Dear R-users,

I'm new to R, so my apologies if this question doesn't make sense.

I've tried the following model in lmer, and it works perfectly:
model<-lmer(aphids~densroot+zone+(1|zone/site), family=quasipoisson)

But if I try the exact same model with a different variable, totmas, the model looks as follows:
model<-lmer(aphids~totmas+zone+(1|zone/site), family=quasipoisson)

Totmas is also a continuous variable just like densroot, but in this case I receive the following message:

CHOLMOD warning: &ß†e
Error in objective(.par, ...) : 
  Cholmod error `matrix not positive definite' at file:../Supernodal/t_cholmod_super_numeric.c, line 613

Moreover, if I test yet another continuous variable "vitality", to my surprise R just crashes completely.


This is a mystery to me, especially because the variables totmas or vitality don't give any problem when I build the exact same models in SAS with proc glimmix... 

Does someone have experience with this type of problem?

Thank you in advance,

Martijn.




-- 
Martijn Vandegehuchte 
Ghent University 
Department Biology 
Terrestrial Ecology Unit 
K.L.Ledeganckstraat 35 
B-9000 Ghent
telephone: +32 (0)9/264 50 84
e-mail: martijn.vandegehuchte@ugent.be 

website TEREC: www.ecology.ugent.be/terec

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