[R-sig-ME] glmm question

[Ken Beath]

Having a random effect with only 3 levels is not recommended, it usually
gives problems fitting. There are also some philosophical questions about
its use as a random effect.

A random effect for field is reasonable, but you may be fitting too many
parameters. With only 57 observations it is easy to overfit the models, and
a standard linear model may be all that is necessary.

On 14 May 2015 at 07:15, Joaquín Aldabe <joaquin.aldabe at gmail.com> wrote:

> Hello, this is Joaquín Aldabe from Uruguay. I´m trying to model shorebird
> counts (Buff breasted Sandpiper, BBSA) with glmm (using lme4 package),
> using continuous variables (grass height, field area, forest cover) and one
> factor variable (presence/absence of other shorebird species: American
> Golden Plover, AMGP). I sampled 19 fields during three years in December.
> I´m interested in identifying predictors correlated with BBSA counts. I
> used Year as a random effect as I´m not interested in Year as a fix effect
> and because fields were counted three times (pseudoreplication).
>
> The model doesn´t converge, and the output showed that the factorial
> variable has not a significant effect. This is weird as in every field I
> observed the Buff breasted Sandpiper I also observed the other
> species. When I take AMGP out, the model runs ok.
>
> This is the model I´m trying to run:
>
>
> mysub3.3<-glmer(BBSA~Grass_height+Field_area+Field_enclosure_700m+Grass_height*Field_enclosure_700m+fAMGP+(1|fYear),family="poisson",
> data=mysub3.2)
>
> continuous variables were scaled.
>
> I can send de data frame if somebody is interested.
>
> Thanks in advanced for helping me on my master thesis.
>
> Cheers,
>
> Joaquín.
>
> --
> *Joaquín Aldabe*
>
> *Grupo Biodiversidad, Ambiente y Sociedad*
> Centro Universitario de la Región Este, Universidad de la República
> Ruta 15 (y Ruta 9), Km 28.500, Departamento de Rocha
>
> *Departamento de Conservación*
> Aves Uruguay
> BirdLife International
> Canelones 1164, Montevideo
>
> https://sites.google.com/site/joaquin.aldabe
> <https://sites.google.com/site/perfilprofesionaljoaquinaldabe>
>
>         [[alternative HTML version deleted]]
>
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-- 

*Ken Beath*
Lecturer
Statistics Department
MACQUARIE UNIVERSITY NSW 2109, Australia

Phone: +61 (0)2 9850 8516

Building E4A, room 526
http://stat.mq.edu.au/our_staff/staff_-_alphabetical/staff/beath,_ken/

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