There are four reasons why comparative data is needed for student ratings of faculty performance: (1) the considerable inflation of student ratings; (2) the great variability in the way students rate different items; (3) because student rating systems must be flexible and comparable; and (4) because of factors (such as student motivation, class size and academic field) which bias student ratings. Studies involving the Instructional Development and Effective Assessment System, the Student Instructional Report, the Purdue University Cafeteria System, the Instructor and Course Evaluation System, and the Course/Instructor Evaluation Questionnaire are used to illustrate these problems with student rating systems. It is concluded that without comparative data, use of student ratings for teaching improvements is misleading and use for personnel decisions is inaccurate at best. A 24-item bibliography is included. (BS)