such as telephone interviewing, proved more useful than had been expected. All processes, from sampling through estimation, were carried out according to well-documented specifications, with quality control procedures (e.g., reviews of the work of clerical matchers and field staff) implemented at appropriate junctures.
Because the quantity being estimated—the net undercount of the population—is very small relative to the total population (1–2%), it is essential that the P-sample survey meet high standards with regard to the completeness of reporting. A high rate of household noninterviews that required extensive adjustments to the sampling weights would be detrimental to the dual-systems estimation that is the key to the A.C.E. A high rate would not only increase variance, but also likely introduce bias due to the likelihood that nonresponding households differ from responding households in systematic ways that are important for estimation.
Overall, the A.C.E. obtained interviews from 98.9 percent of households that were occupied on interview day. This figure compares favorably with the 98.4 percent interview rate for the 1990 Post-Enumeration Survey (PES).2 However, the percentage of occupied households as of Census Day that were successfully interviewed in A.C.E. was somewhat lower—97 percent, meaning that a weighting adjustment had to account for the remaining 3 percent of noninterviewed households.
The lower interview rate for Census Day households is due largely to the fact that households that had been occupied entirely by outmovers at the time of the census were harder to interview than other households. This result is not surprising because the new occupants of such households may know nothing of the people who lived there before, and it may not always be possible to interview a knowledgeable neighbor or landlord. The interview rate for outmover households was 81.4 percent. Such households comprised 4 percent of Census Day occupied households in the P-sample.
Two weighting adjustments were calculated so that interviewed households would represent all households that should have been interviewed: one for the A.C.E. interview day and the other for Census Day. Each of the two