[R-sig-ME] transform glmer model into MCMCglmm: how to define the priors?

[Jarrod Hadfield]

Hi,

You will need priors for this model: the observation-level (units)  
effects are not identifiable. The random effect covariance structures  
are 2x2 matrices. For these models I fix the residual covariance  
matrix at

IJ <- (1/k) * (diag(k-1) + matrix(1, k-1, k-1))

where k is the number of categories in the response:


A starting prior might look something like:

prior=list(R=list(V=IJ, fix=1), G=list(G1=list(V=diag(2), nu=2,  
alpha.mu=c(0,0), alpha.V=diag(2)*1000)))


Although I haven't seen it done (although I don't know the literature  
on multinomial models) it might make sense to have the random effect  
covarinace matrix proportional to IJ and estimate a single variance  
component (rather than a 2x2 covariance matrix). The rational behind  
the IJ matrix is that the propensity to belong to a category varies  
over random effects, but variation in the propensity is equal for all  
categories, and the propensity for two categories are uncorrelated. In  
non-multinomial models such an assumption would be represented by an  
identity matrix (i.e. trait:author) but for multinomial models this is  
a bit trickier because things are parameterised in terms of  
differences from a base-line category.

The error you are getting does not look like it is a MCMCglmm issue.  
Could you post the data (or a reproducible example) so I can check?


Cheers,

Jarrod




Quoting Anne Krause <anne.krause at frequenz.uni-freiburg.de> on Tue, 17  
Mar 2015 18:06:48 +0100:

> Dear list members,
> I am a linguist who would like to use MCMCglmm in order to
> model change in language morphology. I have a 3-level
> dependent variable (unordered) which are 3 realisations of
> a morphological form in German. Say, I call them A, B, and
> C - A and B share the same vowel, B and C share the
> suffixation.
> I did not know about MCMCglmm until recently; therefore I
> worked with two glmer models, looking at vowel and
> suffixation separately. However, I have been criticised for
> the clumsy model interpretation, and I am sure that the
> output of a MCMCglmm would be more straightforward and
> convincing.
>
> My glmer looks like this:
> model <- glmer(vowel~frequency+year+recency+
>                   frequency*recency+
>                   (1|verb)+(1|author), data=imp,
>                   family=binomial)
> - where “vowel” differentiates between AB on the one hand
> and C on the other hand,
> - “frequency” and “year” are numeric fixed variables and
> “recency” a categorical fixed variable (7 levels),
> - “verb” and “author” are categorical random variables
>
> The model for the dependent variable “suffix” is
> practically the same (part of the criticism), “suffix”
> distinguishing between A on the one hand and BC on the
> other hand.
>
> Trying to transform this into an MCMCglmm, I managed to get
> as far as this:
> model2 <- MCMCglmm(form~trait:frequency+trait:recency+
>                   trait:year-1,
>                   random=~us(trait):author,
>                   rcov=~us(trait):units,
>                   data=imp, family="categorical",)
> (“form” now distinguishing between all 3 three levels of
> the dependent variable A, B, and C)
>
> I am well aware that this model is running without priors.
> Whichever prior I tried gave me the error “V is the wrong
> dimension for some prior$G/prior$R elements” and I have no
> idea (after reading through the general description, the
> tutorial, the course notes and entries in this mailing
> list) how these priors are defined. I guess there is no
> rule of thumb, but I hope this short explanation of my
> variables is enough for someone of you to point out a
> solution (or a starting point for me). I also need to
> include the second random from above (“verb”) and the
> interaction between “frequency” and “recency”.
>
> Even though model2 is running, I cannot call the summary
> for it (error: “Error in get(as.character(FUN), mode =
> "function", envir = envir) :   object 'C:\Users\Anne
> Krause\some_directory.Rdata' of mode 'function' was not
> found”), which probably has to do with the fact that I did
> not include priors (?!).
>
> Thank you so much in advance for help and/ or comments,
> pointers or the like!
>
> Best, Anne
> ____________________________________
>
> Anne Krause
> Research Training Group GRK DFG 1624
> "Frequency Effects in Language"
> University of Freiburg
> Belfortstraße 18
> 79098 Freiburg
>
> Phone: 0761/203-97670
> frequenz.uni-freiburg.de/krause
>
> _______________________________________________
> R-sig-mixed-models at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
>
>


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