Student evaluations of teaching may be subject to halo effects, where answers to one question are contaminated by answers to the other questions. Quantifying halo effects is difficult since correlation between answers may be due to underlying correlation of the items being tested. We use a novel identification procedure to test for a halo effect by combining a question on lecture-room capacity with objective information on the size of the lecture room. We confirm the presence of halo effects but show that the responses to the contaminated question remain informative. This suggests that the distortion in the evaluation questionnaires caused by halo effects need not be a concern for higher education institutions.