A recent comment brought up the traditional criticism of grading on curves -- what if 80% of a particular class did really well, and deserves As? what if 80% did badly, and deserves Fs? why not grade objectively rather than comparatively? -- so I thought I'd repost my paean to curves from 2002. (Note that 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.)
Lots of people really oppose curves. Shouldn't people be graded on their own merits, they reason, 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 surely do not know the difference 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:
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 not because this student is unusually bad, but because the exam was just harder than ones from previous years.
Even setting the previous factor aside, I've been in teaching for 12 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. (Likewise, in undergraduate institutions, many classes are traditionally taught by relatively inexperienced teachers.) Where are they going to get the distinction between A-s and B+s?
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. And the variation in class strength, especially classes of 50-100 students (the size of nearly all my non-seminar classes)), is likely to be much less than variation in professor harshness.
The pressures for grade inflation are quite real, and flow from basic human nature: Most people don't like giving students low grades, especially once they've spent many hours with them. 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.
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.
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