[R-sig-ME] GLMM & lack of linearity on the logit

[Simon Blomberg]

Hi Luciano,

In general, categorization or "binning" is a bad thing to do. You are 
throwing away information in the process, and the size and number of 
bins must be subjective at some level. If there is important 
nonlinearity in egg volume, you could consider a Generalized Additive 
Mixed Model (GAMM) with a smoothing term for egg volume. I recommend 
package mgcv.

Cheers,

Simon.

On 05/07/10 10:34, Luciano La Sala wrote:
> Dear R-people,
>
> I have just received from reviewers of a manuscript some harsh comments on
> the statistical procedures. I'm studying risk factors of mortality at the
> nest level among Olrog's Gull nest mates, which is why I used mixed models
> with random intercepts (Nest ID). The outcome of interest if "Death"
> (yes/no) and one of my explanatory variables is "Egg Volume" (continuous).
> Since violation of linearity on the logit was evident I created 4 categories
> using the quartiles of the distribution and modeled them as dummies.
>
> However, one reviewer stated: "It is unclear why you used volume of eggs as
> a factor (i.e. categorized variable) in the analyses. Incorporating this
> predictor as a continuous variable, as was originally measured, would make
> analysis more informative. You stated that you made so "to relax the
> linearity assumption". GLMM are sufficiently robust to accept a continuous
> variable into a categorized model that, with the correct link function and
> the variable transformation, would support well the linearity assumption."
>
> That said, I wonder if (1) categorization is such a bad thing on the one
> hand, and (2) lack of linearity on the logit scale can be handled well by
> GLMM.
>
> In my case, adding quadratic and cubic terms after assessment of the shape
> of the x-y relationship did not improve the fit, so I decided to use dummies
> and thus relax the linearity assumption.
>
> Thank you very much in advance.
>
> Luciano
>
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>    

-- 
Simon Blomberg, BSc (Hons), PhD, MAppStat.
Lecturer and Consultant Statistician
School of Biological Sciences
The University of Queensland
St. Lucia Queensland 4072
Australia
T: +61 7 3365 2506
email: S.Blomberg1_at_uq.edu.au
http://www.uq.edu.au/~uqsblomb/

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