ABSTRACT

This chapter examines the effect size of a statistically significant test using Cohen's d. It describes that effect size standardizes the significant difference between two means and that effect size is interpreted relative to the standard deviation units of the control group. The chapter provides the labels for values of d and the principles and interpretation of effect size. It discusses a specific measure called Cohen's d that is used to measure the effect size for t test of means comparisons. Cohen's d is one of the most widely used statistics for the purpose of standardizing the difference between two means to measure the effect size, which is quite easy to compute. Effect size is measured differently with different statistics. Thus, effect size measures how large an observed effect is by standardizing the size of the difference between two means.