The yardstick used in the immigration bill to determine border control may produce too rosy a picture of how well the Border Patrol is doing in cracking down on illegal crossings, according to an independent study released Monday that threatens to upend the immigration debate.

In their 76-page report, three researchers at the Council on Foreign Relations also said the drop in illegal immigration is only partly a result of tougher border security and about two-thirds because of economic changes in Mexico and the U.S. that have made it less attractive for Mexicans to migrate north. …

“The Border Patrol doesn’t know what it doesn’t know, which is some people are going to come across the border, get across the border unseen,” said Edward Alden, a senior fellow at the Council on Foreign Relations and one of the report’s authors.