ABSTRACT
This chapter introduces the Index of Similarity, which measures the degree to which distributions overlap. It explains two measures "effect size", Cohen's d and the Common Language Effect Size, that measure the substantive size of a difference. Describing a Cohen's d value as "large" or "small" is very much a matter of interpretation and context. The Index of Similarity (IS), which was introduced in 2012 to the behavioral economics literature, is an easily computable and understandable measure of the degree of overlap between two distributions. The IS is an especially useful tool when data are of a discrete or categorical nature—which means that the variable of interest takes on only a very limited number of values. In the psychological literature on behavior, the expression of findings in terms of substantive difference is widespread, standardized, and considered best practice. The implications of various methodologies for the study of sex differences has also been the topic of intense professional discussion.
