[R-sig-ME] (no subject)

[Douglas Bates]

On Wed, Apr 6, 2011 at 8:48 AM, ONKELINX, Thierry
<Thierry.ONKELINX at inbo.be> wrote:
> Dear Yonghao,

> The problem is that your model is too complex for the dataset. So you have two options: 1) simplify the model, 2) collect more data.

A third option is to try to fit the model in lme4a which should be
more robust in fitting GLMMs.  That's not a guarantee that it will
work but it is sometimes worth a try.

I'll send a message in a moment about a class I am teaching in which
students are to do a data analysis project involving mixed-effects
models.  Those who would like to have a student consider their data
are encouraged to respond to that request.  Data confidentiality can
be requested.

>> -----Oorspronkelijk bericht-----
>> Van: r-sig-mixed-models-bounces at r-project.org
>> [mailto:r-sig-mixed-models-bounces at r-project.org] Namens Lim Yonghao
>> Verzonden: woensdag 6 april 2011 15:39
>> Aan: r-sig-mixed-models at r-project.org
>> Onderwerp: [R-sig-ME] (no subject)
>>
>>
>> Dear all,
>>
>> I wish to conduct a piecewise growth model in a dataset
>> (n=40) with 9 timepoints. The dependent variable is
>> vocabulary size. Initially, i ran 3 models (using glmer with
>> family = poisson) comparing models with a linear growth and
>> knots at 2 different timepoints. The knots are suggested both
>> by theory and graphical observation. The models ran
>> successfully and i chose the model with the lowest AIC.
>> However, when i added level 2 predictors to the model, the
>> following error message came up
>>
>> model2 <- glmer(vocab ~ time1_18*seg + time2_18*seg +
>> (time1_18 + time2_18|id), data = growth, family = poisson)
>>
>> Warning messages:
>> 1: In mer_finalize(ans) :
>>   Cholmod warning 'not positive definite' at
>> file:../Cholesky/t_cholmod_rowfac.c, line 432
>> 2: In mer_finalize(ans) :
>>   Cholmod warning 'not positive definite' at
>> file:../Cholesky/t_cholmod_rowfac.c, line 432
>> 3: In mer_finalize(ans) : false convergence (8)
>>
>> I did a brief search of the archives and it seems to suggest
>> that having a small sample size might result in this. I am
>> unsure what is causing this. Any advice on how to resolve this?
>>
>> Cheers,
>> Yonghao
>>
>>       [[alternative HTML version deleted]]
>>
>> _______________________________________________
>> R-sig-mixed-models at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
>>
> _______________________________________________
> R-sig-mixed-models at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
>