Although the Census Bureau makes every effort to count all persons and housing units, it is inevitable that some will be missed. In 1990, post-enumeration surveys showed that 3.1% of New Mexico's population was not counted, giving us the highest undercount rate among the states. After the actual Census 2000 enumeration, the Census Bureau will again conduct a large quality check survey (300,000 households nationally) called Accuracy and Coverage Evaluation (ACE). The results of the ACE survey will then be used to adjust the enumeration for undercount (or overcount in some areas). The Census Bureau will release adjusted numbers, along with some unadjusted data, through the standard data products. Since the U.S. Supreme Court ruled that as-enumerated counts rather than adjusted populations must be used for apportionment, unadjusted data are still required. Additionally, some States will be using unadjusted counts for legislative redistricting.
For the Census Bureau's many reports addressing undercount issues from Census 2000 go to their web site at http://www.census.gov/dmd/www/ace2.html.