Ah yes, I get your point. It looks like no solution is going to entirely iron out these skews.

However, should it be deemed that what I am aiming to...

Good point Tony. Maybe what I should have made clear is that these grades represent each teacher's opinion of the work done in a half term against criteria that are...

That is an interesting point Jeff. Let me try and explain better why I am attempting it.

A student studies some subjects and is awarded grades by each teacher. There...

On futher investigation it seems that there are values that could be null in the column that I want to average. I set those values to null so that I...