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Re: st: Right skewed (positive) dependent variable

From   Maarten buis <>
Subject   Re: st: Right skewed (positive) dependent variable
Date   Thu, 10 Jun 2010 15:51:00 +0000 (GMT)

--- On Thu, 10/6/10, SURYADIPTA ROY wrote:
> However, as I look at my program now, I discover
> the source of the anomaly- my transformatrion
> was newvar=ln(1+oldvar).. that explains.

Are there 0s in your dependent variable (oldvar)?
If there are, then you really have no choice other
than go the -glm- route. There are ways of getting 
a meaningfull interpretation out of a log transformed
dependent variable, but no such way exists for the
transformation log(oldvar + some constant), and 
leaving the constant out is no sollution either, as
that means that he 0s will be recoded to missing
values. This may also explain your non-normality:
is there a spike at 0. If that is the case, than
there can be no transformation that will lead to
a normal distribution. In that case you could 
consider modeling the zero separately using -zip-.
It is usually used for counts, but can also be
used for continuous variables in a Quasi-likelihood
kind of way, by specifying the -robust- option.

Hope this helps,

Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen


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