Dear Adele,

I saw your message and was struggling with kind of the same thing a few days ago for my thesis.

I found this in the Conn handbook (see screenshot): Click on ‘plot effects’ to compute effect-sizes (e.g. connectivity values within each group) averaged across all voxels within each significant cluster.

So I think effect size is average connectivity within each group.

Maybe it helps!

Kind regards,

Fenna