[Excerpt] This CRS report presents data estimating since 1986 the number of unauthorized aliens who have been living in the United States. There have been a variety of estimates of the unauthorized resident alien population over this period, sometimes with substantially different results. This report is limited to analyses of the Current Population Survey (CPS) conducted by the U.S. Census Bureau and the Bureau of Labor Statistics and of the American Community Survey (ACS) conducted by the U.S. Census Bureau so that there are basic standards of comparison over time. Because the CPS and the ACS are both sample surveys of the U.S. population, the results are estimates. Additionally, while the data distinguish between the foreign born who have naturalized and those who have not, they do not identify immigration status (e.g., legal permanent resident, refugee, temporary foreign worker, foreign student, unauthorized alien). Summaries of the detailed analyses of the March CPS, the ACS, and the monthly CPS are presented separately because each of these surveys is based on different questions and sample sizes.