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st: -skewplot- updated on SSC


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: -skewplot- updated on SSC
Date   Tue, 4 Feb 2003 14:18:13 -0000

Thanks to Kit Baum, the program -skewplot- has 
been updated on SSC, so that it now uses Stata 8's 
new graphics. 

At the same time, the previous version of 
-skewplot- has been renamed -skewplot6- 
as a flag that it requires Stata 6 and 
so may be used in either Stata 6 or Stata 7. 
(I am aware of various other ways of keeping 
different versions of a program equally 
accessible, but this was most convenient 
to me in this instance.) 

-skewplot- has some affinity with -symplot-,  
which has been in official Stata for some
time, so let us start from there. The idea behind 
-symplot- is to plot 

	y = upper quantile - median 

versus 

	x = median - lower quantile 

where the upper and lower quantiles
are corresponding pairs such as 

	maximum and minimum 
	second largest and second smallest 
	etc.

A characteristic of a symmetric distribution 
is evidently that y = x, so that this sloping 
line is the natural reference line. 

However, -symplot- as implemented in Stata
supports only the plotting of one set of 
data, so that comparison requires production 
of several graphs. In addition, the sloping 
reference is, in my experience, more awkward 
to work with than the horizontal reference of 
a display of 

	(upper quantile + lower quantile) / 2 

versus 

	upper quantile - lower quantile 

In a symmetric distribution, the first variable 
produces equal estimates of the centre of the 
distribution. 

-skewplot- supports the plotting of several 
variables or of several subsets of a variable. 

To install in Stata 8, type 

. ssc inst skewplot 

or 

. ssc inst skewplot, replace 

as appropriate. 

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


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