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.