[R] drop1 with contr.treatment

Prof Brian Ripley ripley at stats.ox.ac.uk
Fri Aug 20 16:41:06 CEST 2004

Since this was addressed to R-Core (please do read the posting guide!) 
discussion will happen elsewhere.

R-help is not the right place for suggestions of changes to R: R-devel may
be.  As with bugs, please ensure you understand why the existing
behaviour is not both correct and intentional.  In a case like this, see
what the White Book says it does (as referred to on the help page) and
check it against S if you can.

On Fri, 20 Aug 2004, Christoph Buser wrote:

> Dear R Core Team
> I've a proposal to improve drop1(). The function should change the
> contrast from the default ("treatment") to "sum". If you fit a
> model with an interaction (which ist not signifikant) and you
> display the main effect with
> drop1(   , scope = .~., test = "F")
> If you remove the interaction, then everything's okay. There is
> no way to fit a model, let an non signifikant interaction in
> the model and get interpretable main effects with the
> "treatment" contrast.
> One solution would be to change automatically the contrast to
> the "sum" contrast.
> Another possibility is to produce a warning when you use drop1
> with the scope argument to get main effects in the presence of 
> an interaction (even not signifikant) with "treatment"
> contrast. 


Brian D. Ripley,                  ripley at 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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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