Boxplots of the estimated standardized mean differences in 10,000 simulated meta-analyses.

For each sample size range on the horizontal axis, the left gray box was obtained using Cohen’s d, and the right black box was obtained using Hedges’ g. The true between-study standard deviation τ increased from 0 (upper and middle panels) to 0.5 (lower panels). The number of studies in each meta-analysis N increased from 5 (upper panels) to 50 (middle and lower panels). The true standardized mean difference θ (horizontal dotted line) increased from 0 (left panels) to 1 (right panels).