. showed a reduced net undercount, overall, and for such groups as Hispanics, non-Hispanic blacks, children, and renters. Because the only other component of the DSE equation is the number of census people with insufficient information to include in the E-sample (IIs), our attempts to resolve the undercount puzzle centered on that component of the census results. In Chapter 8, we analyze distributions of people requiring imputation and people reinstated in the census (late additions) and determine that people requiring imputation largely explain the reduced net undercount in 2000 for historically less well-counted groups.