Statistical analysis of group-administered intervention data: Reanalysis of two randomized trials

Group-administered interventions often create statistical dependencies, which, if ignored, increase the rate of Type I errors. The authors analyzed data from two randomized trials involving group interventions to document the impact of statistical dependency on tests of intervention effects and to provide estimates of statistical dependency. Intraclass correlations ranged from .02 to .12. Adjusting for dependencies increased p values for the tests of intervention effects. The increase in the p values depended on the magnitude of the statistical dependence and available degrees of freedom. Results suggest that the literature may overstate the efficacy of group interventions and imply that it will be important to study why groups create dependencies. The authors discuss how dependencies impact statistical power and how researchers can address this concern.