Thanks Peter and Paul for the responses. As far as the group sizes are concerned I'm afraid that is out of my hands. The issue I have is not of not completing the assessment at all. We do as you suggest and make it clear that severe penalties can result. The issue I have is of satisfactorily dealing with student responses when they complete the assessment but do not assess everyone in their team. This generally for the understandable reason that they do not feel that they had enough contact with them to score them accurately for a particular characteristic. My current solution is to award the mid-range score in a Likert scale (say 3 from a 5 point scale) to the student not assessed.