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

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.

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“The primary product of a research inquiry is one or more measures of effect size, not p values.”
~ Jacob Cohen

How to manuals

“Statistical significance is the least interesting thing about the results. You should describe the results in terms of measures of magnitude – not just, does a treatment affect people, but how much does it affect them.”
~ Gene Glass