records in various areas of the country, including Puerto Rico)  to estimate the number of people and housing units missed or counted  more than once in the census and to evaluate the final census counts. Although the  bureau’s A.C.E. The four phases of the person matching process were  (1) computer matching, (2) clerical matching, (3) nationwide field follow- up on records requiring more information, and (4) a second phase of  clerical matching after field follow-up. The  woman was coded as “unresolved.”

	Quality Assurance Results Suggest Person Matching Procedures Were Implemented as Planned

The bureau employed a series of quality assurance procedures for each  phase of person matching. The bureau reported that person matching  quality assurance was successful at minimizing errors because the quality  assurance procedures found error rates of less than 1 percent. The Bureau Took Action to Address Some Deviations, but Effect on Matching Results Is Unknown

The bureau carried out person matching as planned, with only a few  procedural deviations. estimates of census undercounts. sample and concluded that matching quality in 2000 was improved over  that in 1990, but that error introduced during matching operations  remained and contributed to an overstatement of A.C.E. The bureau has reported that additional review and analysis  on these remaining uncertainties would be necessary before any potential  uses of these data can be considered. computer systems. to measure the quality of the 2010 Census, it will  be important for the bureau to determine the impact of the deviations and  explore operational improvements, in addition to the research it might  carry out on other uncertainties in the A.C.E.
What GAO Found

The U.S. Census Bureau conducted the Accuracy and Coverage Evaluation (ACE) survey to estimate the number of people missed, counted more than once, or otherwise improperly counted in the 2000 Census. On the basis of uncertainty in the ACE results, the Bureau's acting director decided that the 2000 Census tabulations should not be adjusted in order to redraw the boundaries of congressional districts or to distribute billions of dollars in federal funding. Although ACE was generally implemented as planned, the Bureau found that it overstated census undercounts because of an error introduced during matching operations and other uncertainties. The Bureau concluded that additional review and analysis of these uncertainties would be needed before the data could be used. Matching more than 1.4 million census and ACE records involved the following four phases, each with its own matching procedures and multiple layers of review: computer matching, clerical matching, field follow-up, and clerical matching. The Bureau applied quality assurance procedures to each phase of person matching. Because the quality assurance procedures had failure rates of less than one percent, the Bureau reported that person matching quality assurance was successful at minimizing errors. Overall, the Bureau carried out person matching as planned, with few procedural deviations. GAO identified areas for improving future ACE efforts, including more complete documentation of computer matching decisions and better assurance that problems do not arise with the bureau's automated systems.