http://www.slate.com/articles/life/education/2015/05/a_defense_of_student_evaluations_they_re_biased_misleading_and_extremely.html

Everybody has reasons to hate student evaluations. If they represented the judgments of one individual, rather than being a big heterogeneous data set, we would call that individual sexist. Women face double standards and, often, tougher expectations. Students in some experiments give higher scores to teachers who they think are men. Evaluations may also favor the white and the young, or punish the homely (when we find someone attractive, we tend to rate their other qualities more highly, in what psychologists call the halo effect). And better scores don’t mean that students learned more—not if you go by subsequent performance in sequenced classes (e.g., Spanish 1 and Spanish 2) in which you can measure such things.