Someone puts Cohen's d in a nutshell, please

gianmarco

TS Contributor
Hi,

I came across Cohen's d statistic and I would very appreciate if anyone could provide me with a brief explanation of it and with an example rooted in a context of use.

I understand that it is equal to the difference between two sample means divided by the pooled SD. But, I am wondering what kind of information is Cohen's d actually conveying. Beside, I understand that it can be provided along with t-test. Am I right?

Any help is appreciated.

Gm

Lazar

Phineas Packard
Hi,

I came across Cohen's d statistic and I would very appreciate if anyone could provide me with a brief explanation of it and with an example rooted in a context of use.

I understand that it is equal to the difference between two sample means divided by the pooled SD. But, I am wondering what kind of information is Cohen's d actually conveying. Beside, I understand that it can be provided along with t-test. Am I right?

Any help is appreciated.

Gm
Cohen's d is a measure of effect size. Simply put it indicates the amount of different between two groups on a construct of interest in standard deviation units. It is given for two reasons:
1. It is used as a counter-point to significance tests were it gives an indication of how big or small a significant difference is. This difference can then be compared to Cohen's estimates of what is typical of a small, medium, or large effect.
2. To provide a common metric on which to compare effects for meta-analysis or what not when outcome variables may be measured on different scales.

As an example suppose we found a significant difference in self-esteem in a sample tested before an intervention and then again afterwards. A Cohen's d of .50 would suggest that the intervention program was associated with a half of one standard deviation increase in self-esteem.

and yes it can be used with t-tests.

gianmarco

TS Contributor
Hi Lazar,

Can Cohen'd be used for un-paired t-test, or it only works with paired t-test?

Beside, I guess it cannot be put at work in case of un-equal variance (i.e., Welch't-test) ? right?

Thanks
Gm

Lazar

Phineas Packard
Sorry I should have mentioned that for paired samples one should use a correction for the correlation between the variables over time (see Morris and DeShon - from memory it is equation 6 or 8).

For more info on how to use it in a range of circumstances see the paper above or Cohen's book.

While I am at it this presentation on multilevel meta-analysis might be of some use for calculating d for a range of different things (see the powerpoint slides for detail).

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gianmarco

TS Contributor
Hi Lazar,

I will take a look at all that material.

Thank you.
Regards,
Gm

imk54831

New Member
Lazar,

Apologies for posting on this old thread, but the link to the slides you refer to appears now to be dead.

Do you have a copy of the slides or an updated link please?

Thanks

Ian

Mean Joe

TS Contributor
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