# Thread: Cohen d and Z scores

1. ## Cohen d and Z scores

Hi everyone,

I should start by saying that I posted the same question that I will expose here on the psychological research forum. However, as I did not receive any answer, I wonder whether I misplaced my post. I apologize if this kind of repetition contravenes any of the rules of Talk Stats.

My problem is that I am trying to calculate the size of the difference between the means of two groups in a psychological test. However, I am not using my own data but those published in an article and authors do report test performance in Z scores. Therefore, I am wondering whether or not I can just apply the regular formula of Cohen's d to calculate the size of the observed difference.

Thanks in advance for any possible help

2. ## Re: Cohen d and Z scores

css, some times the initial posts of new contributors get stuck and marked as spam. Don't worry about it, sorry for the hassle.

3. ## Re: Cohen d and Z scores

Is this of any benefit to you, see below. What parameters do you have in particular?

http://www.socscistatistics.com/effe.../Default2.aspx

4. ## Re: Cohen d and Z scores

First of all, thank you hlsmith for your clarification on the posts...

Thanks also for the link, but the webpage you referred to relates to Z tests, no to Z scores.

What I am trying to calculate is the effect size of a difference between males and females in cognitive performance from a published article. Unfortunately, authors did not report the means and SD of raw scores but only mean and SEM from Z scores (and, separately, the N of the groups). The authors also explain how these z scores were calculated "Raw scores were standardized (z-transformed) based on the means and SDs of the entire sample. For ease of presentation, higher z-scores always reflect better performance; z-scores where higher numbers reflected poorer performance (i.e. response time) were multiplied by −1."

Therefore, my question is whether I should use these z scores as "raw scores" for calculating the effect size of the between groups difference. Or should I just calculate the difference between the groups' "z means"? Or none of the previous?

5. ## Re: Cohen d and Z scores

Can you provide the exact data values you are working with, thanks.

6. ## Re: Cohen d and Z scores

Hi again,

First of all, thank you for your interest in helping me out. I do not have direct data to work with. In this case, I am extracting data from figures just to have an impression on the effect size of the reported differences. However, my question is also "general", because I am observing that quite a few articles follow the same strategy of reporting z scores...

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