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RE: st:Transformation for skewed variables with negative values?


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st:Transformation for skewed variables with negative values?
Date   Tue, 15 Aug 2006 22:58:43 +0100

Also, partially echoing Jeph Herrin, if everything is negative, 
consider reversing signs. Sign is often a convention. 

However, I would want really good grounds 
for shifting origins, even if the origin were conventional. 
That is, if x everywhere is greater than a, then (x - a) is 
positive, but if a is just empirically determined, it can be 
difficult to interpret (x - a) or to compare results with other 
studies. 

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

> I agree with Joseph's general stance on this question. 
> Also, consider the alternative of a non-identity link 
> and a -glm-, which often offers the advantages of
> a transformation without the disadvantages. 
> 
> However, more generally, I will add a plug for 
> my package -transint- from SSC, which is just 
> a help file with various comments on transformations. 
> You can install it using -ssc-. 
> 

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