The present article provides a primer on using effect sizes in research. A small heuristic data set is used in order to make the discussion concrete. Additionally, various admonitions for best practice in reporting and interpreting effect sizes are presented. Among these is the admonition to not use Cohen's benchmarks for "small," "medium," and "large" effects, and instead to interpret effects in direct and explicit comparison against the effects in the related prior literature. (Contains 2 tables.)