MEASURES OF EFFECT SIZE

Two different approaches have been used to derive measures of effect s ize. One approach is based on the comparison of treatment means. The s tandardized mean difference is an appropriate measure of effect size w hen one is merely comparing two treatments, but there is no satisfacto ry analogue for comparing more than two treatments. The second approac h is based on the proportion of variance in the dependent variable tha t is explained by the independent variable. Estimates have been propos ed for both fixed-factor and random-factor designs, but their sampling properties are not well understood. Nevertheless, measures of effect size can allow quantitative comparisons to be made across different st udies, and they can be a useful adjunct to more traditional outcome me asures such as test statistics and significance levels.