Among these are the fact that groups studied by researchers have to conform to the rigors of the controlled experiment.
Members have to be randomly assigned to a control group or to the self-help group.
One would have thought that the motivation to participate in a group and the group's effectiveness were inseparable. But this is far from clear. For example groups for migrane headache do not attract members despite their effectiveness, while less effective groups may attract more members. This suggests that motivation for participation and effectiveness may actually come apart in some groups for some conditions. It is an interesting result, if verified, because it would mean that self-selection is not necessarily a vital variable for effective mutual aid functioning, and this would remove one of the strongest objections to the effectiveness studies already on the books.