Before revision: For paired samples Cohen suggests that the d calculated is actually a d', which doesn't provide the correct answer to obtain the power of the test, and that before looking the values up in the tables provided, it should be corrected for r as in the following formula: Glass' Δ In 1976, Gene V. Glass proposed an estimator of the effect size that uses only the standard deviation of the second group 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.
After revision: For paired samples Cohen suggests that the d calculated is actually a d', which does not provide the correct answer to obtain the power of the test, and that before looking the values up in the tables provided, it should be corrected for r as in the following formula: Glass' Δ In 1976, Gene V. Glass proposed an estimator of the effect size that uses only the standard deviation of the second group 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.