In the manual.
Its form, good, med, bad. I'm unsure how it changes as I have left a driver alone while using others and it fluctuate all over the place irrelevant if they've been driving or not, winning or not, winning easily or only just, etc. It is possible its entirely random.
The only correlation I've found is that they go down often when going up a B-spec level group, so from 4-5, 9-10, 14-15. etc (When you can get another driver)