[Date Prev][Date Next][Thread Prev][Thread Next][Date index][Thread index]

Re: st: independnet variable (not normally distributed)

From   Maarten buis <>
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 <> 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

Rise to the challenge for Sport Relief with Yahoo! For Good
*   For searches and help try:

© Copyright 1996–2015 StataCorp LP   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index