Risk-adjusted returns to education?

Here's an economix blog post with a fun graphic comparing the returns to various investments including education, based on this post over at Brookings. As the Brookings post points out, they haven't risk-adjusted the returns, but they say the risks are greater for workers with low education, and presumably they're talking about unemployment risks. The fact that the rate of return for the associate's degree is so high really points something out: these are the rates of return for successfully acquiring the asset in question, and for some the acquisition of an associate's or bachelor's degree is far from certain. You can pay to go to college, but there's no guarantee you'll get the degree. Untangling this might make for an interesting dissertation paper for an enterprising Ph.D. student.