This paper aims to make the techniques of cooperative learning more attractive to teachers by presenting a method of assessment that avoids the drawbacks associated with trying to extract valid and reliable individual marks from cooperative performances. The paper presents an easy-to-use method of assessing an individual's contribution to a cooperative performance. The method makes efficient use of the teacher's time, has a built-in reliability measure, and validity checks. By separating the performance criteria from the assessment process the method also highlights cooperative dynamics within the group and identifies a fundamental learning problem faced by less able students across types of performance and subject content areas. Participants in the study that developed the approach were 57 undergraduate and graduate students in 8 groups. Students were asked to assess the contributions of each member of the group, with the assessment of the project itself kept separate. The product assessment was weighted by the number of students in the group and combined with the confidential peer assessments to arrive at a mark for each person. The variation in the marks received by a group member were a measure of the reliability of the member's marks, and the reasons given by peers were an indication of the construct validity of the group member's mark. Results with the eight groups support the use of the technique. (Contains 2 tables and 18 references.) (SLD)