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Re: st: independnet variable (not normally distributed)


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: independnet variable (not normally distributed)
Date   Sat, 8 Mar 2008 19:53:11 +0000 (GMT)

The independent variables don't need to be normally distributed. If you
log transfom the variable than you think that if you start with a small
dose, an increase has a much bigger effect than when you start with a
larger, which may or may not make sense. Anyhow the real issue with
transforming the indepedent variables is whether the effect is linear,
or gradually slowing down (log) u-shaped, quadratic, etc. As an aside,
if you compute percentile rank scores, you won't make that variable
normal, uniformly distributed.

-- Maarten

--- Mohammed El Faramawi <melfaram@yahoo.com> wrote:
> I have  questions about the independent variable
> transformation. 
> I have an independent variable (lead mg/dl) which is
> not normally distributed. I am trying to run it in a
> cox hazard model the out come is mortality. What
> should I do with lead
> 1) Should I log transform it? I did that but it is
> still not normally ditributed after transformatin.
> 2) Leave it as it is because transformation does not
> change the distribution?
> 3) categorize using quartiles or percentiles? 


-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands

visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------


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