"Assumptions regarding the components of international migration (specifically, emigration, temporary migration, legal migration, and unauthorized migration) contain the largest uncertainty in the demographic analysis estimates. Therefore, evaluating the components of international migration was a critical activity in the DAPE project. This report focuses on the evaluation of the U.S. Census Bureau's estimated residual foreign-born population (including both unauthorized and quasi-legal migrants) in 1990 and 2000. The estimates shown here were calculated in conjunction with estimates of other components of international migration: legal permanent migration and legal temporary migration. These components of international migration, along with assumed deaths and emigrants, are subtracted from a total foreign-born population yielding a residual count. This residual foreign-born population is not an estimate of the number of unauthorized migrants. This estimate also includes people who are here legally but are not yet included in the official estimates of legal migrants and refugees. It also includes people in 'quasi-legal' status who are awaiting action on their legal migration requests. Because the estimate was derived from a residual methodology, any limitations in the methods or in the measurement of other migration components are reflected in the residual number. In addition, our assumptions include a great deal of uncertainty, especially for small migration components. Therefore, the residual may be quite different from the actual number of unauthorized migrants."
Population Division Working Paper #61
Census Bureau: http://www.census.gov/