Dear Albert Romero

I am referring to SE-part of your question:
Lately I had the same problem as you had: SE of my lme-models overlap zero 
to some extent. Of course, it could be that my variables are just bad. But 
from other analysis I knew this could not be. On a bypass (I actually 
wanted to find out something else) I had to find out that the SE-output of 
R corresponds to the usually-called Standard Deviation. (Actually R assumes 
that you are taking multiple data sets from the same population and you are 
comparing their means --> in this case the output SE is correct). Two 
senior bio-statisticians confirmed this. Look at:

G <- lm(rnorm(1000) ~rnorm(1000))
             summary(G)
             vcov(G)

vcov corresponds to the variances of the parameters, in the summary you 
find the standard errors . You will find out that standard error^2 = 
variances, thus SE ~ SD. So, with

SE/sqrt(n)

=> you obtain the SE that you want. I know, it sounds a bit strange that 
this is the problem, but step-by-step calculations convinced me...

Maybe somebody can explain it in a statistically more comprehensive way. 
Nevertheless, the outcome will be the same.

Best wishes

Thomas



>Date: Sun, 31 Aug 2008 18:00:12 +0200
>From: Albert Romero <albertromeropuente@ub.edu>
>Subject: [R-sig-eco] glmer ,    quasipoisson and standard errors of the
>         coefficients
>To: r-sig-ecology@r-project.org
>Message-ID: <48BAC00C.2010205@ub.edu>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>Hello,
>I am trying to simplify backwards a mixed effects model, using lmer
>function from lme4 package. As my data are species numbers and there
>exists overdisperison, I think appropriate to use glmer function with
>error family quasipoisson. I compare one model with its simplification
>through log-likelihood ratio tests.
>Nevertheless, once I have selected a simplified model, I find in the
>summary of this 'significant' model that estimated coefficients  are
>associated  to  very big  standard errors, to the point that no one of
>the coefficients seem to be significantly different from zero.
>
>Here come my questions:
>Can anybody explain this contradiction among standard errors of the
>estimated coefficients and the significance of the model?
>Is unappropriated to use Log-likelihood backwards simplification with
>quasipoisson errors?
>
>Thank you in advance!
>
>--
>Albert Romero Puente
>Departament de Biologia Vegetal-Bot?nica
>Universitat de Barcelona
>Facultat de Biologia
>3a Planta
>Av. Diagonal, 645. (08028) Barcelona
>Tel. 0034 93 402 14 71
>
>
>
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>End of R-sig-ecology Digest, Vol 6, Issue 1
>*******************************************

_______________________________________________

Thomas Sattler

Swiss Federal Research Institute WSL
Bellinzona, Birmensdorf

& Conservation Biology
Institute of Ecology and Evolution (IEE), University of Bern

<http://www.wsl.ch/personal_homepages/sattler>http://www.wsl.ch/personal_homepages/sattler  
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