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st: to Marteen buis (independent variable)


From   Mohammed El Faramawi <melfaram@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: to Marteen buis (independent variable)
Date   Sat, 8 Mar 2008 21:49:05 -0800 (PST)

Hi Marteen,
You wrote Anyhow the real issue with transforming the
independent variables is whether the effect is linear,
or gradually slowing down (log) u-shaped, quadratic,
etc.' would you mind explaining more? especially what
do you mean by slowing down and (log)
Thank you .
--- Maarten buis <maartenbuis@yahoo.co.uk> wrote:

> 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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