Does the group in group psychotherapy matter? A meta-analysis of the intraclass correlation coefficient in group treatment research.

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. Using robust variance estimations, we meta-analyzed 169 effect sizes from 37 group treatment studies. 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. These findings suggest that interdependence in group treatment research is an important concept both theoretically and statistically. (PsycINFO Database Record (c) 2020 APA, all rights reserved).