Student ratings of a course are usually administered during the last days of class, on the day of the final examination, or following receipt of grades. Bias may be introduced simply by choosing a “convenient” time. In this study equivalent random groups of medical students rated an anatomy course at one of three times: immediately before or after the final examination or by mail after receiving grades. The effects of course achievement and timing on favorableness of rating were examined using analysis of variance. Results indicated a significant achievement effect but no difference in rating means due to timing. Students responding to the mail survey represented an academic cross-section of the class. Students present on the last two days of class were higher achievers and on subsequent evaluations gave higher ratings than did absentees, indicating that ratings administered on the last days would have produced a favorably biased response. Thus, comparative analyses among courses may be invalidated by including the ratings of both biased and representative student groups.