evidence: The way Pew Research Center calculates the estimated number of unauthorized immigrants in the United States is the product of decades of work by Jeffrey S. Passel, senior demographer, along with former colleagues at the U.S. Census Bureau and the Urban Institute. Passel has written numerous studies on the demography of immigration and on immigration issues. Following are his answers to some common questions about the research techniques used to derive the unauthorized immigrant population estimate. [...] There have been several studies done by demographers looking at coverage of censuses and surveys. We draw on data from Census Bureau studies of the undercount. We use information from a couple of surveys focused on Mexicans in the U.S. that asked whether they participated in the census or not. And perhaps our most useful set of data comes from surveys and censuses done in Mexico. Mexicans are by far the largest group, now accounting for almost half of the unauthorized immigrants in the U.S., so we feel that if we can get a good handle on how well they’re counted, we can generalize to other groups of people. Mexico has its own censuses; they have a number of large-scale surveys of Mexicans in their country; and they have good historical data. Essentially, almost all of the Mexicans in the world are either in Mexico or the U.S., so by looking at the number in the two countries combined, we can make some inferences on how good the data are in the U.S. and place some limits on how many Mexican immigrants might be missed in U.S. data sources.