FWIW extreme groups (e.g. using upper and lower quartiles) is well understood in its inflation of effect size (there are even formulas to correct this, given an

FWIW extreme groups (e.g. using upper and lower quartiles) is well understood in its inflation of effect size (there are even formulas to correct this, given an extreme groups design). It's definitely related to ecological fallacy in the sense that both underestimate relative error and inflate effect sizes.