From ggrothendieck at gmail.com  Fri Jan 26 06:43:51 2007
From: ggrothendieck at gmail.com (Gabor Grothendieck)
Date: Fri, 26 Jan 2007 00:43:51 -0500
Subject: [Rd] ?mean
In-Reply-To: <20070126112826.7fd9b170@berwin5>
References: <971536df0701250653y47f06f84h9cc24a297250e144@mail.gmail.com>
	<20070126112826.7fd9b170@berwin5>
Message-ID: <971536df0701252143k3e70fc78wabd801aa9b46180@mail.gmail.com>

Good point.  Perhaps what is needed is a Note clarifying all this in ?mean
(unless the software itself is reworked as Martin has discussed).

Regarding var(x), one could use sd(x)^2.

On 1/25/07, Berwin A Turlach <statba at nus.edu.sg> wrote:
> G'day Gabor,
>
> On Thu, 25 Jan 2007 09:53:49 -0500
> "Gabor Grothendieck" <ggrothendieck at gmail.com> wrote:
>
> > The help page for mean does not say what happens when one
> > applies mean to a matrix.
>
> Well, not directly.  :-)
>
> But the help page of mean says that one of the arguments is:
>
>       x: An R object.  Currently there are methods for numeric data
>          frames, numeric vectors and dates.  A complex vector is
>          allowed for 'trim = 0', only.
>
> And the `Value' section states:
>
>     For a data frame, a named vector with the appropriate method being
>     applied column by column.
>
>     If 'trim' is zero (the default), the arithmetic mean of the values
>     in 'x' is computed, as a numeric or complex vector of length one.
>     If any argument is not logical (coerced to numeric), integer,
>     numeric or complex, 'NA' is returned, with a warning.
>
> Since a matrix is a vector with a dimension attribute, and not a data
> frame, one can deduce that the second paragraph describes the return
> value for `mean(x)' when x is a matrix.
>
> As I always tell my students, reading R help pages is a bit of an
> art. :)
>
> > mean and sd work in an inconsistent way on a matrix so that should at
> > least be documented.
>
> Agreed.  But it is documented in the help page of sd, which clearly
> states:
>
>     [....] If 'x' is a matrix or a data frame, a vector
>     of the standard deviation of the columns is returned.
>
> I guess you also want to have it documented in the mean help page?
>
> But then, should `var' also be mentioned in the mean help page?  This
> command also work in an a different and inconsistent manner to mean on
> matrices.
>
> And, of course, there are other subtle inconsistencies in the language
> used in these help pages.  Note that the mean help page talks about
> "numeric data frames" while the help pages of `var' and `se' talk about
> "data frames" only, though all components of the data frame have to be
> numeric, of course.
>
> > Also there should be a See Also to colMeans since that provides the
> > missing column-wise analog to sd.
>
> That's probably a good idea.  What would you suggest should be
> mentioned to provide the column-wise analog of `var'?
>
> Cheers,
>
>        Berwin
>



[Rd] ?mean
Gabor Grothendieck