What’s a good effect size index for comparing the means of two groups?

May 31, 2010

How about the standardized mean difference?

This effect size index very simple to calculate. You subtract the mean of one group from the mean of the other then divide the result by either the standard deviation of the control group (giving you a Glass’s Δ) or by the pooled standard deviation of both groups (giving you Cohen’s d or Hedges’ g depending on the pooling equation used).

You can do this by hand or, easier still, just plug your numbers into an online calculator such as this one.

For more, see The Essential Guide to Effect Sizes, chapter 1.


Where can I find a good effect size calculator?

May 31, 2010

Often when people learn about effect sizes, one of the first things they ask is “what software do I need to do this?”

Many effect size indexes can be calculated on the back of an envelope or using nothing more than a Spreadsheet.

Other effect size indexes are generated automatically by statistical programs such as SPSS or STATA.

Still, for those of you who want something flashy, a set of seven easy to use calculators can be found on the Resources page (or just click here to be taken straight to the calculators).