We've enacted almost all of the border security that the Bush administration had wanted in 2007, dramatically ramping up personnel, detention facilities, border infrastructure, and technology.

Legislators now want to ramp up these efforts even further as part of comprehensive immigration reform. But is our border really more secure because of these additional measures? It's a question that's actually quite difficult to answer, as the American Immigration Lawyers Association lays out in a new report.

The Obama administration has touted success by pointing to the numbers of apprehensions along the southwest border dropping to the lowest level in 40 years in 2010. Violent crime in border cities has also dropped steadily in recent years.

The problem is, the decline in the sheer number of apprehensions doesn't necessarily mean that border security is more effective. Experts point out that illegal immigration as a whole has plummeted since 2007 in significant part because of our economic downturn. Border arrests in 2012, in fact, are up to 356,873 from 327,577 in 2011, the AP reported this week, which could suggest that the number of apprehensions has been contingent at least in part on the economy.

Despite our limited ability to measure success, however, lawmakers have continued to ask for more and more resources. Immigration advocates and experts believe that better metrics are necessary to evaluate whether these additional resources are actually helping. "Missing from these proposals is a proven way to measure when the border is reasonably secure," AILA writes in its new report, arguing that policymakers "need to identify clearer goals for border security and ways to measure success rather than simply increasing resources."

To get a better sense of whether border security itself has improved, it would help to have both the number of apprehensions and the number of border-crossers who have succeeded in getting past border security — a number that's difficult to pin down, but which DHS is "getting closer and closer to knowing," due to improved border cameras, drones, and surveillance, explains the Migration Policy Institute's Doris Meissner, Clinton's former Immigration and Naturalization Services director.

Right now, DHS doesn't even release estimated numbers of the border-crossers who actually succeed in entering the United States, partly because they still don't have good measures and partly because they're "gun shy" to do so, Meissner explains.

Such information is slowly beginning to trickle out, as officials have begun developing more effective metrics for measuring success. The Government Accountability Office released a report this month that calculated border apprehensions as a percentage of total estimated illegal entries in the Tucson area, which has installed a more sophisticated and comprehensive surveillance system to map border crossings electronically. GAO concludes that the effectiveness rate for the Tucson sector improved from 67 percent in FY 2006 to 87 percent in FY 2012. Overall, eight of nine sectors saw more effective border apprehension over the past six years.

But Border Patrol admits that it is difficult to compare performance as different sectors define, collect, and report their data differently. So there's still quite a bit of work to be done — not only only in terms of figuring out how secure the border is, but also what border security measures (agents, fence, cameras, etc.) are actually having the greatest impact.

The other major measure of border security is similarly flawed. Since 2006, "control" of the border has been defined as "operational control," which AILA believes has sets an unrealistic expectation that the border can be 100 percent sealed." In practice, Border Patrol has defined "effective control" as having the ability to detect and stop illegal activity within a part of the border. But as AILA points out, Obama's DHS has become increasingly skeptical about using "operational control" to measure outcomes, which Border Patrol Chief Michael Fisher called an "outdated measure" in 2011. DHS has similarly promised to use more sophisticated, quantitative analysis in lieu of "effective control," using new metrics for risk analysis, but it's still in the process of developing these methods.

Overall, there's still a lot we don't know about how secure the border is and what interventions are the most effective. But despite our spotty knowledge, legislators have continued to equate more resources and money with greater security, and we've continued to pour hundreds of millions of dollars into border security — even far above and beyond what DHS itself has asked for. AILA points out that in 2012, Homeland Security Secretary Janet Napolitano "requested fewer funds for detention beds for FY 2013 compared to the previous fiscal year. She stated that “[DHS had] enough beds to handle the detained population." But Congress increased funding for detention beds in FY 2013 anyway, despite Napolitano's comments and an ongoing climate of austerity.

Defenders of the current approach to border security argue that it's clear the current approach is working given what we do know about the situation on the border, even if our assessment tools are limited. But the risk is that money, time and personnel could be poured into ineffective boondoogles like SBInet, the so-called "virtual fence" whose construction began during the Bush administration. DHS ended the program in 2010 after $1 billion was spent to complete only 2.5 percent of the project, AILA points out.

So while Senate's blueprint calls for "securing our borders" by increasing enforcement once again — more surveillance, more agents, etc. — it will be worth examining how legislators define their end goal and whether the proposals on the table will actually have an impact. "The real question that it begs is we are putting more and more resources into something that's a never-ending spending cycle. We've reached the point of clearly diminishing returns," says Su Kim, an AILA advocacy associate and co-author of the new report.

But such arguments will be challenged by critics who believe that letting up on security measures would be a dangerous form of complacency. "Eternal vigilance is the price of border security," explains Mark Krikorian of the Center for Immigration Studies.