[R] Redundancy contrasts

Robert Espesser Robert.Espesser at lpl.univ-aix.fr
Tue Feb 18 11:12:03 CET 2003

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
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
mailto:Robert.Espesser at lpl.univ-aix.fr

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