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RE: st: RE: Re: Other Box plots


From   "Wallace, John" <John_Wallace@affymetrix.com>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: Re: Other Box plots
Date   Mon, 2 Dec 2002 10:41:52 -0800

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.

I understand what you're saying about the -box- and -box2- ados; I was
uncertain about what the whiskers on the -graph, box- command  represented.

-JW

-----Original Message-----
From: Nick Cox [mailto:n.j.cox@durham.ac.uk] 
Sent: Monday, December 02, 2002 9:32 AM
To: statalist@hsphsun2.harvard.edu
Subject: RE: st: RE: Re: Other Box plots


Wallace, John replied to Fred Wolfe: 
> 
> Thanks Fred, thats a huge improvement.  The whiskers appear
> to behave as
> percentiles however (5th & 95th, perhaps?) rather than 
> functions of the IQR.
> Still a useful display though!
> 

I've got lost in this thread in terms of what
you want. 

-box- and -box2- are wrappers for -graph, box by()-. 
The box plots they produce are exactly those 
produced by -graph, box-: the only difference 
is that a preliminary -sort- command is rendered 
unnecessary (setting aside the fact that the 
sort order of your data may be changed). 

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