[R-sig-ME] Count data

Iker Vaquero Alba karraspito at yahoo.es
Wed Jun 22 20:43:44 CEST 2011

   Dear list:

   This may be quite a naive question: I am fitting a model in which the 
response variable is the number of chicks hatched in nests of different pairs of 
swallows. I assumed that this is count data, so I used family=poisson for the 
model. But when fitting the residuals vs. fitted values, I cannot see the 
structure that I've been told count data normally has. Also, amount of variance 
explained by random effects when assuming count data is this:

   Random effects:
 Groups    Name        Variance   Std.Dev.  
 pair:site (Intercept) 2.0757e-02 1.4407e-01
 site      (Intercept) 6.8285e-17 8.2635e-09
 year      (Intercept) 2.5513e-03 5.0511e-02

   Whereas if I assume gaussian errors, I get this:

   Random effects:
 Groups    Name        Variance Std.Dev.
 pair:site (Intercept) 4.89955  2.21349 
 site      (Intercept) 0.45493  0.67448 
 year      (Intercept) 0.22061  0.46969 
 Residual              0.80047  0.89469 

   I don't know if this is indicative of anything, but I'm concerned I may be 
making the wrong assumptions. Graph of residuals vs fitted is attached.

   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


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