On Fri, Oct 21, 2011 at 5:57 AM, Thaler, Thorn, LAUSANNE, Applied
Mathematics <thorn.tha...@rdls.nestle.com> wrote:
> Dear all,
>
> Today I figured out that there is a neat function called droplevels,
> which, well, drops unused levels in a data frame. I tried the function
> with some of my data sets and it turned out that not only the unused
> levels were dropped but also the contrasts I set via "C". I had a look
> into the code, and this behaviour arises from the fact that droplevels
> uses simply factor to drop the unused levels, which uses the default
> contrasts as set by options("contrasts").
>
> I think this behaviour is annoying, because if one does not look
> carefully enough, one looses the contrasts silently. Hence may I suggest
> to change the code of droplevels to something like the following:This silently changes the contrasts -- eg, if the first level of the factor is one of the empty levels, the reference level used by contr.treatment() will change. Also, if the contrasts are a matrix rather than specifying a contrast function, the matrix will be invalid for the the new factor. I think just having a warning would be better -- in general it's not clear what (if anything) it means to have the same contrasts on factors with different numbers of levels. -thomas -- Thomas Lumley Professor of Biostatistics University of Auckland ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
