[R-meta] effect size similarities

[Viechtbauer, Wolfgang (SP)]

I understand. What I tried to say is that this equation does not apply for a one-sample t-test. Cohen's d is

d = (mean1 - mean2) / SD,

where mean1 and mean2 are the means of two groups and SD the (pooled) SD and hence it reflects the difference between two groups. If there is only one group, then Cohen's d cannot be computed.

Best,
Wolfgang

>-----Original Message-----
>From: Patrizio E Tressoldi [mailto:patrizio.tressoldi using unipd.it]
>Sent: Wednesday, 23 June, 2021 9:24
>To: Viechtbauer, Wolfgang (SP); r-sig-meta-analysis using r-project.org
>Subject: Re: [R-meta] effect size similarities
>
>Il 23/06/2021 09:20, Viechtbauer, Wolfgang (SP) ha scritto:
>The equation for turning a two independent samples t-test statistic into a d-value
>is:
>
>d = t * sqrt(1/n1 + 1/n2)
>
>not t/sqrt(df). But this only applies if the t-statistic comes from a two
>independent samples t-test, not just any t-statistic.
>
>I agree, but I referred to the one-sample condition.
>
>Thanks
>Patrizio