 With this method, the known number of legally documented immigrants to the United States is subtracted from the reported U.S. Census number of self-proclaimed foreign-born people (based on immigration records and adjusted by projections of deaths and out-migration) to obtain the total, illegal immigrant (residual) population.This methodology is used by the US Department of Homeland Security, the Pew Hispanic Center, the Center for Immigration Studies, and the U.S. Census Bureau.Since undocumented immigrants have many reasons for not answering the U.S. Census correctly and since penalties for answering the U.S. Census incorrectly are rarely enforced, it is accepted that it under-counts the number of illegal immigrants.