TIME and again group therapy outcome has proven to be a quagmire for clinical researchers. Reviews of evaluation studies have served primarily to underscore the pitfalls involved, which include not only all those encountered in individual therapy evaluation but several idiosyncratic to group therapy.1-3 Satisfactory outcome measures have not been developed; numerous studies have indicated that many prepost psychometric tests, of value in individual therapy outcome studies, do not accurately reflect the changes undergone by group therapy patients.4,5 Often improvement in the group (group maturation, increased cohesiveness) or improvement of the patient's group behavior has been erroneously equated with patient improvement.6 Clinical studies in which the group therapist evaluates the outcome of his own patients suffer from observer bias.
Another obstacle is the high attrition rate in group therapy, which has forced many investigators into the untenable position of conducting short-term follow-up
Yalom ID, Houts PS, Zimerberg SM, Rand KH. Prediction of Improvement in Group TherapyAn Exploratory Study. Arch Gen Psychiatry. 1967;17(2):159-168. doi:10.1001/archpsyc.1967.01730260031005