The undocumented population in the United States could be twice as large as the most commonly-used estimate, according to a research study published Friday in the scientific journal Plos One.

The paper, led by Mohammad M. Fazel-Zarandi, a researcher at Yale and Massachusetts Institute of Technology, estimates there are 22.1 million undocumented immigrants in the United States.

Fazel-Zarandi's study compared inflows and outflows of immigrants as well as demographic data. According to the report, the number of undocumented immigrants could be as low as 16.5 million, or as high as 29.1 million.

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“We combined these data using a demographic model that follows a very simple logic,” Edward Kaplan, a co-author of the report, told Yale Insights. “The population today is equal to the initial population plus everyone who came in minus everyone who went out. It’s that simple.”

Most previous estimates, based on the U.S. Census Bureau's annual American Community Survey (ACS), place the undocumented population at around 11 million people.

The ACS is generated annually through interviews with about 3.5 million people.

"You find it’s based on one very specific survey and possibly an approach that has some difficulties. So we went in and just took a very different approach," Jonathan Feinstein, another co-author, told Yale Insights.

Still, the new study found similarities in growth patterns of the undocumented population to the ACS reports of the past two decades.

Most immigration researchers have reported a plateau in growth of that population since 2007, spurred mainly by the decrease in Mexican illegal immigration to the United States.

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The reduction in illegal crossings by Mexican citizens coincided with an increase in illegal immigration from Central America and Asia. But the overall undocumented population has leveled off, and possibly slightly decreased.

The Pew Research Center, which provides regular updates on immigration statistics -- based in part on ACS numbers -- estimated last year that the undocumented population dropped to 11.3 million from a high of 12.2 million.

The Yale study, which goes as far back as 1990, found the same upward and downward trajectory as the ACS studies, with the undocumented population ballooning through the 1990s and plateauing after 2007.

“The trajectory is the same. We see the same patterns happening, but they’re just understating the actual number of people who have made it here,” said Fazel‐Zarandi.

The researchers emphasized the new number does not imply a growth in illegal immigration but a longstanding miscount of existing undocumented immigrants.

"We wouldn’t want people to walk away from this research thinking that suddenly there’s a large influx happening now,” said Feinstein. “It’s really something that happened in the past and maybe was not properly counted or documented.”

The researchers also noted if the undocumented population is twice as high as previous estimates, that means the incidence of criminality among undocumented immigrants is half as high as previously reported.

"You have the same number of crimes but now spread over twice as many people as was believed before, which right away means that the crime rate among undocumented immigrants is essentially half whatever was previously believed," said Kaplan.