

   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: 


> 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



   Other times this happened, it always ended up being a problem of a typo in the syntax. But this time, I've double-checked it several times, and I have even written the two models from scratch and then compare them, and I still get the same result. Obviously, there's a difference between the two models, so I can't understand why the AIC, BIC, log Lik and everything else are identical in both models.

> s.hatchmodel1.106<-lmer(prophatch1~sex+inslarge+tlength+briventral+sex:brithr+tlength:brithr+brithr:briventral+(1|site/pair)+(1|year),REML=FALSE)
> s.hatchmodel1.103<-lmer(prophatch1~sex+brithr+inslarge+tlength+briventral+sex:brithr+tlength:brithr+brithr:briventral+(1|site/pair)+(1|year),REML=FALSE)
> anova(s.hatchmodel1.103,s.hatchmodel1.106)
Data: 
Models:
s.hatchmodel1.103: prophatch1 ~ sex + brithr + inslarge + tlength + briventral + 
s.hatchmodel1.103:     sex:brithr + tlength:brithr + brithr:briventral + (1 | site/pair) + 
s.hatchmodel1.103:     (1 | year)
s.hatchmodel1.106: prophatch1 ~ sex + inslarge + tlength + briventral + sex:brithr + 
s.hatchmodel1.106:     tlength:brithr + brithr:briventral + (1 | site/pair) + (1 | 
s.hatchmodel1.106:     year)
                  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


   If anyone has any clue about why this might be happening, I would really appreciate it.
   Thank you very much.


   ________________________________________

   Iker Vaquero-Alba
   Centre for Ecology and Conservation
   Daphne du Maurier Building
   University of Exeter, Cornwall Campus
   Treliever Road
   TR10 9EZ Penryn
   U.K.

   http://biosciences.exeter.ac.uk/cec/staff/postgradresearch/ikervaquero-alba/
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