Pew Research Center’s estimates of the size of the unauthorized immigrant population use a well-accepted residual methodology that is explained in detail in our reports (Passel, Cohn, Krogstad and Gonzalez-Barrera, 2014) and more informally in a Fact Tank blog post.

The first step is to use a national census or survey to determine how many immigrants are living in the U.S. in a particular year. For many years, we chiefly used the Current Population Survey, but we have now switched to the American Community Survey for estimates published in September 2013 and later. Then, using a variety of data sources, mainly from the U.S. government (such as “green card” numbers and refugee admissions), it is possible to estimate how many of these immigrants live in the U.S. legally either as legal permanent residents or as naturalized U.S. citizens. Then we subtract the number of legal immigrants from the total, which gives us an estimate of unauthorized immigration. (That is why our technique is called “residual methodology.”) We update our estimate of the legal foreign-born population each year using information on deaths, departures and new arrivals.

It is more complicated than that, of course. For some years, we’ve reweighted the annual official Census Bureau survey estimates to account for updated information from a later census. Because the initial census or survey always misses some people (and unauthorized immigrants are especially likely to be missed), the final step is to adjust the numbers upward, based on research about omissions from the survey or under-coverage.

In addition, one important step in getting information on the characteristics of the unauthorized immigrant population (as well as the legal foreign-born population) is to assign a specific legal status to each immigrant in the census survey data (“unauthorized immigrant,” for example). These categorizations are based on each individual’s demographic, social, economic and geographic characteristics, and will become the basis of estimating additional characteristics for this population.