[R] Whiskers on the default boxplot {graphics}

[Shi, Tao]

Hi Robert,

Your points are well taken.  However, I reserve mine, b/c I think without this detailed discussion, an average R user would simply confused the "interquartile range" said in boxplot help file with the results of "IQR".  Changing it to "length of box" makes it more exact and consistent, as I stated earlier.  With all these being said, this is up to the R core team to decide.

...Tao





----- Original Message ----
> From: Robert Baer <rbaer at atsu.edu>
> To: "Shi, Tao" <shidaxia at yahoo.com>; Peter Ehlers <ehlers at ucalgary.ca>
> Cc: R Project Help <R-help at r-project.org>
> Sent: Thu, May 13, 2010 7:25:09 AM
> Subject: Re: [R] Whiskers on the default boxplot {graphics}
> 
> > 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.

Rob