In the remainder of this chapter we summarize research from 1990 documenting the superiority of population coverage on mail returns and present largely confirmatory results from 2000. We then analyze changes in mail return rate rates for census tracts between 1990 and 2000 by a variety of characteristics. We knew that the overall reduction in net undercount in 2000 could not likely be due to mail returns, given that the total mail return rate was somewhat lower in 2000 than in 1990.2 However, we thought it possible that changes in mail return rates for particular types of areas, such as those with many renters or minority residents, might help explain the reductions in net undercount rates that the Accuracy and Coverage Evaluation (A.C.E.) Program measured for usually hard-to-count groups.


Research from the 1990 census, based on a match of P-sample and E-sample records in the 1990 Post-Enumeration Survey (PES), found that mail returns were substantially more likely than returns obtained by enumerators to cover all people in the household. Only 1.8 percent of mail returns had within-household misses, defined as cases in which a mail return in the E-sample matched a P-sample housing unit but the P-sample case included one or more people who were not present in the E-sample unit. In contrast, 11.6 percent of returns obtained by enumerators had within-household misses (Siegel, 1993; see also Keeley, 1993). These rates did not vary by type of form: within-household misses were 1.9 percent and 1.8 percent for short-form and longform mail returns and 11.7 percent and 11.3 percent for short-form and longform enumerator-filled returns.

In an analysis of the 1990 PES for 1,392 post-strata, Ericksen et al. (1991: Table 1) found that both the gross omission rate and the gross erroneous enumeration rate were inversely related to the “mailback rate” (equivalent to the mail response rate) for PES cases grouped by mailback rate category.3 The relationship was stronger for omissions than for erroneous enumerations—the omission rate was 3 percent in the highest mailback rate category and 19 percent in the lowest mailback rate category, compared with 4 percent and 10 percent, respectively, for the erroneous enumeration rate. Consequently, the net undercount rate also varied inversely with the mailback rate.


The somewhat lower mail return rate in 2000 than in 1990 is not explained by the larger mailback universe in 2000 (99% of the population and 95% in 1990). We calculated mail return rates for tracts in the 2000 mailback universe, which included people previously enumerated in person and for tracts in both the 2000 and the 1990 universes; the latter rate was only 0.2 percentage point higher than the former. There were larger effects for individual states: for example, in Alaska, the 2000 mail return rate for tracts in both the 1990 and the 2000 mailback universes was 65 percent, compared with 61 percent for all tracts in the 2000 universe.


Ericksen et al. (1991) defined 10 mailback rate categories: one for under 55 percent, eight intervals of 5 percentage points from 55–59.9 percent to 80–84.9 percent, and one for 85 percent and over.

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