With student evaluations of instructor effectiveness playing an increasingly important role in the determination of merit pay, promotion, and tenure, there is a growing interest in what these evaluations actually measure. Faculty members frequently voice doubts about using student evaluations, because it is not clear to what extent they measure the leniency of the instructors, the amount the instructors taught the students, or the performing ability of the instructors. Previous studies of the problem have not been satisfactory. This paper presents a sequential, three-equation model to determine the effects of learning and leniency on evaluations. The variables include: previous knowledge of the concepts of the course; amount of previous course study; amount of related course study; previous academic average; academic year of the student; time the class meets; size of the class; and sex of the student. The model was applied to students in 14 sections of the microeconomics protion of the "Principles of Economics" course at the University of Western Ontario. The results indicate that evaluations do not depend on leniency. (LBH)