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Date:         Thu, 29 Sep 2005 09:31:52 +0200
Reply-To:     Marta García-Granero
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Marta García-Granero
Organization: Asesoría Bioestadística
Subject:      Re: Hedges Unbiased Effect Size Estimator
In-Reply-To:  <>
Content-Type: text/plain; charset=ISO-8859-15

Hi Jonathon,

To my knowledge, this formula is for 2 independent samples. Right now I don't have Egger's book on meta-analysis at hand, but I'll check if that particular topic is covered.

I seem to remember that I read (somewhere...) that the formula could be used for Cohen's d obtained from paired data considering n1=n2=n, but I'm not really really sure, because somewhere else I read that the Hedges estimator is only for independent samples.


I haven't had time to read the following paper slowly, but I think you can find something interesting in it:

(Although it's a Spanish journal, only the summary is in Spanish, the rest of the paper is in English).

LJ> My question relates to the calculation of the confidence intervals for LJ> Hedges Unbiased Effect Size Estimator. Can someone clarify if this LJ> formula pasted below is appropriate for a single group pre-test LJ> post-test design (that is, repeated measures). I ask because Hedges and LJ> Olkin (1985, somewhere between 80-90) use the example of an experimental LJ> and control group (that is, independent groups, not repeated measures). LJ> In addition is anyone is able to offer any references which deal with LJ> the issue, then that would also be appreciated. LJ> I have pasted a portion of syntax provided by Marta Garcia-Granero LJ> below: LJ> COMPUTE poolvar=((n1-1)&*(sd1&**2)+(n2-1)&*(sd2&**2))/(n-2) . LJ> COMPUTE cohen=(mean1-mean2)/SQRT(poolvar). LJ> COMPUTE hedges=cohen&*(1-(3/(4&*(n)-9))). LJ> COMPUTE se_hedge=SQRT((n/(n1&*n2))+((hedges&**2)/(2&*(n-3.94)))).

Regards, Marta García-Granero

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