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random structure in lmer
2 messages · Cerni, Tania, 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