As student evaluations become more commonplace and increasingly influential in assessing teaching, it becomes important to gain skill in interpreting such data. To determine the effect of class hour (one potentially contaminating variable) on students' ratings, an analysis was made of ratings given the same instructor on the same course in the same quarter by students enrolled in different sections. Not only did the statistic used reveal a significant difference from one class hour to another; it also supported the assertion of consistent treatment within groups. It is recommended, therefore, that student evaluations not always be taken at face value. (Author)