Reliability refers to the consistency, stability, and replicability of a measurement. The consistency of student evaluations refers to the extent that students within the same class give similar ratings on a given question. Good consistency is achievable with class sizes greater than 30. Class sizes of 10 or fewer students will probably not produce adequate consistency. The stability of student evaluations refers to the agreement among raters over time.
Student evaluations tend to be fairly stable.
Thus, one can expect to see good agreement between ratings at the end of the semester and ratings given by those same students years after graduation. Our institution spends a lot of time and effort surveying graduates about teaching effectiveness for tenure decisions. The literature suggests that little if any new information is obtained because of the high stability levels of student evaluations. The replicability of student evaluations refers to the extent that the same instructor is rated the same for the same course over a number of semesters and for all his or her courses. Replicability is high for both the same course over a number of semesters and for different courses taught by the same instructor.