A simple effect size indicator for two-group comparisons? A comment on r equivalent.

R. Rosenthal and D. B. Rubin (2003) proposed an effect size, r equivalent, to be used when (a) only sample size and p values are known for a study, (b) there are no generally accepted effect size indicators, or (c) sample sizes are so small or the data so non-normal that the directly computed effect sizes would be more misleading than the simple effect size. The limitations of their proposal, however, are many, and much more serious than the authors suggested, and should be carefully considered before this effect size is applied, as well as in developing other effect sizes using similar methods.