Alternative Approaches to Interpreting Effect size (1 of 2)
Cohen's d'
The number of standard deviations separating two group means can be used as a measure of effect size. This measure, called d', can be computed as follows:
where M1 is the mean for Group 1, M2 is the mean for Group 2, and
is an
estimate of the standard deviation taken from the analysis of variance summary
table. For
example, if the mean of Group 1 were 20 and the mean of Group 2 were
10 and the the standard deviation as estimated by were 5, then d'
would be 2. The means are two standard deviations apart. although
there are no generally accepted criteria for determining whether a
given d' is large enough to be important,
Cohen (1962) made the reasonable recommendation that a d' of 0.20
is a small effect, a d' of 0.50 is a medium sized effect, and a d' of
0.80 is a large effect.