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
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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> 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)))).
Marta García-Granero mailto:email@example.com