Persons as Effect Sizes

Traditional indices of effect size are designed to answer questions about average group differences, associations between variables, and relative risk. For many researchers, an additional, important question is, “How many people in my study behaved or responded in a manner consistent with theoretical expectation?” We show how the answer to this question can be computed and reported as a straightforward percentage for a wide variety of study designs. This percentage essentially treats persons as an effect size, and it can easily be understood by scientists, professionals, and laypersons alike. For instance, imagine that in addition to d or η2, a researcher reports that 80% of participants matched theoretical expectation. No statistical training is required to understand the basic meaning of this percentage. By analyzing recently published studies, we show how computing this percentage can reveal novel patterns within data that provide insights for extending and developing the theory under investigation.