A team of Yale researchers say that the number of undocumented persons in the U.S. is probably about 22 million for 2015, close to double the conventional estimate of 11.3 million. They offer a conservative estimate of 16.2 million, an average estimate of 22.1 million, and a high estimate of 29.5 million.
The conventional estimate draws from annual surveys of the American public, to find an estimate of total foreign born. It then subtracts the formal number of foreign born persons here legally, to get to a residual number of 11.3 million. In contrast, the Yale researchers built a model of illegal immigration flows, from visa over stays and from Mexican border crossings, and then subtracted estimates of voluntary outflows and deaths. They suggest that the conventional method is flawed because of the annual population survey: “It is plausible that undocumented immigrants are more difficult to locate (and survey) than other foreign-born residents of the United States, and if contacted, undocumented immigrants might misreport their country of origin, citizenship, and/or number of household residents fearing the possible consequences of revealing their true status.”
Where are these additional people? What do they do? The Yale study implied that there roughly 7 million more undocumented workers in the U.S. than is conventionally estimated (about 8 million).
They arrive at this higher estimate by, first, agreeing to start with the conventional estimate of 3.5 million persons in 1990.
They then estimate visa over stayers, benchmarking from the first official estimate of visa over stayers, done in 2016. Next, they estimate that from 1990 through 2004, the Mexican border apprehension rate was first very low, about 20%, then came to 39% — in other words, in 2004 61% of attempted illegal crossings were successful. They agree that apprehension rates have since increased
They estimate that 40% of visa over stayers leave within one year, and that further voluntary emigration in later years drops eventually to 1%. Their estimates for border crosser returnees is about the same. They use a mortality rate of 0.7% for undocumented persons in the U.S.
They ran one million simulations of these kinds of estimates to arrive at their conclusions.