[R] Whiskers on the default boxplot {graphics}

Robert Baer rbaer at atsu.edu
Thu May 13 16:25:09 CEST 2010

> Hi Peter,
> You're absolutely correct!  The description for 'range' in 'boxplot' help 
> file is a little bit confusing by using the words "interquartile range". 
> I think it should be changed to the "length of the box" to be exact and 
> consistent with those in the help file for "boxplot.stats".

The issue is probably that there are multiple ways (9 to be exact) of 
defining quantiles in R.  See 'type= ' arguement for ?quantile.  The 
quantile function uses type=7 by default which matches the quantile 
definition used by S-Plus(?), but differs from that used by SPSS.  Doesn't 
fivenum essentially use the equivalent of a different "type= " arguement 
(maybe 2 or 5) in constructing the hinges?

It seems perfectly reasonable to talk about 'length of box' (or 'box height' 
depending how you display the boxplot), but aren't the hinges simply Q1 and 
Q3 defined by one of the possible quartile definitions (as Peter points out 
the one used by fivenum)?  The box height does not necesarily match the 
distance produced by IQR() which also seems to use the equivalent of 
quantile(..., type=7), but it is still an IQR, is it not?

Quantiles apparantly can be defined in more than one "acceptable" way (sort 
of like dealing with ties in rank statistics).  The OP seemed to want an 
"exact" explanation of the wiskers, and I think Peter has pointed us at the 
definition of quartiles used by fivenum, as opposed to the default  used 
with quantile(..., "type=7").

All that said, I'm not convinced that it is wrong to speak of "interquartile 
range" in 'boxplot' help.


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