[R-sig-ME] Reporting main effects in glmer

[Francesco]

Dear all,


fairly simple question this time. I need to report a main effect and or 
pairwise comparisons for a model that has a binary DV 'success' and a 
categorical predictor X with three levels (A, B, and C). My model looks 
like this:

Correct ~ 1 + A + (1 | Part) + (1 | Item)

The original call was:

 >object<-glmer(Correct~1+A+(1|Part)+(1|Item), family=binomial, data=data)


with the following summary output:

Generalized linear mixed model fit by maximum likelihood ['glmerMod']
  Family: binomial ( logit )
Formula: ......
    Data: .....

       AIC       BIC    logLik  deviance
  218.9053  242.0455 -104.4526  208.9053

Random effects:
  Groups Name        Variance Std.Dev.
  Part   (Intercept) 9.0129   3.0021
  Item   (Intercept) 0.7356   0.8577
Number of obs: 756, groups: Part, 45; Item, 18

Fixed effects:
               Estimate Std. Error z value Pr(>|z|)
(Intercept)     -95.14 4122464.75       0        1
A             89.88 4122464.75       0        1
B             90.75 4122464.75       0        1

Correlation of Fixed Effects:
      (Intr)   A
A -1.000
B -1.000  1.000

where Part and Item are random effects for participant and items. 
Basically the three groups are very similar in their response, where one 
group (the reference level group in this analysis) has no 0s and all 1s. 
Releveling generates a Pr(>|z|) of .54 for the difference between group 
B and C so I believe the model is ok.

In this scenario I would not expect A to have a main effect on the model 
but when I compare it to the same model minus the A predictor, R yields 
the following:


 > anova(object,objectminusA)

Data: data
Models:
objectminusA: Correct ~ 1 + (1 | Part) + (1 | Item)
object: Correct ~ 1 + A + (1 | Part) + (1 | Item)
              Df    AIC    BIC  logLik deviance  Chisq Chi Df Pr(>Chisq)
objectminusA  3 221.64 235.53 -107.82 215.64
object  5 218.91 242.04 -104.45   208.91 6.7367      2    0.03445 *

My questions are as follows:
1. Should this be interpreted as there being a main effect but no 
significant difference exists between the three levels of the predictor?
2. How do I report the result in my paper?

Many thanks in advance for any help.



-- 
Frank Romano

Sabanci University
website: http://sabanciuniv.academia.edu/FrancescoRomano