for every dwelling unit and then uses the U.S. Postal Service for delivery of a mail questionnaire. If questionnaires are not returned, the census office then directly contacts the households that have not responded.

As noted above, there are many different specific procedures and widely different levels of intensity for achieving coverage. Some countries distribute their census questionnaires, do a "reasonable" follow-up, and accept some net undercount. In contrast, the U.S. census operation is a very intensive attempt to contact every housing unit and obtain demographic information for every person in the household. As part of its activities, the U.S. census operation involves an intensive effort to prepare an accurate address list, has enumerators active in the field for a longer time than other census operations, and makes as many as six attempts to follow up all nonrespondents. This intensive U.S. census is now very expensive. One major census design alternative, therefore, is for a less intensive, cheaper census.

A NATIONAL REGISTER FOR THE BASIC CENSUS

A national population register could replace the conventional census with the continuous recording of individual data, possibly linked to other records, for every resident of the United States. In order to provide data required for apportionment and redistricting (and for other purposes) the register would need to report the precise geographic location of everyone, that is, his or her residential address.

In spite of the challenges of such a system, population registers have been successfully developed in several nations, including Belgium, the Netherlands, Switzerland, and the Scandinavian countries. The Swedish system has its roots in the seventeenth century, and the Dutch register has been operating for more than a century. A similar system was instituted in the United Kingdom during World War II but was discontinued.1

A population register must be continuously updated to retain its usefulness. Also, a register needs to be linked to other records as people move, change their marital status, change jobs (and incomes), increase their formal education, have children, and die. Such a register can be maintained continuously only if there is a legal requirement for individuals to report all changes of addresses and certain other changes in status. Furthermore, massive record linkage (in order to obtain required information from other administrative record systems) is generally possible at reasonable cost only if all individuals are issued unique identifiers and if these identifiers are recorded on the main administrative system.

A register-based system, by itself, can supply basic demographic information about the population. Basic demographic data could include a person's birth date and birthplace, sex, and race or ethnicity (if requested by some form and if identity is assumed to be fixed). Through record linkage (or mandatory reporting to the register), information can also be obtained on marital status,



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