[R-sig-ME] glmm question

[Joaquín Aldabe]

Thanks a lot Ken for your response. I had decided to use mix model with
random effects because I have repeated measures in each field (one per
year). If I perform a glm, how can I manage the pseudoreplication?
(repeated counts on the same field)

I have one data per field per year. So, no hierarchical or nested structure
of the data. Can I use field as a random effect anyway?

Thanks a lot for your help.

Cheers,
Joaquín.

2015-05-13 19:17 GMT-03:00 Ken Beath <ken.beath at mq.edu.au>:

> 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
>
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>
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