U.S. Census Bureau Measurement of Net International Migration to the United States: 1990 to 2000 [open pdf - 105KB]
"The U.S. Census Bureau produces national population estimates on an annual basis using methodology that incorporates the three main determinants of population change: births, deaths, and net migration. Net international migration plays an integral role in determining both the size and demographic composition of the population at the national level. It is the most complex determinant of population change, and its complete measurement involves the measurement of several sub-components, each one using separate methodology. In view of the fact that these sub-components are estimated separately, each is subject to differing levels of uncertainty or bias. This paper provides a detailed discussion of the definitions, methodologies, assumptions, and data sources used to measure net international migration, including a brief description of estimates for geography and demographic characteristics. Following the discussion of the subcomponents of net international migration, the strengths and limitations of each are presented along with potential research and developments."
Population Division Working Paper #51
Census Bureau: http://www.census.gov/