When it comes to studying unauthorized migration, there is no perfect data set. The fact that undocumented migrants are clandestine means they necessarily fall outside the reach of the usual statistical systems, and the irregularity and marginality of their position in society makes it difficult to gather information on them using alternative methods. The best one can hope for is a triangulation that gets at the truth from multiple angles using diverse data sources that have complementary strengths and weaknesses. Despite obvious challenges, however, demographers have made considerable progress in assessing the numbers, characteristics, and behavior of undocumented migrants in the United States, especially those from Mexico.

Although they are clearly underenumerated, undocumented migrants are nonetheless captured to a significant extent in U.S. demographic surveys, such as the Current Population Survey (CPS) and the American Community Survey (ACS), and to an even greater extent in Mexican surveys, such as the...

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