[R] Why warnings using lmer-model with family=binomial

[Åsa Granberg]

Hi all!

My question is why, and what I can do about that 
I sometimes, but not always, get warning-messages like

nlminb returned message singular convergence (7)
  in: LMEopt(x = mer, value = cv)

or

IRLS iterations for PQL did not converge

when trying to fit a model looking like this: 
lmer<-(cbind(Diseased,Healthy)~Fungus+(1|Family)+(1|Fungus:Family), 
family="binomial") to four similar data sets?

All four data sets consists of four columns; 
Fungus (1 or 2), Family (1-30), Diseased and Healthy.
An example:

Fungus  Family          Diseased        Healthy
1               1               3               7
2               1               1               5
1               2               2               16
2               2               2               14

...and so on up to totally 30 families.

One of the data sets works perfectly fine and 
gives no warnings, but for the others i get 
different amount of warning messages. Between the 
data sets it is only the number of diseased anf 
healthy for each family and fungus that differ.
Does anyone have any idea why this happens and 
how I can get around it (I'm mainly interested in the interaction term...)?

Grateful for any help


/Åsa

__________________________________________________________________

Åsa Granberg, Doktorand / Phd-student

Evolutionär ekologi
Ekologi, miljö och geovetenskap
Umeå Universitet
901 87 Umeå

Evolutionary ecology
Department of Ecology and Environmental Science
Umeå University
901 87 Umeå, Sweden

Phone: +46 (0)90 786 7461
Fax: +46 (0)90 786 6705
e-mail:asa.granberg at emg.umu.se