[R-sig-ME] Interpretation of odd error variances in glmer

[Brendan Halpin]

I'm trying to fit a cross-classified multi-level logistic regression
using glmer and am getting exactly zero variance on one of the grouping
levels. I have observations on grades, nested within class within
department on the one hand, and within student on the other.

Here is an example:

     Generalized linear mixed model fit by the Laplace approximation 
     Formula: gradeA ~ 1 + cao + subj1 + subj2 + subj3 + as.factor(stu_gend)
      + ageentry + as.factor(yrs5) + as.factor(year) + modsize + meancao +
        depfemr + (1 | deptno) + (1 | modinst) + (1 | id)
     
         AIC    BIC logLik deviance
      155671 155901 -77813   155627
     Random effects:
      Groups  Name        Variance Std.Dev.
      id      (Intercept) 1.9624   1.4009  
      modinst (Intercept) 1.6783   1.2955  
      deptno  (Intercept) 0.0000   0.0000  
     Number of obs: 264059, groups: id, 11656; modinst, 6420; deptno, 26

If I change the outcome variable, I get non-zero variance.

How do I interpret this? Is it a model estimation problem?


Regards,

Brendan
-- 
Brendan Halpin,  Department of Sociology,  University of Limerick,  Ireland
Tel: w +353-61-213147 f +353-61-202569 h +353-61-338562; Room F2-025 x 3147
mailto:brendan.halpin at ul.ie  http://www.ul.ie/sociology/brendan.halpin.html