afni.nimh.nih.gov

> how I might calculate and show an effect size for the group level analysis?

If you perform your population-level analysis with Fisher-transformed correlation values, you can covert them back to correlation values:

3dcalc -a Grp_Result+tlrc'[0]' -expr 'atan(a)' -prefix GrpR
3dcalc -a Grp_Result+tlrc'[0]' -expr '(exp(2*a)-1)/(exp(2*a)+1)' -prefix GrpR

Gang