Just this week I was marveling that I had not yet been asked to do any departmental work this summer, then wham.. I got 'asked' to do two things: read an undergraduate honors thesis (by tomorrow, because the professor who was supposed to do it is away) and serve on a grad student committee for an exam. Professors are not paid by the university in the summer, but we are expected to 'volunteer' for these activities because [select explanation from menu of options below].
#1: It's not the students' fault that faculty don't get paid by the university in the summer, yet student schedules for exams, theses, etc. don't always fit into the academic year.
#2: We get paid enough during the other 9 months, so we should work for free during the summer.
#3: We have lots of free time in the summer, so we might as well spend some of it doing administrative work.
The best explanation, at least in the case of faculty who are active year-round in research, is #1, but I've heard the other two expressed. I don't mind doing some student-related work in the summer, but I do mind the situations in which I am asked to do something just because I am here in the department and some of my colleagues are not. Some of my out-of-town colleagues are out of town for research-related reasons, but some are taking the summer off. That's fine, they can do that because they aren't being paid by the university or by grants, but it means more summer 'volunteer' work for the rest of us.
11 years ago