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st: RE: Log transform- SE or std


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Log transform- SE or std
Date   Mon, 23 Jun 2003 19:52:31 +0100

Ricardo Ovaldia
 
> I used a log transform to normalize the distribution
> of a biochemical substance. I then use a ttest to
> compare the transformed mean of cases to controls. I
> now want to present my results listing the means, stds
> and the p-value. I could present the original
> (untransformed) means and stds, but I think that is
> misleading. Now, I know that the transformed mean is
> the geometric mean, so no problem there, but what
> about the standard error. How can I "untransform" it?

I am not at all clear that the se has an analogue 
on the original scale. 

I suggest that it is easier, and possibly 
closer to the underlying problem, to exponentiate 
the confidence intervals given by -ttest- for
the transformed means. 

Another way to approach this is by 

glm response caseorcontrol, link(log) 

by analogy with the principle (e.g. Conroy Stata Journal 
2002) that a t test is equivalent to a regression 
on a binary explanatory variable. 

Nick 
n.j.cox@durham.ac.uk 
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