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The objective of this article is to offer guidelines regarding the selection, calculation, and interpretation of effect sizes (ESs). To accomplish this goal, ESs are first defined and their important contribution to research is emphasized. Then different types of ESs commonly used in group and correlational studies are discussed. Several useful resources are provided for distinguishing among different types of effects and what modifications might be required in their calculation depending on a study's purpose and methods. This article should assist producers and consumers of research in understanding the role, importance, and meaning of ESs in research reports.