Grading leniency is a removable contaminant of student ratings.

It is well established that students' evaluative ratings of instruction correlate positively with expected course grades. The authors identify 4 additional data patterns that, collectively, discriminate among 5 theories of the grades--ratings correlation. The presence of all 4 of these markers in student ratings data (obtained at University of Washington) was most consistent with the theory that the grades--ratings correlation is due to an unwanted influence of instructors' grading leniency on ratings. This conclusion justifies use of a statistical correction--illustrated here with actual ratings data--to remove the unwanted inflation of ratings produced by lenient grading. Additional research can profitably seek other inappropriate influences on ratings to identify more opportunities for validity-enhancing adjustments.