The call for researchers to report and interpret effect sizes and their corresponding confidence
intervals has never been stronger. However, there is confusion in the literature on the definition of
effect size, and consequently the term is used inconsistently. We propose a definition for effect size,
discuss 3 facets of effect size (dimension, measure/index, and value), outline 10 corollaries that
follow from our definition, and review ideal qualities of effect sizes. Our definition of effect size is
general and subsumes many existing definitions of effect size. We define effect size as a quantitative
reflection of the magnitude of some phenomenon that is used for the purpose of addressing a
question of interest. Our definition of effect size is purposely more inclusive than the way many have
defined and conceptualized effect size, and it is unique with regard to linking effect size to a question
of interest. Additionally, we review some important developments in the effect size literature and
discuss the importance of accompanying an effect size with an interval estimate that acknowledges
the uncertainty with which the population value of the effect size has been estimated. We hope that
this article will facilitate discussion and improve the practice of reporting and interpreting effect
sizes.
Keywords: effect size, confidence intervals, research design, research question, reporting results