Mechanisms of change in control group drinking in clinical trials of brief alcohol intervention: implications for bias toward the null

Reductions in control group consumption over time that are possibly related to research design affect the impact of brief alcohol interventions (BAI) in clinical settings.We conducted a systematic review to identify research design factors that may contribute to control group change, strategies to limit these effects and implications for researchers. Studies with control group n30 were selected if they published baseline and outcome consumption data, conducted trials in clinical settings in Anglophone countries and did not censor gender or age.Among 38 studies cited in 20 reviews through October 2009, 16 met criteria (n = 31-370). In 54%, controls received alcohol specific handouts, advice and/or referral. Both the number and depth of assessments were highly variable. The percentage change in consumption ranged from-0.10 to-0.84 (mean-0.32), and effect size from 0.04 to 0.70 (mean 0.37). Published data were insufficient for meta-analysis.Researchers should consider strategies to reduce the impact of research design factors, such as procedures to enhance sample diversity, blind subjects to study purpose to limit social desirability bias, reduce the number and depth of instruments (assessment reactivity), and finally, analytic techniques to decrease the impact of outliers and regression to the mean.This review identifies problems with retrospective analysis of predictors of control group change, and underscores the need to design prospective studies to permit identification, quantification and adjustment for potential sources of bias in BAI trials.