When presidential candidates make campaign promises about improving the country's education system, they should keep this in mind: It will be difficult to prove you made a difference.

We saw that recently with a promise about improving the nation's high school graduation rate, which we concluded should remain In the Works indefinitely because imperfect and delayed measurements won't reflect Obama's impact for years to come.

Here's another example: Barack Obama told voters he would get more high school students taking college-level or advanced placement courses. He said his goal was to increase the number by 50 percent "in the coming years.”

Any time you promise to improve performance, you need a baseline. A new runner might record a mile time with no training, and then record another mile six months down the road.

When it comes to high school students taking college-level courses, no one recorded the baseline number at the start of Obama's term. Actually, to a large extent, no one knows the number even today.

"It's not something that's well tracked on a national basis,” said Christina Clark Tuttle, a senior researcher at Mathematica Policy Research, a nonpartisan policy think tank.

The U.S. Department of Education does conduct one longitudinal study that suggests the percentage of public high school graduates taking college-level courses is increasing: 34.9 percent in 2005 and 41.4 percent in 2009.

In general, secondary education researchers differentiate between two types of college-level courses: those designed to prepare students to take an Advanced Placement exam and so-called concurrent enrollment or dual-credit courses. Obama seemed to be referring to both types in his campaign pledge.

The College Board reports that the number and percentage of public high school graduates taking Advanced Placement exams increased steadily during President George W. Bush's two terms, and the trend continued under Obama. The percentage of public high school graduates taking Advanced Placement exams climbed from 16.8 percent in 2001 to 30.2 percent in 2011.

Now this doesn't quite match Obama's pledge. He referred to increasing the number of students taking Advanced Placement courses, which might correlate with taking the related exams. However, not everyone who takes a course takes the exam and the College Board doesn't track AP course enrollment.  

Here's the trickier question: How many students are taking other kinds of college-level courses and how did that change?

Tuttle, of Mathematica, said states count college-level courses differently and some states don't track college-level courses taken by high school students at all, making it difficult to draw definitive conclusions about nationwide trends.

It's plausible that Obama's signature K-12 education initiative, Race to the Top, has triggered progress on this front, and we just don't know it yet. The program uses the allure of grant funding to compel states to change their own education standards in the hopes of winning federal money. One of the many criteria by which the federal government grades states is the number of students who complete at least a year's worth of college credit.

"Dual enrollments seem to me to be increasing because there are a range of incentives and policies which are encouraging more schools, colleges, and students to participate. My sense is that it's on the rise fairly widely,” said Elisabeth Barnett, a post-secondary education researcher at Columbia University's Teacher College.

Barnett's observation aligned with other experts we interviewed. Even so, no data exists yet to prove more high school students took college-level courses under Obama -- especially not 50 percent more. Until we see evidence to the contrary, we'll leave this In the Works.