Count data

Enviado: mi?,22 junio, 2011 20:43
Asunto: [R-sig-ME] Count data


   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
   U.K.

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