Date: Thu, 24 Oct 2002 15:00:33 0400
ReplyTo: Richard Ristow <wrristow@mindspring.com>
Sender: "SPSSX(r) Discussion" <SPSSXL@LISTSERV.UGA.EDU>
From: Richard Ristow <wrristow@mindspring.com>
Subject: Re: Some formulae needed
InReplyTo: <7D25CCC35D3CD31192BF006008BFB1FE04F572C2@nsofs14.uchc.edu>
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At 01:34 PM 10/24/2002 0400, Burleson,Joseph A. wrote:
>In Cohen's power book, the formula for finding the "harmonic"
>(geometric) mean is:
>
>n' = 2*(n1)*(n2)/(n1 + n2)
>
>Note that when n1 = n2, this becomes the same as the arithmetic mean:
>
>nbar = (n1 + n2)/2
>
>In effect, your "average" sample size per group incurrs a stiffer
>penalty the more discrepant the difference between the two group sizes.
To correct the terminology: This IS the formula for the harmonic mean.
(The defining formula is
1/n' = (1/n1 + 1/n2)/2
but that's less convenient for calculation.)
The GEOMETRIC mean is a different value:
n' = SQRT(n1*n2)
