I rarely use peer evaluations any more, so take what I offer with a
grain of salt.
 
My scoring essentially did what Jim Sibley suggests.
 
I put limits on the number of peers who could be rated in the top
category.  After all, if everyone is contributing reasonably well, there
isn't much room for several people to be outstanding.  In a good group
you expect similar contributions and similar ratings over the long haul.
 
Some of my groups figured out they could rotate the top category among
all the members over the semester.  A group that can figure out how to
work in harmony on their peer evaluations and stick to their agreement
all semester has done something well. If their reports or assignments
are working out well, then they are achieving the substantive goals of
the group.
 
Try giving some explicit guidelines for ratings, eg, "did not
participate in the discussion."  If you do, then everyone in the group
will actually speak up, or their group will ask them to.  Otherwise,
they can't legitimately hold to their agreement to fix the ratings. This
will legitimately give everyone in the group a similar rating.
 
Watch carefully for someone who is doing noticeably worse than their
group on exams and other evaluations.  They might be letting the rest of
the group do all the work.  You can suggest privately that they are
passing up an opportunity to learn by not contributing in a meaningful
way.  Correspondingly, someone who has much better individual RAT scores
than the group score is either not contributing or is not being listened
to by the group and the group may need to be told. 
 
Finally, there shouldn't be much point in worrying about it a great
deal.  Let them have the points and move on. Use this information as a
diagnostic, of group effectiveness.  If a group can conspire to game
your system, then they are working well on something.
 
Regards,
 
David Smith
 
David W. Smith, Ph.D., M.P.H., C.Stat.
Associate Professor, Biostatistics
Fellow, Institute for Health Policy
The University of Texas School of Public Health
San Antonio Branch Campus
voice: (210) 562-5512
e-mail: [log in to unmask]