How do you measure rape and sexual assault? It’s a tricky and loaded question, and the answer impacts a couple of highly charged debates. As in: If you believe the measurements that say sexual violence against women is significantly on the wane—as one prominent national survey shows—then you might argue against spending a lot of money fighting it. Or you might argue, as Slate’s Amanda Hess does, that binge drinking among women doesn’t really explain the problem of sexual assault, since the drinking has increased even as the rape numbers have fallen.

On the other hand, if you’re worried that the same measurement tool—the Justice Department’s National Crime Victimization Survey—is vastly undercounting sexual violence against women, especially when it comes at the hands of men they know and in the company of drinking or drugs, then you might agree with Emily Yoffe that it’s time to stop letting “a misplaced fear of blaming the victim” prevent college educators—and the rest of us—from warning “inexperienced young women that when they get wasted, they are putting themselves in potential peril.”

How helpful, then, that the Justice Department asked the National Research Council (part of the National Academies, which also includes the National Academy of Sciences and the Institute of Medicine) to study how successfully the federal government measures rape. The answer has just arrived, in a report out Tuesday with the headline from the press release: “The National Crime Victimization Survey Is Likely Undercounting Rape and Sexual Assault.” We’re not talking about small fractions—we’re talking about the kind of potentially massive underestimate that the military and the Justice Department have warned about for years—and that could be throwing a wrench into the effort to do the most effective type of rape prevention.

Read it at Slate.