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st: RE: vwidth in old boxplot graphics


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: vwidth in old boxplot graphics
Date   Tue, 23 Aug 2005 18:52:43 +0100

It is still available in Stata as -graph7, box vwidth-. 

Programming this yourself in Stata 8 or 9 would be possible, 
I guess, but not quite trivial. 

In my view, wanting something like this touches on a key 
limitation of box plots: they often leave out far too much 
detail. Box plots, I suggest, are optimal when the number of 
groups concerned is >~ 10 and severe compression is needed to
see "the wood for the trees". With fewer groups, more detail 
is often tolerable and even highly desirable. 

(Incidentally, an example which I owe to a Howard Wainer paper is 
instructive. How do you interpret a box plot like this? 

         +---------------+------------------+
    +----|               |                  |----+ 
	   +---------------+------------------+	

Most people I have asked go for a diagnosis of a short-tailed 
distribution. This forgets that if the half the distribution
is inside the box, then the other half must be outside. In 
this case, the average density in the tails must be much higher than 
in the centre and the best guess has to be a U-shaped distribution. 

Boxplots can be much harder to interpret than you think!) 

Alternatives such as -dotplot- (official), 
-onewayplot- (user-written), -beamplot- (user-written) 
dedicated to the idea of showing one symbol for every 
data point have the advantage that a clear impression of 
the number of data points is given. 

Yet further plots are possible showing all the quantiles. 
-quantile- (official) is here less flexible than -qplot- 
(user-written). The next issue of the Stata Journal will
carry a long diatribe on quantile plots and will 
be accompanied by an enhanced version of -qplot- (and 
also of -distplot-, also user-written). 

A yet further possibility is to hybridise dot and box 
plots. One example is given by Wild and Seber, "Chance 
encounters" p.122. 

Fortuitously, just this afternoon a colleague and I 
came up with our own hybrid. This assumes a categorical 
variable coded by successive integers. I wouldn't defend 
this default design to the limit as it was entirely 
optimised for one particular dataset. However, the main 
point is that your own hybrid design is attainable 
with some coding. (Varying width boxes do sound a bit 
harder.) 

For this to work, you need -onewayplot- from SSC. 

Silly example: 

. sysuse auto
. myboxplot mpg rep78, magic(-0.2) rbar(barw(0.15)) ysc(noreverse) stack h(0.5)

*! 1.0.0 NJC/ISE 23 Aug 2005 
program myboxplot 
	version 9 
	syntax varlist(min=2 max=2 numeric) [if] [in] ///
	[, magic(real 0.4) rbar(str asis) * ] 
	marksample touse
	qui count if `touse' 
	if r(N) == 0 error 2000

	tokenize `varlist' 
	args y cat 

	tempvar median upq loq offset 
	qui { 
		egen `median' = median(`y') if `touse', by(`cat')
		egen `loq' = pctile(`y') if `touse', by(`cat') p(25)
		egen `upq' = pctile(`y') if `touse', by(`cat') p(75)
		gen `offset' = `cat' + `magic'  
	}
	onewayplot `y' if `touse', by(`cat') msy(+) msize(small) ///
	plot(rbar `upq' `median' `offset',            ///
	barw(0.25) blcolor(black) bcolor(gs14) hor legend(off) `rbar' ///
	|| rbar `loq' `median' `offset',              ///
	barw(0.25) bcolor(gs14) blcolor(black) hor `rbar') ///
	xti("`: variable label `y''") ysc(reverse) yla(, noticks) yti("") `options'  
end 





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

> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu]On Behalf Of skreisel
> Sent: 23 August 2005 17:17
> To: statalist@hsphsun2.harvard.edu
> Subject: st: vwidth in old boxplot graphics
> 
> 
> What happend to the usefull Stata 7.0 box-plot command [graph y, box
> by(x) vwidth]? Does anyone know of an alternative to graphically
> highlight the N of each boxed group by adjusting its relative width?
> Regards
> Stefan Kreisel
> *
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