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# Re: st: Right skewed (positive) dependent variable

 From Maarten buis To statalist@hsphsun2.harvard.edu Subject Re: st: Right skewed (positive) dependent variable Date Thu, 10 Jun 2010 14:37:55 +0000 (GMT)

```--- On Thu, 10/6/10, SURYADIPTA ROY wrote:
> My dependent variables are heavily right skewed, and
> originally a logarithmic transformation did not help with
> the normality of the (conditional) distribution of
> the residuals. I transformed the dependent variables
> using  - ladder - (indicate that it did a logarithmic
> transformation also) and after carrying out the regressions
> find that the residuals are normally distributed, -ovtest-
> and -linktest- do not indicate any unexplained variation,
> the level of significance in all cases are significantly
> higher, and the explanatory variable of interest is coming
> out to be strongly significant in most cases (all good
> news!?) I am wondering if someone could suggest me as to how
> much faith I can have in the regressions, and why did my
> original transformation did not yield the same results as

That is very odd, you should get the exactly the same variable:

*------ begin example ---------
sysuse citytemp, clear
gen sqrt2 = sqrt(tempjuly)
assert sqrt1 == sqrt2
*------- end example ----------

Anyhow, if you want to interpret your results you are usually
much better of by using -glm- with the appropriate -link()-
dependent variable. The whole logic behind regression is that
you want to look how the mean of your dependent variable differes
across values of your independent variables. If you (non-linearly)
transform your dependent variable, then you are no longer looking
at the mean of your dependent variable, and back transforming
won't work either. I nice discussion can be found here:

Nicholas J. Cox, Jeff Warburton, Alona Armstrong, Victoria J. Holliday
(2007) "Fitting concentration and load rating curves with generalized
linear models" Earth Surface Processes and Landforms, 33(1):25--39.
<http://www3.interscience.wiley.com/journal/114281617/abstract>

Hope this helps,
Maarten

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

http://www.maartenbuis.nl
--------------------------

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