Weighting Grades based on group size I am having students complete a presentation as a group project. Each student's grade will have two components: an instructor grade, from me, and a group grade from their group.

The group grade is calculated as the sum of five elements, each graded out of five, for a maximum of 25 points from each member of the group. Each student's 'group grade' is then calculated as the average of scores from the group.

The problem I have is that some of the groups have 4 students, some 5, and some 6. This means that weight of the score for students in larger groups is less than for those in smaller groups.

I plan to compensate for this by weighting the instructor grade more heavily. But as I think about it, even this doesn't cancel out the discrepancy.

How can I weight, or adjust, my grading scheme to make sure it's as fair as possible?
I don't see a problem with two-part grades where all instructor grades are weighted equally. Your issue seems to be that individual teammates' opinions carry less weight in the larger teams. But I don't see why that is necessarily unfair. 

My suggestion would be to calculate the grades based on the same instructor weights for everyone and then make sure, before finalizing, that people in larger or smaller groups didn't get systematically different grades.