Dear morphometers, In the search for biological diversity, the study of taxonomy, and the delimitation of species we often look for evidence allowing us to establish concrete boundaries among populations or taxa. For this, we usually obtain measurements of characters like the length (or shape) of the skull or other parts of the body (something similar occurs in the genetic realm of the problem, although this does not concerns the present case).
In this context, my question concerns Cohen's effect size and the standards for defining small, medium or large differences between groups. The problem is that there is apparently no available discussion upon which values of this index should be considered large in the framework of species differentiation. Cohen (in his book Statistical power analysis for the behavioral sciences) is explicit in explaining that his standards for effect size (Cohen's d) may only apply to studies concerning the social sciences, that is d=0.2 (small), d=0.4 (medium), d=0.8 (large). I have tried to find discussions concerning species differentiation and the application of Cohen's d, but there is apparently little or none available. Does anyone have an idea of what could be called large when dealing with species differences? I know there is probably no answer to this question, since there is no standard for the wide range of sizes and shapes in biological entities. In any case, I was hoping someone could provide me with a reference or citation that could bring some light into this issue. Thanks for all your help. Pablo Palo Jarrin Grad. Student Dept. of Biology Boston University -- Replies will be sent to the list. For more information visit http://www.morphometrics.org
