Question glmmADMB

Dear Wouter,

Unfortunately we can?t see your data and code because attachments are removed from emails sent to this list. I?m guessing your convergence problems could be caused by overfitting. Lacking the code and data, I have some questions...

Is subject the same as participant? Do you have multiple observations for some subjects (i.e. participants) or did aggregating remove the repeated measures? If there are not multiple observations per subject, then you do not need the random effect of subject. Make sure you aggregated the data in a logical way.

How many observations do you have in total? Do you have 10 to 20 per term in the model? Are there observations representing all of the interactions? If not, you may need to simplify the model. I would avoid 3-way interactions (i.e.  Context * Outcome * Language).

You don?t necessarily need to have a zero-inflated model. It?s possible that the zeros can be explained by a low mean. See Warton, D. I. (2005). Many zeros does not mean zero inflation: comparing the goodness-of-fit of parametric models to multivariate abundance data. Environmetrics, 16(3), 275?289. http://doi.org/10.1002/env.702 <http://doi.org/10.1002/env.702>.

I would start with a negative binomial (NB) distribution and see if that converges. This could be done with any of the packages you mention. If it does converge, then try zero-inflated Poisson (ZIP) or zero-inflated negative binomial (ZINB) models (mgcv can do NB and ZIP; glmmTMB can do NB, ZIP, and ZINB). Then do model selection using AIC (only for models fit by the same package unless you know they calculate the likelihood in the same way).

cheers,
Mollie

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Mollie E. Brooks, Ph.D.
Postdoctoral Researcher
National Institute of Aquatic Resources
Technical University of Denmark

On 16Jun 2017, at 11:02, Wouter Broos <Wouter.Broos at UGent.be> wrote:

Dear Professor Bolker,


At the moment, I am working on a data set that contains
information on 'number of errors' that are made by participants. Thus
far, I used poisson regression to create generalized mixed effects
models of the data. In order to use poisson regression I aggregated the
data set by participant and by category. There are eight different
categories in total (I've added the data for ease of reference). There
are three main factors that determine the category: 1. Outcome: The
error that is made can be lexical or non-lexical / 2. Context: The
context of the block where the error was made can be mixed or
non-lexical / 3. Language: The language in which the error is made can
be the first language (L1) or the second language (L2). So, the
dependent variable in the model is 'Number of Errors' and the fixed
factors are 'Outcome', 'Context', and 'Language'. All factors
interaction with one another. One potential problem with the data set is
that there is an imbalance in the number of '0 number of errors per
category' and the '1/2/3 number of errors'. However, the zeros in my
data set can be explained in only one way: no mistake was made by that
participant in that category. So my first question is: do I need to add
the zero-inflation component to the model?



I tried using the glmmADMB package that can take zero-inflation into
account in order to see whether there are differences with the normal
poisson model and the zero-inflated model. However, when I ran the
model, I got the error:


*Parameters were estimated, but standard errors were not: the most
likely problem is that the curvature at MLE was zero or negative*

*Error in glmmadmb(Num_Mistakes ~ (Context * Outcome) + (1 | Subject),  : *

*  The function maximizer failed (couldn't find parameter file)
Troubleshooting steps include (1) run with 'save.dir' set and inspect
output files; (2) change run parameters: see '?admbControl';(3) re-run
with debug=TRUE for more information on failure mode*

*In addition: Warning message:*

*running command 'C:\WINDOWS\system32\cmd.exe /c glmmadmb -maxfn 500
-maxph 5 -noinit -shess' had status 1*



I tried leaving out one or two factors, googled the problem and tried
several 'solutions' but nothing works. So my second question is: how can
I solve this problem? I also used other packages where zero-inflation
could also be inserted (glmmTMB and mgcv). When I run the glmmTMB model,
zero-inflation works (but only for the interaction Context*Outcome, not
when I try to include Language) but the standard errors for the
zero-inflated model are rather large (and huge for the interaction)
leading to p-values of 1. An additional problem with glmmTMB is that I
cannot use all three factors because there are 'extreme or very small
eigenvalues'. The R-script is added as an attachment to the e-mail so
that you can see what I did. Would you be willing to help me out or do
you have any suggestions as to what I can do next? Thank you.



Kind regards,

Wouter Broos?

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