For the average graduate, going to college is a wonderfully profitable investment. The evidence is unambiguous. Even after subtracting tuition and all the years of foregone salary, the pay boost from a degree will still pay for itself, and then some. The problem is that the “average” college student doesn’t really exist; she’s an imaginary amalgam of state school grads and Ivy League alums, of education majors and engineering nerds.

Once you ignore averages, and start looking across the entire earnings spectrum, the question of whether higher education is financially worthwhile for everybody becomes more complicated. Recently, researchers from the Federal Reserve Bank of New York noted that the bottom 25 percent of college degree holders basically earn no more than the median worker who ended his or her education after high school.

“While we can’t be sure that the wages of this group wouldn’t have been lower if they had never gone to college,” the New York Fed’s researchers wrote, “this pattern strongly suggests that the economic benefit of a college education is relatively small for at least a quarter of those graduating with a bachelor’s degree.”

Other researchers have made similar observations. In a lengthy review of the literature on the economic value of college, for instance, Philip Oreopoulos and Uros Petronijevic point out that among Americans between the ages 30 and 50, the median college graduate earns less annually than the top 10 percent of high school grads.

And what about over a lifetime? A widely cited study by economists Christopher Avery and Sarah Turner found that, among men, some exceptionally well-paid high school grads could expect to make more money during the course of their career than lower-earning bachelor’s recipients. This summer, writer and economist Allison Schrager mimicked their analysis in Bloomberg Businessweek, but included women and part-time workers in the math. She found that, long-term, a college graduate at the 25th percentile of income should expect to earn less than the typical working high school grad.

The bottom line: A large minority of college grads earn like high school grads. So what do we make of that fact?

One possible interpretation is that college simply isn’t worth it for a chunk of students, even if they graduate. That may be true to a degree. There are some young adults who would probably end up wealthier if they learned a trade or pursued a technical degree from a community college than they would by majoring in communications at, say, a nonselective state school.

But there are other issues to consider. The New York Fed says “we can’t be sure” if low-earning college graduates would have made even less money without their degree. But we can be close to sure. The strongest academic evidence suggests that regardless of their background, any given individual does boost his pay by going to college, even if he’s only a so-so student. The question is whether that earnings bump will outweigh the cost of school. In one important study of marginal students who were barely admitted to Florida’s state university system, the answer was yes. But at a more expensive private institution, the return on investment might turn out to be nil (or negative).

The useful question isn’t whether college is worth it to some students. It’s whether individual colleges offer a worthwhile return to the population of students they serve. The fact that some college graduates earn less than high school grads is one more reason for us to worry about the cost of tuition, not to doubt the value of higher education as a whole. And we should call B.S. whenever a school markets itself to potential undergrads based on the average income that a college grad can make.