When available, we present data as percentages (e.g., percent abstinent) in the active and control treatment groups. For meta-analyses, which use standardized effects to combine results from multiple studies, we present the effect sizes. These include Cohen’s d, the difference between two means divided by the pooled standard deviation. Cohen’s d=0.2 is a small effect size, 0.5 a medium effect size, and 0.8 a large effect size. Effects smaller than d=0.2 are trivial, despite statistical significance. Hedges g is similar to Cohen’s d, but uses pooled weighted standard deviations, which provide greater accuracy in estimating very small effect sizes.
