LETTER TO THE EDITOR REGRESSION TO THE MEAN: WHAT DOES IT MEAN?

Johnsson and Berglund’s recent article evaluating two interventions for high-risk drinking in university freshman could be regarded as a model of appropriately conservative interpretation of experimental results (Johnsson and Berglund, 2006). The authors found statistically significant reductions in drinking from baseline to one-year follow-up but little evidence of differences between the two, randomly assigned intervention conditions. Given these findings they make the point that the results could be due to regression to the mean, an admission that is rare in the clinical trials litera-ture. However, some benefit might be derived from consider-ing alternate explanations for this pattern of results. One explanation is provided by the authors—that the results can be explained by regression to the mean. This is a statistical concept which is used to describe the phenomenon that