and death records for previous years, immigration and emigration data, and previous censuses.
Demographic estimates are the primary means for comparing coverage for censuses over time for the nation as a whole. Demographic analysis has two main methodological weaknesses. One is deficiencies in the data—particularly the immigration and emigration data. The magnitude of illegal alien flows into and out of the United States in recent decades is unknown, as is the magnitude of emigration of citizens and legal resident aliens. The second shortcoming is that demographic analysis cannot provide estimates by states or other subnational areas. Demographic estimates of coverage (see Table 2.1) show that the net national undercount (the number of people omitted minus the number over-counted) was estimated at 7.0 million in 1940, 6.3 million in 1950, 5.6 million in 1960, 5.5 million in 1970, 2.8 million in 1980, and 4.7 million in 1990. The undercount rate dropped from 5.4 percent in 1940, to 4.1 percent in 1950, 3.1 percent in 1960, 2.7 percent in 1970, and 1.2 percent in 1980, then rose to 1.8 percent in 1990 (Robinson et al., 1993:13).
Based on the criterion of net undercount, the 1990 census was somewhat worse than the 1980 census. Comparisons of net undercount, however, fail to reveal some other kinds of deficiencies in census counting. Net undercount figures reflect three elements:
People who were not counted in the census or who were omitted from the census in their proper place or residence. These people are called omissions.
People who were enumerated more than once, were ineligible to be counted in the census (e.g., babies born after Census Day), or were counted at their incorrect place of residence. These people are called erroneous enumerations.
People whose existence was ascertained (by the judgment of the enumerator or other evidence) but whose characteristics were missing and so had to be "borrowed" from another enumerated person. These people are called Substitutions.
The combination of erroneous enumerations and substitutions (b and c) can be added together to produce the total number of counting errors, which in 1990 were estimated to include 16 million people. A substantial proportion of counting errors are people who were incorrectly placed in the wrong geographic areas, resulting in an omission in the correct location and an erroneous enumeration in the incorrect location. From the national perspective, people assigned to an incorrect location were counted in the population (and thus not part of the national