[R] Redundancy contrasts

[Robert Espesser]

Is it meaningful to run 2 different sets of contrasts on a model , or is
there some redundancy somewhere ?
For example,I have a model :
tcons ~ group

where group is a factor with 3 levels ( A, B, C)
I first run the model with the default contrasts (treatment), 
so I  tested   (A vs B)   and (A vs C); 
but is it meaningful to also carry on a 2nd analysis with an other set
of
contrasts, to  test B vs C  ?  ie   c(0,-1,1)  ,  in fit.contrasts
gregmisc notation.

I'v heard  that the  first  set of contrast ( ie treatment default in
the example)
 extracts all the "information"
in the model, and that  a second analysis with an other set of contrasts
was not meaningful.

Thanks for help. 
-- 
Robert Espesser     
Laboratoire Parole et Langage  UMR 6057, CNRS
29 Av. Robert Schuman  13621 AIX    (FRANCE)
Tel: +33 (0)4 42 95 36 26   Fax: +33 (0)4 42 59 50 96
http://www.lpl.univ-aix.fr/~espesser
mailto:Robert.Espesser at lpl.univ-aix.fr