Editing Effect size (section)

==== Glass' Δ ====
In 1976, [[Gene V. Glass]] proposed an estimator of the effect size that uses only the standard deviation of the second group<ref name="HedgesL1985Statistical"/>{{Rp|p=78|date=November 2012}}
<math display="block">\Delta = \frac{\bar{x}_1 - \bar{x}_2}{s_2}</math>

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.

Under a correct assumption of equal population variances a pooled estimate for ''σ'' is more precise.