Specific reasons for errors are more likely to be determined if the Census Bureau saves as much contextual information as possible from the 2010 census. It will need assessments of which individuals and households were enumerated in error, along with various characteristics of persons and households, and of the census processes that gave rise to each enumeration. In Chapter 4 we present some initial ideas on what data might be useful to save, and we plan to provide more specific guidance on what data to save in 2010 for this purpose in our final report. One possibility is to design a comprehensive master trace sample database (see National Research Council, 2004a: Chapter 8).

As we have pointed out, the 1999 Supreme Court decision (Department of Commerce v. United States House of Representatives, 525 U.S. 316) precluded the use of adjustment based on a sample survey for congressional apportionment, and time constraints strongly argue against the feasibility of using adjusted counts (based on a PES) for redistricting (see National Research Council, 2004a: p. 267). Furthermore, the current approach to adjustment estimation has a number of remaining complications that continue to present a challenge to the production of high-quality estimated counts, including the treatment of movers, matching errors, the treatment of missing data, and the heterogeneity remaining after poststratification of match and correct enumeration status (resulting in correlation bias).

In addition, the use of adjustment is also complicated by the multitude of numbers needed, since one needs adjusted counts at relatively low levels of demographic and geographic aggregation. A decision whether to use adjusted counts for any purpose must therefore rest on an assessment of the relative accuracy of the adjusted counts compared with the census counts at the relevant level of geographic and/or demographic aggregation. The Census Bureau’s decision not to adjust the redistricting data, due for release by April 1, 2011, was based on the difficulty of making this assessment within the required time frame.