**Glass’s delta** (Glass et al. 1981) is a measure of effect size. Glass’s delta uses only the control group’s standard deviation (SD_{C}).

## When to Choose Glass’s Delta

Use Glass’s Delta when standard deviations are significantly different between groups.

If your standard deviations are not significantly different between groups, use Hedges’ g or Cohen’s d instead. If your sample size is small (less than 20) choose Hedges’ g over Cohen’s d. For samples 20 or greater, the two statistics are roughly equivalent.

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