[R] Orthogonal polynomial contrasts

Peter Dalgaard p.dalgaard at biostat.ku.dk
Thu Apr 10 14:22:55 CEST 2008

Chris Bennett wrote:
> How do you remove one of the terms from an ordered polynomial contrast in
> your linear model.  For example, I have significant terms for linear and
> cubic but not quadratic, how would i remove the quadratic term from
> lm(response~treatment)
> Cheers,
> Chris
Are you _sure_ you want to do that? The interpretation can get tricky, 
but if you insist, just set the contrast matrix for the term 
accordingly, e.g.,

C(myterm, contr.poly(4)[,-2])

   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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