Evaluation of college instructors often centers on course ratings; however, there is little evidence that these ratings only reflect teaching. The purpose of this study was to assess the relative importance of three facets of course ratings: instructor, course and occasion. We sampled 2,459 fully-crossed dyads from a large university where two instructors taught the same two courses at least twice in a 3-year period. Generalizability theory was used to estimate unconfounded variance components for instructor, course and occasion, as well as their interactions. Meta-analysis was used to summarize those estimates. Results indicated that a three-way interaction between instructor, course and occasion that includes measurement error accounted for the most variance in student ratings (24%), with instructor accounting for the second largest amount (22%). While instructor -- and presumably teaching -- accounted for substantial variance in student course ratings, factors other than instructor quality had a larger influence on student ratings.