[R-sig-ME] Comparing mixed models

[Alex Fine]

The problem (depending on what you're trying to do) with comparing model 1
and model 2 is that, if you observe, say, a large change in the AIC, it's
not clear what to attribute the change to.  It could be driven either by
the fixed effect of Sector or by the random intercept for Cage.  Maybe it
doesn't matter in your case.

On Sat, May 7, 2016 at 11:42 AM, Carlos Barboza <carlosambarboza at gmail.com>
wrote:

> yes I agree, my question was just a numerical doubt about comparing AIC
> values. My approach was to show that, the AIC value from a model including
> the single fixed effect has a smaller AIC value than any other model only
> including the interecept effect in the fixed structure
>
> thank you
>
> 2016-05-07 12:34 GMT-03:00 Ben Bolker <bbolker at gmail.com>:
>
> >
> >   My only other comment would be that my standard approach would be to
> > retain all random effects in the model unless they are causing difficulty
> > in model fitting -- this depends on your goal (confirmation/testing,
> > prediction, exploration)
> >
> > On Sat, May 7, 2016 at 11:26 AM, Carlos Barboza <
> carlosambarboza at gmail.com
> > > wrote:
> >
> >> Dear Dr. Ben Bolker
> >>
> >> My name is Carlos Barboza and I am a Marine Biologist from the Rio de
> >> Janeiro University, Brazil. First it's a pleasure to again have the
> >> opportunity to send you a message.The reason for it is a simple doubt:
> >> Can I compare AIC from:
> >>
> >> 1. glmmADMB: Density ~ 1 + 1|Site
> >>
> >> 2. glmmADMB: Density ~ Sector + 1|Site + Cage
> >>
> >> Note that they have different random and fixed structures. I know that
> >> this is not the best choice to model selection but, I think that the AIC
> >> values can be compared.
> >>
> >> thank you very much for your attention
> >>
> >>
> >>   is Cage a random effect?  Are you intentionally leaving out the
> >> intercept in the second case (it will be included anyway unless you
> >> use -1)?  In any case, I don't see any obvious reason you can't
> >> compare AIC values; see
> >>
> >>
> https://rawgit.com/bbolker/mixedmodels-misc/master/glmmFAQ.html#can-i-use-aic-for-mixed-models-how-do-i-count-the-number-of-degrees-of-freedom-for-a-random-effect
> >>
> >>   Follow-ups to r-sig-mixed-models at r-project.org, please ...
> >>
> >> sorry, yes, cage was included only to examplify a different random
> >> structure in the second case...it should be coded (1|Site) + (1|Cage)
> >> yes, I know that the intercept will be included in the second model
> >>
> >> it's an example of comparing AIC values from mixed models with different
> >> fixed and random structures:
> >>
> >> 1. Density ~ 1 + 1|Site
> >>
> >> 2. Density ~ Sector + 1|Site + 1|Cage
> >>
> >> comparing AIC...I beleive that both values can be compared
> >>
> >> again, thank you very much for your very fast message
> >>
> >>
> >>
> >>
> >
>
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>
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-- 
Alex Fine
Ph. (336) 302-3251
web:  http://internal.psychology.illinois.edu/~abfine/
<http://internal.psychology.illinois.edu/~abfine/AlexFineHome.html>

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