... is a rare thing, but Eric Rasmusen is an unusually clear thinker:
IU policy is for professors to allocate 10 minutes of a class for students to fill in a rather silly 20-question questionnaire to rate them on a scale from 1 to 7. This is really the only way the University keeps track of teaching quality. And it matters, for salary and for keeping one's job if one does not yet have tenure.
Same at UCLA, except we use a 9 point scale. Eric then observes some serious problems with using student evaluations to "grade" professors.
Why, then do we rely so heavily on student evaluations? It is hard to believe that professors and administrators do not realize how weakly they measure the amount a teacher has taught his students. Even if they did not, if good teaching was the objective, surely we would pay some attention to the syllabi and what kind of tests were given and use objective evaluators -- students or faculty observing single class sessions -- which we do not do in any serious way. Rather, I think that "good teaching" means "contented students" for the people who rely on student evaluations. Student evaluations are indeed a good way to measure this. And it is a reasonable objective. Administrators are trying to sell a product, and if you view the student as a customer rather than as someone to whom you have a moral obligation, you want to design a product that he wants. The student will likely want a course that has a low workload and gives him a pleasant feeling of accomplishment while being described as difficult course on an advanced topic. Professors have incentives similar to administrators-- it is more fun teaching contented students, and while it is quite difficult to know how to make students learn (I know that after 20 years I still don't know when I have succeeed and when I have failed, or even whether I, as opposed to the students' own efforts, make much difference), it is much easier to figure out how to make students pleased.
This is a key problem with higher education. The system creates strong incentives for us to gear our teaching to producing happy rather than well-educated students. Go read the whole thing.