[Rd] under certain conditions, model.matrix appears to lack one
column (PR#646)
Prof Brian D Ripley
ripley@stats.ox.ac.uk
Mon, 28 Aug 2000 12:26:21 +0100 (BST)
On Mon, 28 Aug 2000, Rashid Nassar wrote:
> Dear Professor Ripley,
>
> Thank you very much for your kind explanation. If I may lamely say
> something in my defence, even as I apologize for my error: I mistook the
> sentence "the (quoted) name of a function" to mean "optionally quoted"
> because of the parentheses surrounding "quoted", and was encouraged by the
> fact that R did not complain when I gave the unquoted form and that it did
> implement a change (although not the desired one), and (even more lamely)
> the fact that none of the examples on the help page show the function
> quoted, although I now realize that in the example the function has an
> argument, and is not given simply the name of the function.
>
> Again, thank you for taking the time to reply and point out my mistake.
That was not what I was trying to do! If this had not already been fixed
as a result of another bug fix, it would be a bug we needed to address.
>
> Rashid Nassar
>
>
> On Mon, 28 Aug 2000, Prof Brian D Ripley wrote:
>
> > Date: Mon, 28 Aug 2000 07:28:19 +0100 (BST)
> > From: Prof Brian D Ripley <ripley@stats.ox.ac.uk>
> > To: rnassar@duke.edu
> > Cc: r-devel@stat.math.ethz.ch, R-bugs@biostat.ku.dk
> > Subject: Re: [Rd] under certain conditions, model.matrix appears to lack one column (PR#646)
> >
> > On Sun, 27 Aug 2000 rnassar@duke.edu wrote:
> >
> > > Dear R Team,
> > >
> > > # Summary of the problem: setting contrasts as
> > > > contrasts(g) <- contr.treatment
> > > or > contrasts(g) <- matrix(c(1,-1,0),ncol=1)
> > > (i.e. without quotes around `contr.treatment' or `contr.sum', etc.)
> > > and fitting an lm model without an intercept results in a model matrix
> > > that lacks one column.
> >
> > Right, but first is not allowed by (?contrasts)
> >
> > ctr: either a matrix whose columns give coefficients for contrasts
> > in the levels of `x', or the (quoted) name of a function
> > which computes such matrices.
> >
> > and the second is arguably correct: you chose to use only two columns.
> > However, the behaviour is different in the development version,
> > which does not code x in models such as this, and so does not
> > have the problem.
> >
> > That is not precisely what S does, but seems a sane way of interpreting the
> > contradictory information given (which is that you don't want an intercept
> > and that you only want two columns for the coding matrix for x! That
> > applies to the function case too: there the coding matrix is evaluated
> > before knowing if contrast are required, that is using argument contrasts =
> > FALSE).
> >
> > --
> > Brian D. Ripley, ripley@stats.ox.ac.uk
> > Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
> > University of Oxford, Tel: +44 1865 272861 (self)
> > 1 South Parks Road, +44 1865 272860 (secr)
> > Oxford OX1 3TG, UK Fax: +44 1865 272595
> >
> >
>
--
Brian D. Ripley, ripley@stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272860 (secr)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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