Reporting effect size is becoming increasingly popular yet I cannot find a solution to this question anywhere. The most common calculations, such as Cohen's d, only compare the means of 2 groups. How does one calculate the effect sizes between multiple groups without having to resort to several individual calculations?
If you could formulate what exactly you would understand as an "effect" in this case, the answer would possibly already evident. If not, we can still help you - but first you must clearly and precisely state what you actually mean with the "effect for more than 2 groups" (for which the size should be estimated/calculated)!
Clarification:
Is there a way to calculate Cohen's d for multiple groups all at once? Currently I am doing individual cohen.d (package: effsize) equations between all my groups.
library(effsize)
set.seed(1492)
treatment1
I did not say that there is a "Cohen's for multiple groups at once" - that makes no sense because Cohens'd is about a difference, and a difference is not uniquely defined for more than 2 groups. If you look for some "effect" comprising the relationship between three or more groups you must find something different, and my question was aiming at what you have in mind to be a useful specification of such an "effect".
Okay, you are right. I can see how I worded my question stupidly. Very sorry!!
What I am looking for is a way to calculate Cohen's d for all my mean pairs in one fell swoop. In the same way that a post-hoc analysis calculates p-values by comparing every group against every other group.
Does this make sense?
I don't see a very smart solution. However, one could solve this using a list and nested for-loops:
L
It has certainly been a long time since the beginning of this thread, but I dare to add an idea: does it make sense to evaluate the effect size between that pair of groups in which there is a greater difference (according to post-hoc)?
A different approach I guess would be something like an "average effect size", I mean, a weighted mean of Cohen's d for each pair of groups. Again, does make it sense or is it just a silly idea?
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