In praise of grading on a curve

I was recently reminded of the traditional criticism of "grading on curves," which is to say grading a class in such a way that, say, 20 percent get As, 60 percent get Bs, and 20 percent get Cs (perhaps with similar breakdowns for the +/- grades): What if 80 percent of a particular class did really well, and deserve As? What if 80 percent did badly, and deserve Cs? Why not grade "objectively" rather than comparatively? Shouldn't people be graded on their own merits, rather than based on how other students have done? After all, they ask me, don't you know the difference between an A exam and a C exam?

Well, yes, I do—but I can't tell, just by looking at the exams, where to draw the line between an A- exam and a B+ exam. And this ties in to some of the reasons why grading on a curve is the lesser of evils:

1. Sometimes I draft a hard exam and sometimes an easy one. I often can't tell which is which, since they're all easy to me—I know the material, after all! So something might look to me like a C exam rather than a B- exam not because this student is unusually bad, but because the exam was just harder than ones from previous years. (I write a new exam each year, rather than reusing questions.)
2. Even setting the previous factor aside, I've been in teaching for 21 years now—but many professors are new, and don't even have the data points that I have. In some areas, such as legal writing, the typical teacher has even less experience. Where are they going to get the distinction between A-s and B+s?
3. Perhaps the curve is unfair to a class that consists of unusually strong students—but the absence of a curve is unfair to a class that has an unusually harsh professor (which includes a class that has a professor who grades in a way that you see as fair, but that is harsh compared to other unusually lenient professors). And the variation in class strength, especially classes of 50-100 students—the size of most non-seminar classes—is likely to be much less than variation in professor harshness.
4. The pressures for grade inflation are quite real, and flow from basic human nature: Most teachers don't like giving students low grades, especially once they've gotten to know their students relatively well. When I have small classes that can't be curved as easily (since there are so few data points that there's a higher chance that the class is unusually strong or weak), I feel this pressure myself, even if the class is still blind-graded. And of course if a professor is known for resisting this pressure, then fewer and fewer students will end up taking his class.
5. Some people argue that the curve makes things harder for students to get jobs, but I don't think that's right. To be sure, a curve that's harder than the curve at other comparable schools might make things harder for students, since employers might erroneously think that someone with a B+ average at school 1 has done worse than someone with an A- average at school 2, even if both grades are (say) at the 70th percentile in each school, and the difference is just a result of different curves. But that's a reason to deliberately align the curves with your competitors, which in my experience law schools tend to do, not to abolish the curve (which could likewise lead to differences in median among schools because of differences in grading cultures). Indeed, a curve makes it easier to make sure that the median grade at your school is comparable to the median grade at competitors schools.

Of course, there may also be tweaks to the curve that might be worth making, like setting the norm for elective classes to be equal to the average GPA of the students in the class: This would take into account classes that attract unusually strong students or unusually weak students, and it would work because schools generally don't have classes in their first semester of law school. But this is a small change on the margins, which might or might not be worthwhile in practice.

There are, I'm sure, many more advantages to the curve; and I think these advantages vastly outweigh the disadvantages. Like democracy, grading on a curve may be the worst possible system—except for all the alternatives.

A few notes:

A. By "curve" I mean a range of ways to assure a relatively consistent distribution of grades from class to class. One model, for instance, would be to have 20 percent Cs (or below), 60 percent Bs, and 20 percent As, with similar subdivisions within each grade (though perhaps with some flexibility, whether +/-3 percent on each category, or with grades of A+ and C- being entirely optional).

B. This discussion is about curves in large classes, of about 30 students or more; for smaller classes, such as 12-student seminars, the curve is not apt, though of course there's some controversy about where the cutoff size should be.

C. The discussion doesn't deal with unusually low grades. To my knowledge most top law schools don't have any mandatory Ds or Fs; the mandatory distribution is just for the As and Bs, and the Ds or Fs (if the instructor chooses to give any) would come out of the C or C- allotment.

UPDATE: Some commenters object that curved classes are too easy, so that people who just care about, say, getting a B won't do much work in a class that they know will be 80% Bs and above (or maybe even 95% Bs and above). But that's a concern about the likely grade distribution, not about the curve generally.

You can have the same problem even if classes aren't curved, but it's known that a particular professor (or even all professors in a department or a school) gives very few grades below C. Conversely, if you have a curve in which, say, 50% of the class get Cs or below, then most students will know that they have to work to get a higher grade (unless they are very confident that they are so good that they can get a B without any work, and all they want is a B—in which case they won't work hard even in an uncurved class where they still know, from past experience, that the instructor will give Cs to at most half the class).