Cohen's d

From Wikiversity

Cohen's d is an effect size used to indicate the standardised difference between two means. It can be used, for example, to accompany reporting of t-test and ANOVA results. It is also widely used in meta-analysis.

Cohen's d is an appropriate effect size for the comparison between two means. APA style strongly recommends use of ESs. Partial eta-squared covers how much variance in a DV is explained by an IV, but that IV possibly has multiple levels and hence partial eta-squared doesn't explain the size of difference between each of the pairwise mean differences. Cohen's d can be readily calculated as the difference between the means divided by the pooled SD, and this can be done using available spreadsheets since its not available in SPSS. Calculating Cohen's d also gives particularly useful information for discussion (e.g., allows ready comparison with meta-analyses and the size of effects reported in other studies). There's no hard and fast rules, but if you are reporting about differences between two means, then a standardised mean effect size (such as d) would be an appropriate accompaniment. This approach, however, is not necessarily representative of the traditional teaching of ANOVA in part because understanding use of effect sizes is still relatively poor - and because Cohen's d is not readily available in commonly used statistical software packages.

[edit] See also

• Cohen's d (Wikipedia)