R help - Oct 2010 - general construction of 'all pairwise comparison' contrast in ANOVA

Hi R people

I am using regsubsets() to fit large numbers of models and collect summary
statistics in order to perform a Bayesian analysis of multi-way ANOVA with
specific prior information. In general the variables have differing numbers of
levels >=2.  This works well but with variable of more than 2 levels there
are naturally some arbitrary decisions about which treatment contrasts to
include in the model matrix.  It's obviously easy to choose the
'standard' contrasts using

options( contrasts = c( "contr.sum", "contr.poly" ))

with contr.helmert, contr.sum or contr.treatment as required.

However, what I would really like is a set of all pairwise comparison contrasts,
and preferably in a form that can be fed straight to regsubsets() so that all
labelling, fitting, output etc is automatically dealt with.

Are there any functions or libraries in R that will allow me to do this without
too much grief?

Thanks in advance


Michael Hopkins
Algorithm and Statistical Modelling Expert
 
Upstream
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London
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