<P> The second group may be regarded as a control group, and Glass argued that if several treatments were compared to the control group it would be better to use just the standard deviation computed from the control group, so that effect sizes would not differ under equal means and different variances . </P> <P> Under a correct assumption of equal population variances a pooled estimate for σ is more precise . </P> <P> Hedges' g, suggested by Larry Hedges in 1981, is like the other measures based on a standardized difference </P> <Dl> <Dd> g = x _̄ 1 − x _̄ 2 s ∗ (\ displaystyle g = (\ frac ((\ bar (x)) _ (1) - (\ bar (x)) _ (2)) (s ^ (*)))) </Dd> </Dl>

What is a good cohen's d effect size