[R-sig-ME] Random slope

Ben Bolker bolker at ufl.edu
Fri Jun 11 14:09:00 CEST 2010

fengsj at mail.utexas.edu wrote:
> I am in the process of selection models. If the random slope for A is  
> decided in the model, but the fixed effect of A is not significant. Is  
> that ok if I remove fixed A? I don’t feel very comfortable with it.   
> Is the random slope like the interaction between the fixed and random  
> factor?  If the interaction term is in the model, we better also keep  
> all the main effects even if they are not significant. Does it also  
> apply in this situation?
> Thanks
> Shujuan

  Your analogy is good.
  Keeping a random slope while discarding the fixed effect of A is
equivalent to assuming that the population mean slope is *exactly* zero
while the variance is non-zero: it's possible that it could make sense
for some special cases, but in general it doesn't.

  [Insert usual caveats about model selection algorithms here]

  Ben Bolker

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