When you team-teach a course, the students are supposed to try to evaluate each faculty separately at the end of the term, but in most cases it is not realistic to expect your particular contribution to be considered completely separately from that of your fellow teacher(s).
It may be very obvious to you what your specific contribution to the course was, and for you, as a professor, there is a dramatic difference in your experience depending on whether you are standing in front of the classroom talking or whether you are somewhere else during class (e.g., sitting in the classroom; working in your office; snorkeling around a tropic atoll). The combined efforts of the various instructors, however, created the class as a whole, and the students' overall experience in the class can't be perfectly compartmentalized.
This can be both good and bad for any individual instructor. Consider a course team-taught by two faculty with different teaching abilities: one good and one not so good. I am not going to discuss today what could be done to salvage a course that is partially taught by a bad teacher, nor dwell on how awful it is for students to be taught (or team-taught) by a poor instructor (though I do sympathize). In this discussion, I am going to focus only on the impact of disparate teaching abilities in a team-taught course on the teaching evaluations of the good teacher.
For the sake of discussion, let's assume that you are the good teacher. If I had time, I would graph the results, but for now, here are some possible scenarios for how your overall teaching evals might turn out:
If your colleague's teaching is truly ghastly, two possible outcomes for you are:
(a) Students will be so disgruntled about the awfulness of part of the course that no matter how good you are, your evaluations may suffer as students register their dissatisfaction with the course as a whole; or
(b) Students are so appreciative that you are not a ghastly teacher like your colleague that they will give you awesome evaluations.
The collective experience of some close colleagues and I have included outcomes (a) and (b).
I have personally benefited in my evaluations from teaching with a ghastly teacher, but I think that I benefited in large part because we divided the course exactly in half, and my colleague taught the first half. By teaching the second half of the course, I had many weeks to get the students feeling more positive about the class and this showed when they filled out their evaluations. Perhaps if I had taught the first half of the class, and then my colleague had spent the second half systematically destroying the students' psyches, they would not have had recent memories of liking my teaching. Their overall opinion of the course would have been negative. I don't know if my positive effect on the second half of the class made the students more forgiving of my colleague, but I rather think not, as his evaluations were rather savage.
I have a colleague who is an excellent teacher and who has also team-taught with an appalling teacher. In his case, though, the two instructors alternated teaching throughout the term, so it was more difficult for my colleague to have an overall positive impact on the course. I think that the students enjoyed the days he taught, but he had hints from student comments during the course that they might blame him for not fixing the problem of the bad teaching of the other instructor. In fact, some of the comments indicated that students thought that he was the real professor, and the other professor (a woman) was his assistant who substituted for him when he was off snorkeling in the Maldives*. Some students may have blamed him for inflicting his atrocious 'assistant' on them. In that case, the chances of benefiting from the better-by-comparison effect in teaching evaluations are not so good.
The variables in any team-teaching scenario include how you divide up the course in terms of responsibilities and teaching schedule, and what the absolute and relative magnitudes of the instructors' teaching abilities are.
Whatever the case, departments should carefully consider how team-teaching arrangements are structured, particularly when assigning assistant professors to team-teach courses. Team-teaching early in your career can be a great experience (and not just in terms of teaching evaluations; you may also learn important things about teaching if paired with an experienced teacher with whom you interact in a positive way) or it can be a bad one from which it is difficult to recover.
* He was not snorkeling in the Maldives (or anywhere). He was doing other academic activities on campus (research, advising, doing essential service activities etc.).
12 hours ago