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Undocumented immigrantsCounting 11 million undocumented immigrants is easier than you think

By Jennifer Van Hook

Published 3 November 2016

News organizations widely report that there are 11 million unauthorized immigrants living in the United States. But where does this figure come from? Donald Trump has falsely asserted: “It could be three million. It could be 30 million. They have no idea what the number is.” In the third debate, Hillary Clinton said, “We have 11 million undocumented people. They [undocumented parents] have 4 million American citizen children. 15 million people.” The confusion is warranted – but demographers have figured out a simple and effective way to estimate the number of unauthorized immigrants.

News organizations widely report that there are 11 million unauthorized immigrants living in the United States. But where does this figure come from?

Donald Trump has falsely asserted: “It could be three million. It could be 30 million. They have no idea what the number is.” Inthe third debate, Hillary Clinton said, “We have 11 million undocumented people. They [undocumented parents] have 4 million American citizen children. 15 million people.”

The confusion is warranted. After all, the U.S. Census Bureau does not ask people about their immigration status, so how can we know much about the unauthorized foreign-born population?

Well, demographers have figured out a simple and effective way to estimate the number of unauthorized immigrants. In the last five years, my colleagues Frank D. Bean, James D. Bachmeier and I have conducted a series of studies that evaluate this method and its assumptions. Our research on the methods used to estimate the size of this group indicates that these estimates are reasonably accurate.

Here’s how it works.

A simple formula
Beginning in the late 1970s, a group of demographers consisting primarily of Jeffrey Passel, Robert Warren, Jacob Siegel, Gregory Robinson and Karen Woodrow introduced the “residual method” for estimating the number of unauthorized immigrants living in the country. At the time, Passel and his collaborators were affiliated with the U.S. Bureau of the Census and Warren with the Office of Immigration Statistics of the Immigration and Naturalization Service. Much of this work was published in the form of internal reports, but some of it appeared in major journals.

The residual method uses an estimate of the total foreign-born population in the country (F), based on U.S. Census data. Researchers then subtract from it the number of legal immigrants residing here (L), estimated from government records of legal immigrants who receive “green cards” minus the number that died or left the country. The result is an estimate of the unauthorized population (U):

F – L = U

Various adjustments are typically made to this formula. Most adjustments are minor, but a particularly important one adjusts for what researchers call “coverage error” among the unauthorized foreign-born. Coverage error occurs when the census data underestimate the size of a group. This can occur when people live in nonresidential or unconventional locations – such as on the streets or in a neighbor’s basement – or when they fail to respond to the census.