A meta-analysis of the effectiveness of placebo manipulations in alcohol-challenge studies.

Our meta-analysis examined the impact of specific alcohol placebo procedures on two manipulation checks (participant reports of number of alcohol drinks consumed and subjective intoxication) to determine which procedures produced the smallest effect sizes in comparisons between alcohol and placebo conditions. Databases for the years 1990-2007 yielded 44 studies that met inclusion criteria. These were subjected to detailed coding of procedures pertinent to placebo effectiveness. Alcohol versus placebo condition comparisons generally produced large effect sizes for both manipulation checks, but they were moderated by double-blind procedures and by peak breath-alcohol concentration (BrAC) attained in the alcohol condition. Other procedures moderated only the estimated number of alcohol drinks consumed. Implications for selection and assessment of alcohol placebo manipulations and for interpretability of experiments using them are discussed.