The problem is that students are terrible judges of who’s a good teacher. Because learning is not always pleasant, they end up punishing teachers who teach the most and rewarding the instructors who challenge them the least. An extensive body of research shows no correlation—or even a negative correlation—between how students do on objective learning assessments and how they score their professors. One experiment found that Harvard physics students learned more from “active learning” instruction but thought they learned more by passively listening to a lecture. Another study demonstrated that Air Force Academy students who were taught by highly rated professors tended to do worse in subsequent classes.
Evaluations are also vulnerable to just about every bias imaginable. Course-evaluation scores are correlated with students’ expected grades. Studies have found that, among other things, students score male professors higher than female ones, rate attractive teachers more highly, and reward instructors who bring in cookies. “It’s not clear what the evaluations are measuring, but in some sense they’re a better instrument for measuring gender or grade expectations than they are for measuring the instructor’s actual value added,” Philip Stark, a UC Berkeley statistics professor who has studied the efficacy of teacher evaluations, told me.