Part of the debate about immigration reform is that several principles don’t kick in until the borders are “secure.” What does that mean?

A recently released Government Accountability Office report reflects the complexity of determining what progress is made in securing the borders. It also shows that the devil, as usual, is in the details.

In fiscal years 2006 to 2011, arrests across Southwest borders decreased 69 percent. That mirrored an estimated decrease in known illegal entries. Customs and Border Protection attributed the decrease in part to the recession in the United States and the implementation of increased resources along the Southwest border.

But in fiscal year 2012, arrests increased by 30,000 to about 357,000. The GAO said it is too early to say whether the change represents a trend.

Arrests are just one measure of effectiveness. Take how the Border Patrol measures success. Until recently, each of the nine Border Patrol sectors on the Southwest border decided how it would collect data on such things as “turn backs” (entrants who illegally crossed the border but were not apprehended because they crossed back into Mexico) and “got aways” (entrants who crossed the border and continued traveling in the United States). The data even varied by agent experience, resources and terrain. The Border Patrol, according to the report, is in the process of developing performance goals for assessing success but has yet to identify milestones or time frames for implementing them.

Homeland Security Secretary Janet Napolitano has said other parts of an immigration bill, such as a path to citizenship for undocumented immigrants, should not be contingent on achieving a defined number for border security. As a news conference last week touting the Senate immigration bill, Minnesota Sen. Al Franken was asked what his definition of a secure border was.

“To assign a trigger is one of the things we wanted to avoid,” Franken said. “We didn’t want to delay the path [to citizenship] until somebody said, ‘Now the border is secure.’ ”