ABSTRACT
The magnitude of a difference when it is expressed on a standardized scale is referred to as the effect size. The statistic d is one of the most popular for describing the effect size of the difference between two means. To understand the need to consider the effect size of a difference, consider a practical problem in interpreting two sets of research findings that can be resolved using the statistic d. For the results of the two studies to be comparable, they need to be standardized so that both differences can be expressed on the same scale. The most straightforward way to do this is to express both differences in terms of standard-deviation units. Within each of the two examples in this topic, the two standard deviations are equal. When they are unequal, a special that results in a pooled standard deviation should be used.
