Comparing the earnings of college graduates and nongraduates is an imperfect way of measuring the value of a degree, as I noted in my Tuesday column. Hypothetically, college graduates could earn more because they’re innately smarter — and would have earned just as much if they had never attended college or even any school. As a reader named Shawn posted in the comments section of the column, “The useful comparison is not between those who do and do not attend college but, rather, between equally intelligent students who do and do not choose to attend college.”

Fortunately, a large amount of economic research has attempted to make this comparison. And the results point overwhelmingly in one direction: College brings a large return to the vast majority of students who attend it.

Conducting this research is not easy, because finding otherwise similar people with different levels of educational attainment is not easy. But economists have found several ways to try to get around the problem. Many of these studies look for natural experiments in which otherwise similar people have different rates of college attendance and graduation.