Introduction to Effect Size (d)

To understand the need to consider the effect size of a difference, consider a practical problem in interpreting research. Suppose that Experimenter A administered a new treatment for depression (Treatment X) to an experimental group, while the control group received a standard treatment. Furthermore, suppose that Experimenter A used a 20item true-false depression scale (with possible raw scores from 0 to 20) and obtained the results on the posttest shown in Table 1.1 Note that the difference between the two means is 5 raw-score points.