Why do you use an effect size of d=0.40 as a cut-off point and basically ignore effect sizes lower than 0.40?

Effect sizes below d=0.40 aren’t ignored however a decision has been made to not look at what works (d=0.00 – 0.40) but what works best (d>;0.40). Ninety percent of all effect sizes in education are positive (d>; 0.0) and this means that almost everything works. The effect size of d=0.40 looks at the effects of influences in achievement in such a way where we can notice real-world and more powerful differences. It is not a magical number but a guideline to begin discussion about what we can aim for if we want to see student change.