

   Dear list: 


   I am implementing a model with lmer in which the dependent variable is the number of chicks fledged by a number of barn swallow couples. I am using the simplification method, and have realized that the order in which different elements are introduced in the model has a great important, to the extent of making a "false convergence" warning message disappear. After doing all the simplification, I arrive to this model:

   s.hatchcoltailmodel1.76<-lmer(nhatch1~sex+tlength+brithr+briventral+cond+weatherpc1+weatherpc1:briventral+weatherpc1:cond+(1|site/pair)+(1|year),family=poisson,na.action=na.omit)

   This one, the LAST one (and the simplest) gives me this error message:

   Mensajes de aviso perdidos
1: In mer_finalize(ans) :
  Cholmod warning 'not positive definite' at file:../Cholesky/t_cholmod_rowfac.c, line 432
2: In mer_finalize(ans) :
  Cholmod warning 'not positive definite' at file:../Cholesky/t_cholmod_rowfac.c, line 432
3: In mer_finalize(ans) : false convergence (8)


   No matter the order in which I write the elements. 


   I really cannot understand how it is possible that I have been able to make all the simplification from the maximal model (18 parameters), and after a long and careful process, exactly when I arrive to my minimal model (8 parameters and only 2 interactions) is when everything collapses. It is really annoying. 


   Any suggestions, comments or ideas'll be much appreciated.

   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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