The adjacent values are the most extreme
values within 1.5 iqr of the nearer
quartile. This definition goes back
to Tukey and is, I think, that most
commonly used in box plots.
That is, calculate
iqr = upper quartile - lower quartile
and identify
(1) the largest value <= (upper quartile + 1.5 iqr)
(2) the smallest value >= (lower quartile - 1.5 iqr)
This leaves open two overlapping issues:
(a) exactly how the quartiles are calculated: [R] summarize
gives Stata's rule
(b) other definitions of the box plot e.g. see
Frigge, M., Hoaglin, D.C. and Iglewicz, B. 1989.
Some implementations of the box plot.
The American Statistician 43: 50--54.
Having said that, I don't think that
-graph box- offers any handles for
varying this definition.
Recently a program -adjacent- was posted
on SSC which list adjacent values according
to this definition. More useful, but not
yet in the public domain, are -egen- functions
-adjl()-, -adju()- and -outside()- which
emit the lower and upper adjacent values
and (any) values outside those. These make
it a little easier to program your own variants on
the box plot.
Nick
n.j.cox@durham.ac.uk
John Wallace
> I've misplaced my graphics manual at the moment, and
> I'm trying to figure out two things:
> 1) what are the default values for the adjacent
> values (location of the end of the whiskers on a boxplot)
> 2) Can/how are they altered?
> I thought it was -graph box, cwhiskers(...)-, but that appears to be
> for the appearance of the whiskers (line weight, cap appearance, etc)
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/