[R] model.matrix for non-hierarchical models
Peter Dalgaard
p.dalgaard at biostat.ku.dk
Fri Nov 4 17:04:57 CET 2005
Lukas Meier <meier at stat.math.ethz.ch> writes:
> Dear R-users,
>
> Using the function model.matrix I noticed the following
> behaviour (example below): Using the formula "~ a + a:b" will
> give a matrix of the same dimension as using "~ a * b". In the
> first case there are additional columns for the interaction (to
> compensate for the missing main-effect).
>
> dd <- data.frame(a = gl(3,4), b = gl(4,1,12))
> model.matrix(~ a*b, dd, contrasts = list(a="contr.sum",
> b="contr.sum"))
> model.matrix(~ a + a:b, dd, contrasts = list(a="contr.sum",
> b="contr.sum"))
>
> Is there any way to get the design matrix corresponding to "~ a
> + a:b" or do I have to do this manually?
The design matrix corresponding to ~ a + a:b is what you go above. If
that wasn't what you wanted, perhaps you should tell us what you did
want.
--
O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk) FAX: (+45) 35327907
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