I needed to put together a simple little Excel calculator for these two common effect sizes. While there are many different online calculators out there, I like the idea that I can go in and verify the calculations if necessary, and add things to it (I would eventually like to add in confidence intervals for both effect sizes, if I can figure it out).
I have this sheet calculating both Cohen’s d and Hedges’ g because of other people’s preference. Most recognize Cohen’s d, as it is very common to use for pairwise comparison of means. Hedges’ g is a little less recognizable, but it is a very useful adaptation of the original Cohen’s d calculation. There are two adjustments made in the Hedge’s g calculation- first, the standard deviations are weighted by the sample size of each group; second, there is a positive bias to Cohen’s d with smaller sample sizes that is adjusted for in Hedge’s g. I don’t know what the “official cut-off” is what constitutes a small sample size, but I would guess it is in the neighborhood of <20 samples.
Calculations for the spreadsheet were pulled from “The Essential Guide to Effect Sizes”, by Paul D. Ellis. This is a spectacular book, one worth putting into your library. It’s also really cheap for a textbook. Click here to check it out.
You can download the spreadsheet here:
Paired Effect Size Calculator v1.1.xlsx (fixed calculation of Hedges’ g- was originally “double correcting” the effect size).