Dear All
I am trying to do a repeated measures analysis using lmer and have a number
of issues. I have non-orthogonal, unbalanced data. Count data was obtained
over 10 months for three treatments, which were arranged into 6 blocks.
Treatment is not nested in Block but crossed, as I originally designed an
orthogonal, balanced experiment but subsequently lost a treatment from 2
blocks. My fixed effects are treatment and Month, and my random effects are
Block which was repeated sampled. My model is:
Model<-lmer(Count~Treatment*Month+(Month|Block),data=dataset,family=poisson(link=sqrt))
Is this the only way in which I can specify my random effects? I.e. can I
specify them as: (1|Block)+(1|Month)?
When I run this model, I do not get any residuals in the error term or
estimated scale parameters and so do not know how to check if I have
overdispersion. Below is the output I obtained.
Generalized linear mixed model fit by the Laplace approximation
Formula: Count ~ Treatment * Month + (Month | Block)
Data: dataset
AIC BIC logLik deviance
310.9 338.5 -146.4 292.9
Random effects:
Groups Name Variance Std.Dev. Corr
Block (Intercept) 0.06882396 0.262343
Month 0.00011693 0.010813 1.000
Number of obs: 160, groups: Block, 6
Fixed effects:
Estimate Std. Error z value Pr(>|z|)
(Intercept) 1.624030 0.175827 9.237 < 2e-16 ***
Treatment2.Radiata 0.150957 0.207435 0.728 0.466777
Treatment3.Aldabra -0.005458 0.207435 -0.026 0.979009
Month -0.079955 0.022903 -3.491 0.000481 ***
Treatment2.Radiata:Month 0.048868 0.033340 1.466 0.142717
Treatment3.Aldabra:Month 0.077697 0.033340 2.330 0.019781 *
---
Signif. codes: 0 *** 0.001 ** 0.01 * 0.05 . 0.1 1
Correlation of Fixed Effects:
(Intr) Trt2.R Trt3.A Month T2.R:M
Trtmnt2.Rdt -0.533
Trtmnt3.Ald -0.533 0.450
Month -0.572 0.585 0.585
Trtmnt2.R:M 0.474 -0.882 -0.402 -0.661
Trtmnt3.A:M 0.474 -0.402 -0.882 -0.661 0.454
Any advice on how to account for overdispersion would be much appreciated.
Many thanks in advance
Christine
----------------------
Christine Griffiths
School of Biological Sciences
University of Bristol
Woodland Road
Bristol BS8 1UG
Tel: 0117 9287593
Fax 0117 925 7374
christine.griffi...@bristol.ac.uk
http://www.bio.bris.ac.uk/research/mammal/tortoises.html
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