

   Dear Ben: 


   Thank you very much. I simplify that term only to find out which is the effect of each of the main terms taking part in the interaction in which I'm really interested in, but I don't pretend to simplify it really. The thing is that I had 3 significative 1st order interactions, and the term "brithr" was included in all of them, being the one that generated the error. When simplifying the rest of the main effects taking part in one or more interactions (but never in ALL of them like "brithr"), everything worked fine. I wonder wether this could be the problem. 


   So, I guess, I simply can't report the effect of the main term "brithr". Is that right?

   Thank you very much.

   Iker



________________________________
De: Ben Bolker <bbolker@gmail.com>
Para: r-sig-mixed-models@r-project.org
Enviado: lunes 26 de septiembre de 2011 22:37
Asunto: Re: [R-sig-ME] Chisquare=0, p-value=1

Iker Vaquero Alba <karraspito@...> writes:

>     Dear list:    I am implementing a model and simplifying it
> step-wise. Everything goes fine until I get the minimum adequate
> model. When I try to simplify one of the main effects and compare
> the model with or without that effect, I get a chisquare value of 0
> and a p of 1:

  I still don't think you should do stepwise simplification, but ...

This has come up as recently (with a very similar subject line) as
http://article.gmane.org/gmane.comp.lang.r.lme4.devel/6649

> 
> > s.hatchmodel1.106<-update(s.hatchmodel1.103,~.-brithr)
> > anova(s.hatchmodel1.103,s.hatchmodel1.106)
> Data: 
> Models:
> s.hatchmodel1.103: prophatch1 ~ sex + inslarge + tlength +
>  briventral + brithr + 
> s.hatchmodel1.103:     (1 | site/pair) + (1 | year) +
>  sex:brithr + tlength:brithr + 
> s.hatchmodel1.103:     briventral:brithr
> s.hatchmodel1.106: prophatch1 ~ sex + inslarge + tlength + 
> briventral + (1 | site/pair) + 
> s.hatchmodel1.106:     (1 | year) + sex:brithr + 
> tlength:brithr + briventral:brithr
>                   Df     AIC     BIC logLik Chisq Chi Df Pr(>Chisq)
> s.hatchmodel1.103 13 -130.36 -98.915  78.18                        
> s.hatchmodel1.106 13 -130.36 -98.915  78.18     0      0          1
> 

   The problem is that you are dropping a main effect, "brithr",
while at least one interaction with brithr is still in the model.
There are (very) unusual circumstances where it could make sense
to drop a main effect while the interaction was still present,
but R's default way of constructing interactions will guarantee
that the main effect is still implicitly included in the model.
See the thread referenced above.

  Ben Bolker

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