I have problems with z-scores and effect size. I think that you use a z-score if you want to know how a score compares to the mean or standard derivation. And you use the effect size if you want to see the difference between two groups of score and if the difference is statistically significant.

But don't you also use z-scores for this? It's kind of difficult for me to see the clear difference between those.

I'm not sure if I understand it.

What is the difference of a z-score and an effect size? When use what?

Thank you