Peer and self-ratings have been strongly recommended as the means to adjust individual contributions to group work. To evaluate the quality of student ratings, previous research has primarily explored the validity of these ratings, as indicated by the degree of agreement between student and teacher ratings. This research describes a Generalizability Theory framework to evaluate the reliability of student ratings in terms of the degree of consistency among students themselves, as well as group and rater effects. Ratings from two group projects are analyzed to illustrate how this method can be applied. The reliability of student ratings differs for the two group projects considered in this research. While a strong group effect is present in both projects, the rater effect is different. Implications of this research for classroom assessment practice are discussed.