[R-sig-ME] (no subject)

[ONKELINX, Thierry]

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 at 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 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
>  		 	   		  
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> 
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