Impaired boxplot functionality - mean instead of median

I'd like to add two comments to Martin's sensible response.

1. I've seen several intro-stats textbooks that define a
boxplot to have whiskers to the extreme data values
and then define Tukey's boxplot as a "modified" boxplot.
I wish authors wouldn't do that.

2. I've also seen boxplots used for sample sizes as small
as -- are you ready for it? -- n = 2!! (Admittedly, only in
plots comparing several groups.) The help page for
stripchart() points out that stripcharts "are a good
alternative to boxplots when sample sizes are small".
My own rule-of-thumb: n > 20 for single boxplots, n > 12
for multiple boxplots.

Peter Ehlers

Martin Maechler wrote:

Boxplots were invented by John W. Tukey and I think should be
counted among the top "small but smart" achievements from the
20th century.  Very wisely he did *not* use mean and standard deviations.

Even though it's possible to draw boxplots that are not boxplots
(and people only recently explained how to do this with R on this
 mailing list), I'm arguing very strongly against this.

If I see a boxplot - I'd want it to be a boxplot and not have
the silly (please excuse)  10%--------90% whiskers  which
declare 20% of the points as outliers {in the boxplot sense}.

If you want the mean +/- sd plot, do *not* misuse boxplots
for them, please! 

Martin Maechler, ETH Zurich

"Evgeniy" == Evgeniy Kachalin <ka4alin at yandex.ru>
   on Thu, 01 Dec 2005 19:04:47 +0300 writes:

    Evgeniy> Hello to all users and wizards.
    Evgeniy> I am regulary using 'boxplot' function or its analogue - 'bwplot' from 
    Evgeniy> the 'lattice' library. 

 [there's the lattice *package*  !]

    Evgeniy> But they are, as far as I understand, totally 
    Evgeniy> flawed in functionality: they miss ability to select what they would 
    Evgeniy> draw 'in the middle' - median, mean. What the box means - standard 
    Evgeniy> error, 90% or something else. What the whiskers mean - 100%, 99% or 
    Evgeniy> something else.
    Evgeniy> Is there any way to realize it? Or is there any other good data 
    Evgeniy> visualization function for comparing means of various data groups? 
    Evgeniy> Ideally I would like to have a bit more customised function for doing 
    Evgeniy> that. For example, 'boxplot(a~b,data=d,mid='mean').


    Evgeniy> -- 
    Evgeniy> Evgeniy, ICQ 38317310.