---
title: Cohen’s d
description: "Probably the most used effect size measure at least in the psychology, MH and therapy realms. A \"standardised effect size\" describing the effect size of a difference in means between two independent groups: it's that mean divided by the SD.…"
url: "https://www.psyctc.org/psyctc/glossary2/cohens-d/"
updated: "2026-02-02"
category: All OM book glossary entries
---

# Cohen’s d

Probably the most used effect size measure at least in the psychology, MH and therapy realms. A "standardised effect size" describing the effect size of a difference in means between two independent groups: it's that mean divided by the SD.

 d=meanSDd = \frac{\text{mean}}{\text{SD}}

That looks nice and simple but the devil is in the choice of standard deviation. Cohen also had a formula for d for change or, more generally, for the mean difference in within subject, repeated, paired data but that needs another entry.

#### Details

I think (I would) that there's a good introductory summary in my [Rblog](https://www.psyctc.org/Rblog/) post: [Hedges’s g and Cohen’s d](https://www.psyctc.org/Rblog/posts/2024-01-19-hedgess-g/). Beyond that the main issues are about the choice of standard deviation for the denominator of the simple equation above. The usual choice, I would say the only choice for the between groups value is the "common" (sometimes called, a bit misleadingly, the "pooled" SD: it's not the SD you get pooling the observations in both groups). See that blog post for more on that SD. Things are a bit more complicated for the repeated measures situation and I'll give that its own Rblog post when I can.

#### Try also

[Effect size](https://www.psyctc.org/psyctc/glossary2/effect-size/)
[Hedges's g](https://www.psyctc.org/psyctc/glossary2/hedgess-g/)
Repeated measures
"Standardising"

#### Chapters

We didn’t put it in the book but Chapter 8 and service comparisons would probably be where you might encounter effect sizes.

#### Online resources

My [Rblog](https://www.psyctc.org/Rblog/) post about [Hedges’s g and Cohen’s d](https://www.psyctc.org/Rblog/posts/2024-01-19-hedgess-g/)

#### Dates

First created 21.i.24.