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Re: st: Elimination of outliers

From   Ronan Conroy <>
To   "" <>
Subject   Re: st: Elimination of outliers
Date   Tue, 7 Jun 2011 10:37:21 +0100

On 2011 Meith 6, at 21:24, Nick Cox wrote:

> I don't think what happens in contrived simulations hits the main
> methodological issue at all. As a geographer, some of the time, an
> outlier to me is something like the Amazon which is big and different
> and something that needs to be accommodated in the model.

In medicine, outliers often warn us that there are unusual (perhaps pathological) processes at work in some cases, so we are seeing a mixture of normal variation plus smaller groups of people with unusual characteristics. In pregnancy, for example, blood pressure is a mixture of three distributions: normal blood pressure response to pregnancy, and two abnormal blood pressure responses: gestational hypertension (high flow, low resistance) and pre-eclampsia (high resistance, low flow). 

They remind us of Bacon's observation: we learn once when we examine the normal, and we learn once more when we examine the exceptions. 

Ronán Conroy
Associate Professor
Division of Population Health Sciences
Royal College of Surgeons in Ireland
Beaux Lane House
Dublin 2

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