Does the Group in Group Psychotherapy Matter? A Meta-Analysis of the Intraclass Correlation Coefficient in Group Treatment Research

Objective: Over the last 3 decades, group treatment researchers have become increasingly knowledgeable of the impact of within-group dependency on analyses of group treatment data and of mutual influence processes that occur within therapy groups. Despite these advancements, there remains a lack of consensus on the magnitude of mutual influence, or group effects, in group treatment research. As such, this study sought to estimate the size of group effects on members' posttreatment outcomes by meta-analyzing the intraclass correlation coefficients (ICCs) in group treatment research. In addition, we tested several moderators of the ICC, including outcome type, outcome reactivity, outcome specificity, group format, treatment length, and group size. Method: Using robust variance estimations, we meta-analyzed 169 effect sizes from 37 group treatment studies. Results: Findings indicated an average ICC of 0.06. Group size, group format, treatment length, outcome specificity, and outcome type did not significantly moderate the ICC; however, we did find evidence to suggest that the ICC varies as a function of outcome reactivity, with observer-rated outcome measures resulting in the largest ICC. Conclusion: These findings suggest that interdependence in group treatment research is an important concept both theoretically and statistically. What is the public health significance of this article? This study found that therapy groups account for 6% of the variability in group treatment outcomes. Outcome reactivity significantly moderated the intraclass correlation coefficient (ICC) in group treatment research, such that the largest ICCs were associated with observer-rated outcome measures. Interdependence in group treatment research is important both theoretically and statistically.