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Date:         Thu, 3 Feb 2005 17:10:59 +0100
Reply-To:     Marta García-Granero
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Marta García-Granero
Organization: Asesoría Bioestadística
Subject:      Re: Confidence intervals for geometric mean
In-Reply-To:  <>
Content-Type: text/plain; charset=ISO-8859-15

Hi Christian,

Working with GM is the same as working with the arithmetic mean of the log(data). Compute CI for the mean of log(data), then take the antilog of the limits.

I don't know of any nonparametric test that compares GM. If the log(data) are roughtly normal, then run a t test on them and antilog the limits of the CI for mean diferences (be careful with the interpretation of the antilog results, you will get CI for the ratio of both GM).

Take a look at these on-line articles (from British Medical Journal):

Transforming data:

Transformations, means and confidence intervals:

The use of transformation when comparing two means:

CB> I am looking some script to estimate the confidence intervals at 95% for CB> geometric mean (GM) values. In addition, I have two groups and I am CB> thinking to apply a non-parametric test to compare their GMs.If you have CB> others ways to compare GM, please go ahead, welcome all ideas and comments.

HTH Marta

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