This is from an excellent article by Andrew Hacker in the NYRB. He puts to bed various myths. Both tables tell a story.
Of course, bad teaching has varied causes. But it may be more apparent in STEM fields, where fixed material has to be covered; the reactions of many students suggests that many professors apparently see no need to make their teaching appealing. While student ratings have drawbacks, those in Table A hint at serious problems in STEM classrooms (see below). In mathematics, few freshmen meet a full-time professor close up. A recent survey by the American Mathematical Society found that 87 percent of all classes are taught by graduate assistants, adjuncts, or instructors on annual contracts.