ISSUE OF STATISTICAL POWER IN COMPARATIVE EVALUATIONS OF MINIMAL AND INTENSIVE CONTROLLED DRINKING INTERVENTIONS

An analysis of recent studies of minimal and intensive cognitive-behavioural treatments for problem drinking was undertaken to decide to whether a lack of statistical power explains the failure of the majority of studies to find a difference in outcome between these two types of treatment. Although the sample sizes have typically been small (n = 12-21), the analysis suggests that low statistical power is unlikely to be the explanation for the majority of null findings. It seems more likely that the difference in outcome between one positive study and the majority of null results reflects some combination of differences in the type of clients who were treated, the therapists' experience, and the type of intensive therapy that was provided. The low power of these studies demonstrates the desirability of researchers calculating the sample size required to an effect before commencing an outcome study. If they continue to undertake studies with small sample sizes, then they should refrain from inferring that the failure to reject a null hypothesis means that there is no difference between treatments. © 1991.