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RE: st: RE: Re: Other Box plots


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: Re: Other Box plots
Date   Mon, 2 Dec 2002 19:01:51 -0000

Wallace, John
> 
> In Probability and Statistics (JL Devore, Duxbury Press, MA 
> 1995) the author
> describes a "Boxplot Rule" for evaluating outliers.  Two 
> outlier limits are
> calculated using the Interquartile range: the first at 1.5 
> * IQR(measured
> symmetrically from the 25th and 75th percentiles), and the 
> second at 3*IQR.
> "Mild" outliers, in this scheme are between the two limits 
> defined above (at
> either tail).  "Extreme" outliers are beyond the 3*IQR limits.
> Note that this is a different application that just a 
> pictorial summary of
> the data (which the various plots we've been talking about 
> do), the plot I'm
> describing allows you to make judgements about how likely 
> it is that suspect
> points in your dataset are outliers.  The whiskers of the 
> plot would span a
> larger range than the data itself, given a "normal" sample. 
>  For that
> reason, I would want the whiskers out to 3*IQR replaced by 
> the data points themselves.
> 

What you want would need some original programming. 
I think it is manageable with various tricks, 
but by the same token it would be tricky. If 
you've not programmed in Stata before, it 
would be a formidable first problem. 

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