What is Effect Size?
Effect size is a parametric test, belonging to the GLM family that allows to compare the size of an effect between two groups (or for that matter two conditions in one group). The test can be used in all kinds of research designs (experimental and non-experimental) that employ different kind of measurements or/and manipulations. Given that the study itself is not biased, it offers an easy solution to compare the effect of different manipulations with each other. It also gives an indication of the real-life value of a given intervention (when compared to baseline). The concept of effect size is one of the core paradigms in scientific evidence-based therapy research and has stimulated the development of effective therapies, by giving a “golden” standard on which effect can be compared. Recently Cohen’s system of labelling different effect sizes has been criticized, still I would consider it to be scientific mainstream knowledge and the wide use and acceptance in the scientific community reinforces its use.