What follows are some reflections on the usage -- not the meaning -- of the metaphor "grading on a curve." When people say that the media are grading Donald Trump on a curve, the meaning is quite clear: He gets a passing grade if he doesn't come across as a blithering idiot. That, I think, is quite different from what teachers -- and students -- mean by "grading on a curve."

Students worry about being graded on a curve because it means that, no matter how well all of them do in some objective sense, the curve is going to force the teacher to give some of them "bad" grades -- worse grades than they would have received had the course been graded according to some objective measure of performance. If a class is graded objectively, when everyone in the class gets a 90 or above on the exam, everyone might get an A, whereas if the class is graded on a curve the students who get "only" 90 on the exam will get low grades. And, conversely, if everyone does badly in some objective sense, grading on a curve means that someone who does just a little bit less badly than everyone else will get an A. Grading on a curve forces teachers to give bad grades to people who perform objectively well, and to give good ones to those who perform objectively badly, depending entirely upon what other students in the class do.

The usage of "grading on a curve" in discussions of Donald Trump seems to me just the reverse: The media are applying an objective standard to his performance. (One indication is that no one seems to think that Trump is performing better than Clinton in the contexts in which the metaphor has been used, which means that, if we really were grading on a curve, he'd being getting a bad grade.) The metaphor for that, which has also appeared, is "setting a low bar" -- that is, making it easy to get a passing grade.