Glass’s delta (Glass et al. 1981) is a measure of effect size. Glass’s delta uses only the control group’s standard deviation (SDC).
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.