I have watched families lean on a ranking that was never built for them. The Academic Ranking of World Universities began in Shanghai in 2003 out of a project to benchmark China’s top universities against U.S. research universities, part of the country’s push to build world-class institutions. It measures research output and prestige: Nobel and Fields winners, highly cited researchers, papers in Nature and Science, and indexed publications. Each year it scores more than 2,500 universities and publishes only the top 1,000.

In my experience, some admissions consultants cite that same ranking to advise students who have no intention of a research career. It is the wrong tool applied to an entirely different question.

National rankings like U.S. News have a different flaw. The problem is not that any single criterion is wrong. It is that crushing many criteria into one number destroys information while manufacturing false precision. And once schools start chasing that one number, it gets worse. As the Cambridge anthropologist Marilyn Strathern put Goodhart’s Law in 1997, “When a measure becomes a target, it ceases to be a good measure.”

Ratings, which score a school on specific measures such as graduation rate or earnings, can help. Rankings, which collapse everything into a single place in line, mostly mislead. Knowing the difference changes which schools you take seriously.

This is Chapter 21 of the book. Read the full case in the book, or compare schools on investment terms in the model.

Read next: Is College Worth It? The argument in full, or this breakdown of when a degree costs more than it returns.