title: Cohen's d needs to be readily interpretable: comment on Shieh (2013).
abstract: Shieh (2013) discussed in detail δ*, a proposed standardized effect size measure for the two-independent-groups design with heteroscedasticity. Shieh focused on inference-notably, the large challenge of calculating confidence intervals for δ*. I contend, however, that the standardizer chosen for δ*, meaning the units in which it is expressed, is appropriate for inference but causes δ* to be inconsistent with conventional Cohen's d. In addition, δ* depends on the relative sample sizes in the particular experiment and, thus, lacks the generality that is highly desirable if a standardized effect size is to be readily interpretable and also usable in meta-analysis. In the case of heteroscedasticity, I suggest that researchers should choose as standardizer for Cohen's δ the best available estimate of the SD of an appropriate population, usually the control population, in preference to δ* as discussed by Shieh.
authors.affiliation: Statistical Cognition Laboratory, School of Psychological Science, La Trobe University, Melbourne, Victoria, 3086, Australia, g.cumming@latrobe.edu.au.
authors.forename: Geoff
authors.initials: G
authors.lastname: Cumming
mesh_terms: D016001:Confidence Intervals; D006801:Humans; D007361:Intelligence Tests / Q000592:standards / Q000706:statistics & numerical data; D007600:Judgment; D008347:Manifest Anxiety Scale / Q000592:standards / Q000706:statistics & numerical data; D015201:Meta-Analysis as Topic; D015233:Models, Statistical; D012107:Research Design; D018401:Sample Size