# Is there a difference between z-score and effect size?

#### mathsucks

##### New Member
I just started teaching statistics to myself (I never had it in school but will need it when I start University).

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

#### hlsmith

##### Less is more. Stay pure. Stay poor.
Effect size is a general term which z -score can fall under. For example an odds ratio can also be considered an effect size measure.

#### noetsi

##### No cake for spunky
Effect size is actually the mean difference normally between two levels of the IV on the dependent variable [well it can be other things like median difference too I believe, but mean is most common]. A z test is just a statistical test if the effect size is statistically signficant. It tells you if the effect size is large enough that it is not likely due to random error [that it actually exists in the population].

I am not sure if you mean the Z score to be the effect size for a Z test or not.

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