I like to use a scoring system from 0-3 (Hattip to Professor Dave Garvin at Harvard)
3- added significantly to the discussion with an insight that shows evidence of synthesis and/or deep insight
2 – contributed in a meaningful professional way that demonstrates appropriate mastery of the subject and evidence of preparation for class
1- attended but was a free rider in the discussions, did not contribute to the learning of others
0 – did not attend
Some people learn by talking and hearing their thoughts and through collaborative dialogue. If they say it 9 times wrong and then 1 time correct when they finally get it, did their 10 “contributions” outweigh the participative value of the introvert who has carefully prepared their insight, and offers a single cogent, deeply penetrating insightful summary that makes us all smarter?.
Participation grades are not “merely” opinion. A professional opinion, inhabiting the land between pure objectivity and pure subjectivity, is an opinion that is informed by both theory and practice and carries more weight than “just an opinion”. This point of view reflects a consideration of teaching as “craft” in which informed judgment is more than “just an opinion”. The judgment is not absolute, can be judged by peers and students, and be subject to calibration and standards of evidence like all craft work.
One of my concerns with grading participation concerns the motivation to participate: we want people to participate as a way to encourage an inquiring mind for its own sake, and not in order to meet a minimum number of speaking events in public to secure a grade. It seems to me that the effect of the contribution on others and as a window into the preparation and thought processes of the student is more important than the motivation behind the offering, and so, to “reward” the participator, and to respect the effort they put into the participation, it seems fair to assign grades for participation based on professional judgment. Studies of allowing anonymous peer grading demonstrate that in adult education peers are pretty well aligned with teacher judgments about quality of contribution.