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RE: st: Elimination of outliers


From   Nick Cox <n.j.cox@durham.ac.uk>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Elimination of outliers
Date   Mon, 6 Jun 2011 15:17:32 +0100

1. Transformation means using a transformed scale (e.g. logarithms) for one or more of your variables. 

2. A non-identity link function in a generalized linear model means what it says: the help for -glm- is the place to start and points to other documentation. 

Otherwise, I assert that elimination of outliers is a very bad idea _unless_ you know from independent evidence that they arise from serious and irremediable problems of measurement, in which case chopping the tails of the distribution is _not_ the way to do it. In most fields I know, the outliers that stick out are genuine and important (the Amazon in hydrology, USA or China wherever it is in economics, and so on, and so on) and leaving them out is in my view lousy science and lousy statistics. 

If you disagree, well, we disagree, but I am not going to tell you how to do this in Stata. 

Nick 
n.j.cox@durham.ac.uk 


-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Achmed Aldai
Sent: 06 June 2011 15:07
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: Elimination of outliers

Hi

sorry I cannot really understand why it is a bad idea. I want to eliminate the outliers beacuse I think they cause a bias in my results. 

How can I transform my predictors and what do you mean by that?

What is a non-identity link function?

Thank you

FElix
-------- Original-Nachricht --------
> Datum: Mon, 6 Jun 2011 13:39:20 +0100
> Von: Nick Cox <njcoxstata@gmail.com>
> An: "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
> Betreff: Re: st: Elimination of outliers

> In general, a very bad idea. Consider transforming your response or  
> predictors or using a non-identity link function in a generalized  
> linear model or some flavour of robust regression as more measured  
> tactics.
> 
> Nick
> 
> On 6 Jun 2011, at 12:46, "Achmed Aldai" <Hauptseminar@gmx.de> wrote:
> 
> > Hi
> >
> > I am currently working on a do file where I want to eliminate  
> > outliers which have the highest and the lowest values regarding  
> > certain variables. Here it is e.g. at and lt. In general I have  
> > 150000 observations and out of these observations I want to delete  
> > 25 observations from the upper and lower boundaries. But it might  
> > also be better to do it relatively meaning that I dont take the  
> > highest and lowest 25 but the lower and upper 1% of the  
> > corresponding variables.
> >
> > gvkey           at           lt
> > 1001            1120         231
> > 1001            1230         312
> > 1210            57           32
> > 1210            67           25
> > 1354            789          560
> > 1368            650          500
> > 1481            1230         900
> > 2930            21           30
> > 3201            234          213
> > 3201            256          220
> > 3210            267          320
> > 4510            4335         3214
> >
> > I hope this became clear.

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