(no subject)
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]]
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