The lack of a sound statistical measure for border security could muddy the debate over whether an immigration bill should contain a security threshold that must be reached before other parts of the bill can be enacted.
Apprehensions show how many people are being picked up by Border Patrol agents. But the figure is so relative to staffing and strategy that it is essentially meaningless without greater context.
"A real measure is more qualitative," Napolitano said. "Really, when you step back and think about the border, what you want is the ability to spot illegal traffic, particularly in the highly trafficked areas -- some parts of the border are virtually not trafficked at all -- and then the ability to respond.
"Using that measure of the plans we have, we're confident that the border is as secure as it's ever been," she continued. "But there is no one number that captures that. That is the problem, if you're looking for just one number."