Testing for group membership effects during and after treatment: The example of group therapy for smoking cessation

Behavioral interventions often are administered in groups, yet the effects of group membership rarely have been evaluated. The current research examined 33 groups of clients (M = 5.5 clients per group, SD = 2.5) volunteering for a group smoking cessation intervention. The intervention consisted: of 6 group therapy sessions over an 11-day period. Attendance at the sessions and smoking behavior during the 11-day period were the dependent variables. Hierarchical linear modeling (HLM) revealed a statistically significant (p < .05) degree of interdependence among group members (intraclass correlation = .44 and .32 for attendance and smoking, respectively). Groups were characterized disproportionately by high attendance and infrequent smoking or low attendance and relatively frequent smoking, with fewer moderate groups than would be expected by chance. Group membership effects dissipated within a month following treatment. These findings suggest the need for assessing such effects and for future research examining their causes.