# Cohens d effect sizes

I'm using Cohens d effect sizes as a means of providing a magnitude based inference in a study I'm conducting.

I just wanted to clarify if I'm correct here: Anything between .0 and .2 is small. Anything between .2 and .8 is moderate Anything above .8 is large

If anybody could let me know if this is correct that would be much appreciated.

Those values are largely arbitrary. I believe Cohen surveyed some published studies in his field and found that the mean $d$ was $.5$ with a standard deviation of $.3$. However, the means and SDs will vary by field. Moreover, they aren't necessarily the same thing as importance for anything. That is, people will naturally assume that a large effect is important and a small effect is not, but you will get a very large effect by comparing the heights of adults and 3 year olds, without having made an important discovery. In general, I think the only person who can say what is "large" in your research is you.