Dear mixed modellers,

 

I want to check if I have overdispersion in my GLMM (poisson). Is it
measured in the same way as for GLM? i.e. 
                                                         residual
deviance
                                                   n -  p degrees of
freedom.
 
The GLMM output below derived using lmer (package lme4) is given below, 

#Generalized linear mixed model fit by the Laplace approximation 

#Formula: spidS ~ elev + slope + cancov + totgrv + unstorey + totgrv *
factor(fortype) + factor(fortype) + (1 | fsite)

 

#   AIC   BIC logLik deviance
# 55.99 74.38 -19.00    37.99
#Random effects:
# Groups Name        Variance   Std.Dev.  
# fsite  (Intercept) 8.5232e-14 2.9194e-07
#Number of obs: 57, groups: fsite, 20
 
#Fixed effects:
#                          Estimate Std. Error z value Pr(>|z|)    
#(Intercept)              5.7834659  1.5565787   3.715 0.000203 ***
#elev                     0.0017985  0.0007788   2.309 0.020933 *  
#slope                   -0.0166407  0.0059833  -2.781 0.005416 ** 
#cancov                  -3.4359767  1.6478168  -2.085 0.037054 *  
#totgrv                   0.5659024  0.2387836   2.370 0.017791 *  
#unstorey                 0.3487673  0.1807072   1.930 0.053605 .  
#factor(fortype)2         0.8585417  0.2953460   2.907 0.003650 ** 
#totgrv:factor(fortype)2 -1.5961442  0.4304812  -3.708 0.000209 ***
#Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

 

If the above formula is correct then overdispersion is estimated in the
following way: 37.99/(57-9) = 0.79?

I am new to all of this and appreciate any help given - and apologies if
this is a trivial question for this forum, as yet I have not managed to
find a sufficient answer in texts books or on the web.

 

Many thanks,
 
Anne



Dr Anne Oxbrough

Postdoctoral Researcher

ForestBio Project

PLANFORBIO Research Programme

Dept. Zoology Ecology and Plant Science (CEC)

University College Cork

Distillery Fields

North Mall

Cork

Ireland

Tel  +353 21 490 4663

Fax +353 21 490 4664

http://www.ucc.ie/en/planforbio/

 

 

 


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