I have had a couple of experiences with cohort groups. The first was an experience where I had not truly learned class control yet. That particular group was very hard to keep in line and was a big distraction for the rest of the class. Since that point I have learned to find who the cohort groups were and use them to my advantage. For example if I were teaching transmissions, I would first find out who that students were that had been to another trans. class or had field experience. I would then separate that group and make lab team leaders out of them. This tends to have a great outcome in the lab as well as the class, because in most cases that lab group would then come back to class and sit near eachother. This keeps them from getting bored and causing distractions because thaty are kept busy.