expanded outreach; and (4) aggressive recruitment of enumerators for followup of nonresponding households. The effectiveness of a fifth innovation—greater reliance on computers for data editing and imputation for inconsistent and missing responses—cannot be determined until information is available with which to evaluate the effects on data quality.

The sixth innovation was to develop the Master Address File (MAF) —a listing of all potential residences in the country—from new and multiple sources, including the U.S. Postal Service’s Delivery Sequence File and interaction with local and tribal governments. Although the concept behind this MAF-building process made sense, there were problems in execution that may have increased duplicate and other erroneous enumerations. An ad hoc operation had to be mounted in summer 2000 to reduce duplication in the census from the MAF. Further evaluation is required to assess the success of that operation and the quality of the overall MAF.

Mail Response Rates

The mail response rate in the 2000 census—the percentage of all addresses from which a census questionnaire was received—held steady at the 1990 level. This was an important achievement in light of the decline in the response rate over several past censuses and given the expense and time required to visit addresses lacking a completed questionnaire. The mail return rate, a more refined indicator that focuses on occupied households by excluding vacant and nonresidential addresses, was very close to the 1990 rate. There was a marked decline in the return rate for recipients of the long-form questionnaire, which could have adverse affects on the quality of the long-form data. Mail return patterns were similar between 1990 and 2000: most areas that were hard to count and had low mail return rates in 1990 had low mail return rates in 2000.


The A.C.E. is, like the census itself, a set of complex procedures and operations. The panel finds that the A.C.E. was well planned and documented and that it seems generally to have been well executed. However, until the Census Bureau completes additional studies of error in the A.C.E., the panel cannot offer a definitive assessment of it.


The net undercount of the population, as measured by the A.C.E., declined from about 4 million people (1.6 percent of the population) in 1990 to about 3.3 million people (1.2 percent of the population) in 2000. Moreover, net undercount rates were considerably reduced for such historically less-well-counted

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