
Thanks for the advice. I will definitely try out lme4a. 

As for the error message, is there anywhere i can find out what it actually means? 

Cheers,
Yonghao

> Date: Wed, 6 Apr 2011 12:44:52 -0500
> Subject: Re: [R-sig-ME] (no subject)
> From: bates@stat.wisc.edu
> To: Thierry.ONKELINX@inbo.be
> CC: oahgony@hotmail.com; r-sig-mixed-models@r-project.org
> 
> On Wed, Apr 6, 2011 at 8:48 AM, ONKELINX, Thierry
> <Thierry.ONKELINX@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@r-project.org
> >> [mailto:r-sig-mixed-models-bounces@r-project.org] Namens Lim Yonghao
> >> Verzonden: woensdag 6 april 2011 15:39
> >> Aan: r-sig-mixed-models@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]]
> >>
> >> _______________________________________________
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> >> https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
> >>
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> > https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
> >
 		 	   		  
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