[R-sig-eco] Question about poisson GLM

[Bob O'Hara]

On 27/10/14 15:38, rosanna di maggio wrote:
> Dear all,
>
> I'm trying to perform a poisson GLM with lme4 using the number of
> individuals as dependent variable and i) years of study and ii) different
> study areas as factors... So far so good..
>
> But the results are quite curious...  The estimates and standard errors are
> huge.. Is is normal? I don't think so...
>
>
> This is an example of what happen:
It's difficult to say without seeing that data, but a couple of things 
spring to mind:

1. If the covariates are meant to be continuous, then did you centre and 
scale them?
2. If they are meant to be factors, did you tell R this? e.g. anno <- 
factor(anno).

Bob

>
> Call:
> glm(formula = NTOT ~ areaN * anno, family = poisson)
>
> Deviance Residuals:
>      Min       1Q   Median       3Q      Max
> -3.7465  -1.0629  -0.5000   0.3132   6.8188
>
> Coefficients:
>                Estimate Std. Error z value Pr(>|z|)
> (Intercept) 140.786646  27.766187   5.070 3.97e-07 ***
> areaN        71.479294  11.876992   6.018 1.76e-09 ***
> anno         -0.069199   0.013823  -5.006 5.56e-07 ***
> areaN:anno   -0.035572   0.005912  -6.017 1.78e-09 ***
>
>
>
>
> I cannot understand the reason of these results... Someone can help me?
>
>
>
> Thank you
>
> Rox
>


-- 

Bob O'Hara

Biodiversity and Climate Research Centre
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