[sebastiennight]

So there might be zero effect?

If it's purely a correlation, then maybe those students would be more successful than average even without the study group. They're already the most motivated kids. Maybe they just do "motivated kid stuff" and would still outperform.

[cgearhart]

Yes, that’s what I think at this point. There is no effect of the study group except as a support group. (That’s all it was for me when I was a student and joined the self-organized study group.)

[sebastiennight]

My self-organized study group was quite a mix of characters, and while I don't doubt that a couple of my friends's grades did benefit from having me there to help them out, I do definitely think that most of the stuff I remember learning there was outside the boundaries of what any of our parents would have approved :)