Security along the U.S. border with Mexico is perhaps the key factor in the debate over comprehensive immigration reform. Those who believe the border is mostly secure already are more inclined to support the plan of the bipartisan Gang of Eight in the Senate — legalization first, followed by enhanced security. Those who believe the border is still far from secure are more likely to oppose the Gang of Eight’s approach, insisting that heightened security measures be in place before the nation’s estimated 11 million illegal immigrants are legalized.

Now, a new report from the Council on Foreign Relations could have a significant effect on the conversation — and cast real doubt on whether the government’s border security statistics are reliable. If the report is correct, more illegal immigrants are making it past U.S. authorities than officials say. And just as important, the report suggests it is nearly impossible to have an informed debate about border security because the government does not reveal the most basic information about illegal border crossings.

“The Department of Homeland Security releases only a single output number: the total arrests, or apprehensions, made by Border Patrol agents of unauthorized crossers in the vicinity of the border,” the authors write. “Such basic questions as the apprehension rate for unauthorized crossers or the estimated number of successful illegal entries cannot be answered simply by counting arrest totals.”

No, they can’t. To find some of the answers that Homeland Security won’t provide, the authors looked to other data — interviews with people who have tried to cross the border illegally; analysis of people who have been caught attempting to cross multiple times; and what is called “known flow,” that is, the actual observations by the Border Patrol of people trying to cross into the United States.

Putting together all the evidence, what they found is that U.S. authorities are catching somewhere between 40 percent and 55 percent of the people who try to cross the border illegally. That’s more than in the past, when the Border Patrol had less manpower, but it’s still just somewhere around half, or even less.

And nobody is quite sure if that is accurate; 40 to 55 percent of what? Knowing that answer requires knowing the total number of people who have tried to enter the U.S. illegally in any given year, including the ones who successfully evaded Border Patrol. The DHS estimate of that group, the so-called “gotaways,” is almost surely too low, and new information from drone-based surveillance suggests more illegal immigrants may be getting through than previously thought.

To that 40 percent to 55 percent who are apprehended, DHS adds another number referred to as “turnbacks.” Those are people who try to enter illegally, make some progress, and then retreat back into Mexico. There are estimates that the turnback rate across the whole border is about 23 percent.

Some border experts put those two numbers together — apprehensions and turnbacks — and come up with what is called an overall effectiveness rate. If the apprehension rate is, say, 47 percent, and the turnbacks are 23 percent, then that would be a 70 percent effectiveness rate. (Assuming the government knows the total number that tried to get in, which it doesn’t.)

DHS has been claiming a pretty high success record lately. “Right now, what is our effectiveness rate … in high-risk border sectors as defined by the [Gang of Eight] bill?” asked Sen. Tom Carper, D-Del., of Border Patrol chief Michael Fisher at a recent hearing. “It’s approximately between 80 and 85 percent,” Fisher answered.

Privately, some Republican lawmakers scoffed at that number. But the bottom line is that, given the secretiveness of Homeland Security, it’s hard to make a really knowledgeable argument one way or the other.

The authors of the Council on Foreign Relations report find it “puzzling” that DHS does not release more data; what is available, they say, is “distressingly sparse.” Maybe that’s just a bureaucratic oversight, and maybe it’s not. For years, DHS has resisted coming up with a clear way to measure border security. Why would they voluntarily produce reams of data about something they don’t want to know?

But things might change. Recently the House Homeland Security Committee unanimously passed a bill that would force DHS to come up with a clear, definite measurement of border security. It’s absolutely critical. Until there is such a measurement, the immigration reform debate is taking place in the dark.

Byron York is chief political correspondent for The Washington Examiner.