How did we estimate the U.S. unauthorized immigrant population?
Pew Research Center bases the estimates in this report on a “residual method” similar to those employed by the U.S. Department of Homeland Security’s Office of Immigration Statistics and nongovernmental organizations, such as the Center for Migration Studies and the Migration Policy Institute. Those organizations’ estimates are generally consistent with ours. Our estimates also align with official U.S. data sources, including birth records, school enrollment figures and tax data, as well as Mexican censuses and surveys.
The first step in our method is to use U.S. census counts and government surveys, such as the American Community Survey, to calculate how many immigrants live in the U.S. in a particular year. Next, we use official counts of immigrant admissions and other demographic data (death rates, for example) to determine how many of these immigrants live in the U.S. legally. Then we subtract those lawful immigrants from the total to get an estimate of the unauthorized immigrant population.
Based on experience and research, we know the census counts and other official surveys tend to miss some people. Unauthorized immigrants are especially likely to be missed. Therefore, we do a further assessment of potential undercounts or undercoverage. Based on this additional research, our final estimate of the U.S. unauthorized immigrant population includes an upward adjustment for undercount.