The Government Accountability Office, in a report last December, published previously unreleased data showing that the Border Patrol’s effectiveness rate for fiscal 2011 was 84 percent. By that yardstick, getting to 90 percent isn’t a huge stretch, noted former DHS official Roberts.

The Border Patrol hasn’t released turn-back or got-away data for fiscal 2012, and hadn’t responded by deadline to The Republic’s request for that information.

Outside researchers say efforts to come up with a better approach to accounting for undocumented migration run smack into Homeland Security’s unwillingness to let academics analyze its data.

Last year, for example, a panel of leading statisticians, economists and demographers at the National Academy of Sciences conducted a study on illegal immigration at the request of Homeland Security. But the DHS refused to provide the panel key apprehension data, such as coded fingerprint figures that would identify precise numbers of repeat crossers. The DHS had demanded that researchers promise not to disclose that data to the public. Panel members said keeping the information classified would impair the quality of their work; they declined, and didn’t get the data.

That study, which included data from Mexican governmental sources and previous U.S. academic studies, suggested that about three-quarters of those who decide to cross keep trying until they make it. Other outside studies have found 85 or even 90 percent make it.

“Almost everybody who really tries eventually gets in,” said Jeffrey Passel, a member of the panel and a senior demographer at the Pew Hispanic Center, a nonpartisan research organization in Washington, D.C., that studies the U.S. Hispanic population.