No one knows exactly how many undocumented aliens are in the United States, or from where they come. Most estimates range wildly, from 10 million to 22 million.

What we do know, from various studies and estimates, is that Central Americans illegally crossing the U.S.-Mexico border are not the only source of our problem.

Undocumented aliens come here from all over the world — and visa overstays are as great a problem as those illegal border crossings shown nightly on TV and debated endlessly in Congress.

The inherent difficulty in calculating the numbers is that undocumented aliens usually avoid drawing attention to their status, to avoid being deported.

In February 2018, the Center for Migration Studies estimated 10.8 million undocumented aliens living here in 2016. Last December, the Department of Homeland Security (DHS) estimated 12 million undocumented aliens as of January 2015. On June 3, Pew Research Center put the number 10.7 million in 2016.

None of these estimates seems reliable. They are based on Census Bureau data: The number of legal immigrants in the country was subtracted from the number of census participants who acknowledged they were not born in the U.S.

Yet, Census information is self-reported by survey participants with no independent verification. There’s no way of knowing if a significant percentage of the undocumented alien population participated. And it is utterly unrealistic to expect undocumented aliens to admit their status in surveys conducted by the federal government.

Professors from MIT and Yale, using more sophisticated methodology, estimated in September 2018 that 22.1 million undocumented aliens are in the U.S. Their estimate is based on operational data such as border apprehensions, deportations, visa overstays and demographic figures, as well as mortality and emigration rates. They evaluated the data with a mathematical model that estimates and tracks population inflows and outflows.

Yet, some of these factors are not wholly reliable. For instance, the professors acknowledged concern about the accuracy of apprehension data: "We don't know the number of people who cross the border successfully — we only know when people get caught trying."

The visa overstay numbers they used may not be accurate, either.

Read more at https://thehill.com/opinion/immigrat...order-crossers

Published originally on The Hill.

Nolan Rappaport was detailed to the House Judiciary Committee as an executive branch immigration law expert for three years. He subsequently served as an immigration counsel for the Subcommittee on Immigration, Border Security and Claims for four years. Prior to working on the Judiciary Committee, he wrote decisions for the Board of Immigration Appeals for 20 years. Follow him on Twitter @NolanR1