[R-sig-ME] fitting models with poisson distributed data

Page E. Van Meter vanmete7 at msu.edu
Sat Oct 25 00:37:09 CEST 2008

Now that I have the code figured out, I hoping for some help on defining 
my model. I might be guilty of trying to fit an overly complex model to 
my data, although my model seems very simple in comparison to what has 
been discussed here. I'm hoping for feedback on my model design. Thanks 
in advance!

I have some pretty ugly longitudinal data measuring hormones and 
behaviors from individual hyenas over many years (355 samples from 39 
individuals). We collect hormone samples based on opportunity and have 
several samples from each individual (3-9 samples per hyena). My 
ultimate goal is to see if my hormone data explains any of the variation 
we see in the behavior data (aggression, I'll call it aggs). My 
dependent measurement is a behavior rate, count of aggs over time just 
prior to hormone sample collection. It is very zero heavy (lots of 
individuals did not aggress prior to hormone sample donation) and 
resistant to transformation to normality, but seems to be a pretty 
poisson distribution. My predictors are hormones and reproductive state 
(pregnant or lactating, which effect both aggression and hormones).

m2<-lmer(aggs~reprostate+hrm1+hrm2+(1|id), family=poisson, aggs)

Generalized linear mixed model fit using Laplace
Formula: aggs ~ reprostate + hrm1 + hrm2 + (1 | id)
   Data: aggs
 Family: poisson(log link)
   AIC   BIC logLik deviance
 12369 12387  -6179    12359
Random effects:
 Groups Name        Variance Std.Dev.
 id     (Intercept) 4.9353   2.2216 
number of obs: 307, groups: id, 39

Estimated scale (compare to  1 )  7.682887

Fixed effects:
            Estimate Std. Error z value Pr(>|z|)   
(Intercept) -3.07625    0.39575  -7.773 7.65e-15 ***
reprostate 2.32056    0.07724  30.044  < 2e-16 ***
hrm1        0.12575    0.04020   3.128  0.00176 **
hrm2       -0.80434    0.04770 -16.862  < 2e-16 ***

Correlation of Fixed Effects:
         (Intr) statct ecent
statecat -0.381             
ecent    -0.085  0.271      
acent     0.136 -0.339 -0.079

Page E. Van Meter
Michigan State University
Department of Zoology
vanmete7 at msu.edu

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