
Dear Thierry,

Thank you for your response. I have already simplified the model and i don't think there will be any opportunity to collect more data anytime soon.

Cheers,
Yonghao

> From: Thierry.ONKELINX@inbo.be
> To: oahgony@hotmail.com; r-sig-mixed-models@r-project.org
> Subject: RE: [R-sig-ME] (no subject)
> Date: Wed, 6 Apr 2011 13:48:00 +0000
> 
> 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.
> 
> Best regards,
> 
> Thierry
> 
> ----------------------------------------------------------------------------
> ir. Thierry Onkelinx
> Instituut voor natuur- en bosonderzoek
> team Biometrie & Kwaliteitszorg
> Gaverstraat 4
> 9500 Geraardsbergen
> Belgium
> 
> Research Institute for Nature and Forest
> team Biometrics & Quality Assurance
> Gaverstraat 4
> 9500 Geraardsbergen
> Belgium
> 
> tel. + 32 54/436 185
> Thierry.Onkelinx@inbo.be
> www.inbo.be
> 
> To call in the statistician after the experiment is done may be no more than asking him to perform a post-mortem examination: he may be able to say what the experiment died of.
> ~ Sir Ronald Aylmer Fisher
> 
> The plural of anecdote is not data.
> ~ Roger Brinner
> 
> The combination of some data and an aching desire for an answer does not ensure that a reasonable answer can be extracted from a given body of data.
> ~ John Tukey
>   
> 
> > -----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
> >  		 	   		  
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> > 
> > _______________________________________________
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> > https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
> > 
 		 	   		  
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