# [R] Overdispersion using repeated measures lmer

Christine Griffiths Christine.Griffiths at bristol.ac.uk
Mon May 18 13:26:16 CEST 2009

```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:

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 ***
Treatment3.Aldabra       -0.005458   0.207435  -0.026 0.979009
Month                    -0.079955   0.022903  -3.491 0.000481 ***
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.

Christine

----------------------
Christine Griffiths
School of Biological Sciences
University of Bristol