[R-sig-ME] mixed model?

[Ben Bolker]

Minor correction: if GrassHeight is a continuous variable then you
need (GrassHeight|Field) to model the among-Field variation in the
effect of grass height.  If GrassHeight is categorical, then
(GrassHeight|Field) will also work, but it will fit an unstructured
variance-covariance model (n*(n+1)/2 parameters for an n-level
categorical predictor), whereas (1|Field/GrassHeight) would fit a
(positive) compound-symmetric model for the variation in grass height
effects among fields (2 parameters instead of n*(n+1)/2)

On Tue, May 2, 2017 at 1:36 PM, Joaquín Aldabe <joaquin.aldabe at gmail.com> wrote:
> Thankyou very much Evan. I´ll try that!
> Cheers,
> Joaquín.
>
> 2017-05-02 14:17 GMT-03:00 Evan Palmer-Young <ecp52 at cornell.edu>:
>
>> Joaquin,
>> It looks like you could use Year and Field as random effects, since there
>> might be variation in bird abundance across years, and similarly, variation
>> across fields.
>>
>> So in this case your model is
>> Birdmodel<- glmer(Presence~ GrassHeight * ForestCover + (1|Year) +
>> (1|Field), data=BirdData, family =                  "binomial")
>>
>> Alternatively you could use Year as a fixed effect, if you are interested
>> in particular years.
>> Another option is to include interaction terms as random effects, eg
>> (1|Field:GrassHeight), to allow the effect of GrassHeight to vary across
>> fields.
>>
>>
>> On Fri, Apr 28, 2017 at 9:32 AM, Joaquín Aldabe <joaquin.aldabe at gmail.com>
>> wrote:
>>
>>> Dear all, I'm analysing bird presence/absence in 16 grassland fields over
>>> 4
>>> seasons (different years) and want to know the effect of grass height and
>>> forest cover on presence/absence of the species. Grass height varied among
>>> season but not forest cover in each field. So we have a spatial dimension
>>> and a time dimension. I tried a binomial glm but wonder if I should use
>>> generalized linear mixed models with field identity as the random as I
>>> have
>>> repeated measures (bird counts) in each field.
>>>
>>> I appreciate your opinion.
>>>
>>> Thanks in advanced,
>>>
>>> Joaquin Aldabe.
>>>
>>> --
>>> *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]]
>>>
>>> _______________________________________________
>>> R-sig-mixed-models at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
>>
>>
>>
>>
>> --
>> Evan Palmer-Young
>> PhD candidate
>> Department of Biology
>> 221 Morrill Science Center
>> 611 North Pleasant St
>> Amherst MA 01003
>> https://scholar.google.com/citations?user=VGvOypoAAAAJ&hl=en
>> https://sites.google.com/a/cornell.edu/evan-palmer-young/
>> epalmery at cns.umass.edu
>> ecp52 at cornell.edu
>>
>
>
>
> --
> *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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