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Increasing emphasis has been placed on the use of effect size reporting in the analysis of social science data. Nonetheless, the use of effect size reporting remains inconsistent, and interpretation of effect size estimates continues to be confused. Researchers are presented with numerous effect sizes estimate options, not all of which are appropriate for every research question. Clinicians also may have little guidance in the interpretation of effect sizes relevant for clinical practice. The current article provides a primer of effect size estimates for the social sciences. Common effect sizes estimates, their use, and interpretations are presented as a guide for researchers.

By the 1990s, statisticians had been aware for some time that null-hypothesis significance testing (NHST) was, in many respects, insufficient for interpreting social science data (Berkson, 1938; Cohen, 1994; Loftus, 1996; Lykken, 1968; Meehl, 1978; Snyder & Lawson, 1993). Subsequently the Wilkinson Task Force (Wilkinson & Task Force on Statistical Inference, 1999) recommended the reporting of effect sizes and effect size confidence intervals (CIs). Nonetheless, the use of effect size measures remains inconsistent (Fidler et al., 2005; Osborne, 2008; Sink & Stroh, 2006). Researchers and clinicians may find themselves with little guidance as to how to select from among a multitude of available effect sizes, interpret data from research, or gauge the practical utility of reported effect sizes. The current article seeks to provide a primer for clinicians and researchers in understanding effect size reporting and interpretation.

NHST, has long been regarded as an imperfect tool for examining data (e.g., Cohen, 1994; Loftus, 1996). Statistical significance of NHST is the product of several factors: the “true” effect size in the population, the size of the sample used, and the alpha (p) level selected. Limitations of NHST include sensitivity to sample size, inability to accept the null hypothesis, and the failure of NHST to determine the practical significance...