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.

At a time when the success or failure of the immigration bill depends on the security level of the border, the authors said it’s surprising how little is known about border security and how little effort the administration and Congress have made to try to get it under control.

“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.

Still, the report said security has increased over the past decade and that the likelihood of being caught crossing the border illegally is 50 percent to 60 percent — up from 30 percent to 45 percent in 2000.

The Senate Judiciary Committee began working through the 867-page immigration bill last week and has closed out the border security section, making only minor changes to the bill.

The panel shot down repeated Republican attempts to stiffen border security measures or to insist that the borders be measurably secure before illegal immigrants are given legal status. Those measures were defeated on a 12-6 vote with two Republicans joining all 10 of the panel’s Democrats in opposition.

The committee did adopt one key Republican-authored change that would set a goal for the Homeland Security Department to apprehend or deter 90 percent of illegal crossers along the entire southwestern border. As originally written, the bill required the 90 percent standard only in high-traffic sectors.

The Council on Foreign Relations report, though, questioned the measure the Border Patrol uses to calculate the 90 percent “efficiency” rate.

Efficiency is calculated by taking the number of illegal crossers the Border Patrol captures and the number it estimates turned back, and then guessing how many got away cleanly. A 90 percent efficiency rating would mean nine out of 10 who crossed illegally were apprehended or forced to turn back.

The problem, according to critics on Capitol Hill and to the report’s authors, is that it’s tough to estimate “turn-backs” and those who got away.

Border Patrol figures suggest an 80 percent efficiency rating right now.

But the Council on Foreign Relations researchers looked at other methods of tracking illegal crossings, including surveys administered to those trying to cross and Border Patrol statistics on recidivism. By those numbers, the Border Patrol was catching no more than 60 percent of illegal crossers at the end of the last decade.

Mr. Alden and his co-authors, Bryan Roberts and John Whitley, said the problem is that Congress and presidents of both parties generally have measured border security by the amount of money spent — “whether the United States has enough Border Patrol agents, enough surveillance, enough fencing.”

But there always has been reluctance to measure outcomes, such as whether added manpower and technology are producing results.

The Council on Foreign Relations also attributed the drop in illegal crossings mostly to factors other than border security, with one-third of the decline resulting from the recession and unemployment in the U.S., and one-third attributed to Mexico’s rapidly expanding economy.

Lawmakers on Capitol Hill were reticent about the report Monday. Messages seeking comment from a half-dozen lawmakers spanning both sides of the issue went unreturned.

The Council on Foreign Relations report could reopen many of those tricky questions — particularly in the Republican-controlled House, where leaders have said they will insist that the Homeland Security Department come up with real security measurements.

Sen. Marco Rubio, a Florida Republican who is considered a linchpin in the immigration debate, said last week that he believes the bill as written is adequate for border security but that he is willing to alter the legislation if it makes voters more confident that it will work.

“I want us to get a consensus on that issue that I’ve outlined, which is this balance between, does Congress dictate to [the Homeland Security Department] specifically what it needs to do on the border, versus allowing them the flexibility — the people on the ground — to decide what the best practices are,” Mr. Rubio said.

“I still believe at the end of the day the bill as it’s currently structured will work, because if you give the experts — and it’s not just [Homeland Security], it’s the Border Patrol and the people down there — if you’re giving them access to $5½ billion to secure the border, I believe it will improve the border,” the senator said. “I personally believe that. But the important thing is to build a consensus of confidence, so people actually believe it’s actually going to happen, and it’s actually going to work.”

The Senate Judiciary Committee continues its work Tuesday with a second day of votes on amendments. This time, the panel will turn to the legal immigration system and to the bill’s plan for legalizing the estimated 11 million illegal immigrants already in the U.S.

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