College is the best investment you can make, President Obama told students last month at the University of Colorado.

As a metaphor for the benefits of education, that statement is fine. But taken as a claim about the financial returns of a college degree, it poses two problems.

The first is that students and their families still lack sufficient data to estimate long-term returns for specific college degrees the way investors do with stocks and bonds.

The second problem is one of investment risk. Stock investors can manage risk by buying a diverse basket of shares, but a college student bets on a single asset: himself.

That makes it crucial that students and their families understand the factors that affect the risk and return of a college investment in order to swing the math in their favor.

College brings higher pay: $1,053 a week for the median bachelor's degree holder last year, versus $638 for a high school graduate with no college, says the U.S. Bureau of Labor Statistics. Last year's unemployment rate was 4.9% for college grads versus 9.4% for those with no college.

The College Board, a not-for-profit association, calculated in a 2010 report (based on 2008 data) that a typical student who enters a four-year college at age 18 and borrows his way through earns enough by age 33 to make up for his costs, including foregone wages and loan interest.

If a bond paid for itself that quickly, the return would be between 5% and 6% a year. That's a handsome payoff; stocks have historically returned around 7% a year after inflation. And it says nothing of college's other benefits, such as enlightenment, fun and higher job satisfaction.

Two big caveats: The College Board math assumes everyone goes to a public college. Those usually cost less than private ones