[R-sig-ME] random structure in lmer

[Ben Bolker]

On 14-02-11 06:07 AM, Cerni, Tania wrote:
> Dear all, I'm  a PhD student in Cognitive science. I'm working on an
> analysis with mixed models with R (lme4 and lmerTest)  but I'm  still
> in trouble with random factors. I have read some Baayen's works and I
> tried to follow his methods, but every time, after comparing models
> with different slopes, the output of lmer is the summary of the model
> preceded by:
> 
> "Asymptotic covariance matrix A is not positive!"
> 
> I want to ask you what kind of problems can I have with this message
> and, most of all, I'm writing to you to ask if you can advise me a
> shared procedure for random structure.

  For what it's worth, this message comes from lmerTest, not
lme4, and I suspect it actually has to do with your fixed-effect
structure, or the interaction between your fixed-effect structure
and your random-effect structure, not your random-effect structure
per se.  I suspect there is some near-perfect multicollinearity
among some of your factors, but I'm not sure.  Have you thought
carefully about your experimental design to make sure that all
of the parameters you want to estimate are really identifiable ... ?

   Ben Bolker

> 
> I have this kind of models: with 3way interaction or 2way
> interaction.
> 
> lex0 = lmer(rt~Trial +first_letter+letters+w.pw*group*manipulation+
> (1|Sj)+ (1|Word),lex)
> 
> the main thing is to see if there is an interaction between groups
> and manipulation.  The other fixed factors are predictors
> 
> 
> 
> Random effects:
> 
> Groups   Name        Variance Std.Dev.
> 
> Word     (Intercept) 0.006321 0.0795
> Sj       (Intercept) 0.022424 0.1497
> Residual             0.032240 0.1796
> 
> Number of obs: 7169, groups: Word, 240; Sj, 32
> 
> 
> 
  [snip]


> How can I adjust my random structure? Thanks, Tania