Using effect sizes for research reporting: examples using item response theory to analyze differential item functioning.

The psychological literature currently emphasizes reporting the "effect size" of research findings in addition to the outcome of any tests of significance. However, some confusion may result from the fact that there are three distinct uses of effect sizes in the psychological literature, namely, power analysis, research synthesis, and research reporting. The authors review these uses of effect sizes and develop a case study of the description of effect size for research reporting in the context of item response theory. For many parametric models, hypotheses are tested by comparing the values of directly interpretable parameters. The authors show that the size of the effect can be expressed by a presentation of the values of the parameter estimates derived from the fitted model. Studies that use item response theory to detect differential item functioning provide illustrations.