The promise: In the 2008 election, Barack Obama pledged to create secure borders. He said that would mean "additional personnel, infrastructure and technology on the border and at our ports of entry."

Four years later, do we have secure borders?

To adequately answer that question, we would need to know what percentage of people who try to enter the country illegally get caught.

Unfortunately for politicos who love clarity, all we have are imperfect measures.

"We don't really know the denominator," said Louis DeSipio, a political scientist who researches immigration at the University of California, Irvine.

In other words, we know how many we catch, but we don't know if that's most, half, or just a sliver of the total number of people crossing illegally.

Here's what we do know:

- The number of people we're catching as they try to cross the border illegally has decreased dramatically.

- The number of people we're removing per year is at an all-time high.

- Following a trend initiated by Presidents Bill Clinton and George W. Bush, the Obama administration has added more border patrol agents, fencing and detection technology to prevent border crossings.

- Two years ago, slightly less than half the Southwest Border met Homeland Security's border security standard of under "operational control."

- Changes in enforcement have had an effect on reduced illegal immigration, but not all the credit should go to policymakers: The down economy in the U.S., especially in the housing sector, played a major role in the decline.

Last year the number of apprehensions near the border (340,000) was the lowest since 1971. That's in contrast to slightly more than 1 million apprehensions, the number in 2005.

Apprehensions correlate closely with illegal immigration patterns, said Jeffrey Passel, a demographer with the Pew Hispanic Center.

As apprehensions go down, one should expect that illegal immigration would go down as well.

A word of caution though: It's really a measure of how many people Border Patrol catches, not how many people attempt to cross. Imagine two scenarios:

Scenario 1: Fewer people from Mexico try to cross the Southwest border, so Border Patrol reports fewer people caught.

Scenario 2: Border Patrol cuts back on staff, changes reporting procedures, or begins using detection technology that malfunctions frequently -- and fails to catch some people who cross. Apprehensions go down, but illegal immigration does not.

Thus it's conceivable the current apprehensions trend is misleading.

In this case though, "every other measure tells you is that immigration is on the decline," said Gordon Hanson, an economist who researches immigration at the University of California, San Diego. "We've got a whole bunch of imperfect measures and they all tell a similar story."

For instance, the number of removals in 2009 and 2010 were higher than in any year under Bush or Clinton. The reason removals matter in a discussion about illegal immigration? Because many people actually enter the country on a legal visa, but stay after it expires.

The number of illegal immigrants in the U.S. declined from a peak of about 12 million in 2007 to about 11.1 million in 2009. Since then, the number has remained relatively flat, said Passel, the demographer.

Homeland Security's "operational control" measure is the strongest evidence touted by groups who favor stricter immigration controls and say the borders aren't secure.

We spoke with Mark Krikorian, executive director of Center for Immigration Studies, who referenced a 2010 report on border security by the Government Accountability Office, a nonpartisan analytical arm of Congress.

The report says that 44 percent of the 2,000-mile Southwest border is under "operational control," a term that represents the level of borders security provided through the number of border patrol agents, fencing and detection technology, such as heat sensors and drones.

Another 37 percent of the border is monitored, with a high chance of detecting crossings, but Border Patrol isn't able to respond because of obstacles such as lack of transportation access or difficult terrain. The report also mentions that the percentage under "operational control" had been increasing by an average of 126 miles per year between 2005 and 2010.

"I don't think it has much scientific merit," said Hanson, the economist. "It's a measure of investment. It's not a measure of return on investment."

Valid or not, the most recent figure we have on "operational control" is two years old because Homeland Security scrapped the measure in favor of a new index that the department said would debut in 2012. It hasn't yet.

Some Republicans are tired of waiting for the new measure. Rep. Candice Miller, R-Mich., introduced a bill in June calling for Homeland Security to reinstate its "operational control" reporting.

Krikorian said his definition of secure borders would -- at the very least -- mean a Southwest border 100 percent under "operational control."

"Secure borders" probably does not mean no illegal immigration though. Even in a scenario where the country marshalled all its resources to border security, it's plausible that people would still find a way to cross.

Edward Alden, an immigration researcher at the Council on Foreign Relations, a nonpartisan think tank, said that even in the Cold War-era when East Germany tried to keep its residents from escaping to West Germany, there were still people who managed to cross.

"You cannot imagine a scenario where it would be impossible to cross the border," Alden said.

We also think it's worth noting that even though Obama's subject heading for the promise was "create secure borders," his specific, measurable pledges were about investing in additional staffing and resources.  

As we reported last year, personnel and other resources to stop illegal crossings of the U.S.-Mexico border have increased dramatically in recent years. One vivid statistic from Homeland Security: The number of border patrol officers more than doubled from about 10,000 to about 21,000 between 2004 and 2012.

In our last update on this campaign promise, we warned that it's difficult to parse the effect of increased enforcement from the effect of changing economic conditions. Hanson, who has run statistical models to estimate factors that explain changes in illegal immigration, says the two biggest factors are enforcement and the economy, and they are equal in weight. In the case of the economy, the U.S. recession and the slowdown of home construction -- which employed many illegal immigrants -- is closely associated with less people trying to immigrate to the U.S.

Without a clear-cut definition on what "secure borders" means, it's difficult to grade Obama on his performance. Many signs point to significant progress on stemming illegal immigration, including added staff and resources in border security. But reports have indicated that a sizeable portion of the border is not under "operational control." We rate this promise a Compromise.