How to Use SPSS®, 12th Edition

Appendix AEffect Size

Many disciplines are placing increased emphasis on reporting effect size. While statistical hypothesis testing provides a way to tell the odds that differences are real, effect sizes provide a way to judge the relative importance of those differences. That is, they tell us the size of the difference or relationship. They are also critical if you would like to estimate necessary sample sizes, conduct a power analysis, or conduct a meta-analysis. Many professional organizations (e.g., the American Psychological Association) are now requiring or strongly suggesting that effect sizes be reported in addition to the results of hypothesis tests.

Because there are at least 41 different types of effect sizes,¹ each with somewhat ...