[R-sig-ME] interaction terms interpretation

[Ken Beath]

There is actually a continuous third order interaction, which are just
about impossible to interpret.

As well as the graphs, which you always should start with, think about what
is sensible for the model. Should any of the predictors be log-scaled? As AMGP
is a count it wouldn't be surprising it it is, or use log(1+x) if there are
zeroes.

Look at diagnostic plots and make sure that everything is fine, especially
that there are no outliers which might be causing the interaction.

Ken

On 10 August 2015 at 17:02, Thierry Onkelinx <thierry.onkelinx at inbo.be>
wrote:

> Dear Joaquin,
>
> Your mail is mangled because you send it in HTML. This makes the model
> output very hard to read.
>
> It's impossible to interpret model output without thorough knowledge of the
> dataset. I suggest that you try to make some graphs of the model.
>
> Best regards,
>
> ir. Thierry Onkelinx
> Instituut voor natuur- en bosonderzoek / Research Institute for Nature and
> Forest
> team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
> Kliniekstraat 25
> 1070 Anderlecht
> Belgium
>
> To call in the statistician after the experiment is done may be no more
> than asking him to perform a post-mortem examination: he may be able to say
> what the experiment died of. ~ Sir Ronald Aylmer Fisher
> The plural of anecdote is not data. ~ Roger Brinner
> The combination of some data and an aching desire for an answer does not
> ensure that a reasonable answer can be extracted from a given body of data.
> ~ John Tukey
>
> 2015-08-07 21:27 GMT+02:00 Joaquín Aldabe <joaquin.aldabe at gmail.com>:
>
> > Hi all, I´m running a mixed model with poisson error distribution as I
> have
> > count data of a shorebird species. My predictors are AMGP (another
> > shorebird that is usually associated to "my" species and probably has
> > ecological influence on it), grass height and field distance to the
> lagoon.
> >
> > I´m having a hard time trying to interpret second order interactions. I
> > appreciate your comments of references recommendations. Here is the
> output
> > of my model. All the best, Joaquín.
> >
> > Call:glmmadmb(formula = BBSA ~ AMGP * Grass_height *
> > Distance_to_lagoon +     (1 | Field_name) + offset(log(Field_area.o)),
> > data = my4S,     family = "nbinom1", zeroInflation = TRUE)
> > AIC: 226
> > Coefficients:                                     Estimate Std. Error
> > z value Pr(>|z|)    (Intercept)                           -5.8797
> > 1.2250   -4.80  1.6e-06 ***AMGP
> > 3.3086     1.0246    3.23   0.0012 ** Grass_height
> >      -4.1592     1.9961   -2.08   0.0372 *  Distance_to_lagoon
> >            -0.5029     1.3929   -0.36   0.7180    AMGP:Grass_height
> >                   2.9442     1.5142    1.94   0.0518 .
> > AMGP:Distance_to_lagoon               -2.4695     0.9632   -2.56
> > 0.0104 *  Grass_height:Distance_to_lagoon        0.0339     2.0065
> > 0.02   0.9865    AMGP:Grass_height:Distance_to_lagoon  -4.0363
> > 1.3264   -3.04   0.0023 ** ---Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01
> > ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
> > Number of observations: total=64, Field_name=16 Random effect
> > variance(s):Group=Field_name            Variance StdDev(Intercept)
> > 0.03721 0.1929
> > Negative binomial dispersion parameter: 20.633 (std. err.:
> > 7.4042)Zero-inflation: 1e-06  (std. err.:  6.8336e-09 )
> > Log-likelihood: -102
> >
> >
> > --
> > *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
> >
>
>         [[alternative HTML version deleted]]
>
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>



-- 

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

Phone: +61 (0)2 9850 8516

Level 2, AHH
http://stat.mq.edu.au/our_staff/staff_-_alphabetical/staff/beath,_ken/

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