Understanding and Interpreting Effect Size Measures (Research Note) (Report‪)‬

Statistical methods are the hallmark of quantitative research. Examining whether a result is statistically significant is standard content in social work research statistics courses. In all of social science, statistical significance testing has been the accepted procedure for examining results. Despite ongoing efforts aimed at encouraging researchers to report some index of magnitude that is not directly affected by sample size--for example, effect size statistical significance testing appears to remain the standard. In 1994, the APA Publication Manual provided "encouragement" to authors to report effect sizes with little impact (see for example Keselman et al., 1998; Kirk, 1996; Thompson & Snyder, 1998). The current Publication Manual of the American Psychological Association (2001) states that "it is almost always necessary to include some index of effect size or strength of relationship" (p. 25). This position was influenced by the APA Task Force on Statistical Methods that recommended researchers report alpha levels and effect sizes (Wilkinson &Task Force on Statistical Inference, 1999). This was also the stance advocated by Dr. Jacob Cohen (1990), statistical power expert, who argued that "the primary product of a research inquiry is one of measures of effect size, not p values" (p. 12). The field has slowly responded and effect sizes are increasingly more visible (Fidler et al., 2005; Vacha-Haase & Thompson, 2004). Indeed, some journal editors, especially in psychology, now require authors to report effect size measures (Harris, 2003; Snyder, 2000; Trusty, Thompson, & Petrocelli, 2004). The reporting of effect size measures is also increasing in social work journals, however, it is still common to find studies void of effect size indices (for example, Claiborne, 2006; Engelhardt, Toseland, Goa, & Banks, 2006; Padgett, Gulcur, & Tsemberis, 2006; Perry, 2006). There do not appear to be any social work journals that require effect size reporting.