Should you grade on a curve or not? If you are student, the answer is clear: go by whatever the instructor does. Otherwise, you have a choice. I don't like to tell other instructors or faculty what to do because I respect their freedom. For my classes, there is no curve. Why? Well, the question really is: "why grade on a curve?" I don't know the exact reason for particular instructors, but I can come up with some possible reasons.
This is a very common curving reason. The basic idea is that the class is a competition between the students. The strong survive. The weak fail. In this case, class is sort of like the olympics in that not everyone can win. But it is also a case that not everyone can fail. This is why many students ask to be graded on a curve (he can't fail everyone, can he?).
I like to think that learning is not a competition. Competition in learning would discourage collaboration and students helping students (which I think is a great way for them to learn). Without a curve, it is possible for everyone to succeed.
I like this reason better, although I still don't agree. Some faculty say they curve because maybe the test was just too difficult. That can clearly happen. The question I think instructors should ask themselves "what are the key ideas I want to evaluate students?" Also: "what questions would be able to show that students know that stuff?" If you really know the answers to these two questions, then you should be able to make a test that is fair.
Example: After finishing the stuff on Newton's second law, I would expect that students could:
- Correctly draw a free-body-diagram
- Solve for an unknown force on an object in equilibrium or with constant acceleration
- Determine the motion of an object with known forces on it
- etc....
So, a reasonable test question would be to perhaps find the acceleration of a block sliding down an inclined plane. One mistake I have seen many faculty make is that they are afraid. Afraid that everyone will get an A on the test. Trust me, that is not going to happen. That problem may seem trivially simple to you, but perhaps you forgot what it was like to to an introductory physics course. If that is a reasonable objective, it is a reasonable objective.
On a related note, I try not to make my tests too long. My general rule of thumb: I write the test, then I make out the key (with complete solution). If I can make the key in 10-15 minutes, then a student should be able to complete the whole test in 50 minutes (well, most students). I am not a big fan of time limits, but sometimes you have no choice.
Ok, back to curving. The idea of the curve in this case is to assume the following:
- Any given class is a good normal distributions of students from the whole population of students.
- The whole population of students has a normal distribution of abilities. Where by normal distribution I mean that most students would be in the C range. There would be fewer in the F and A ranges.
So, the instructor writes a test. Assuming the students are normal, you can essentially determine the universal difficulty of the test and adjust the grades accordingly. That seems like an ok idea, but when you have smaller classes it is possible that the class is not a good representation of the entire population of students. Also, doing this essentially makes class a competition (whether you want that or not).
There are some instructors (not any that I know, but I am sure they exist) that curve just because. They curve because that is they way they were graded and that is what other instructors do. Perhaps they curve because the students want that. On a side note, students in introductory astronomy for non-science majors WANT multiple-choice tests even though they perform significantly poorer on them.
If an instructor (by instructor I mean anyone who teaches a course) falls into this category, he or she should think about why they do what they do.
