[R-sig-ME] On what basis one of the predictors/fixed factors should be included in a random slope model?

[Saudi Sadiq]

I read this paper  'Random effects structure for confirmatory hypothesis
testing: Keep it maximal' by Barr, Levy, Scheepers and Tily (2013) and
enjoyed it. I tried, then, to keep it maximal with a dataset I am analysing
but I am facing a problem (this may look naive). I hope you will help me
solve it. The dataset includes 9 predictors and I am using R. The model I
ran is:

modle1<- glmer(convergence ~ gender + age.group
+education+residence+sound_before+sound_after+ part.of.speech +
grammatical.gender + style + (1|speaker) + (1|item), data = qaaf,
family='binomial', control = glmerControl(optimizer = "bobyqa"), nAGQ = 1)

As you see, I have two random factors (speaker and item).
Q1: On what basis one of the predictors/fixed factors should be included in
a random slope model?
The model with random slopes could be like model2 or model3:

modle2<- glmer(convergence ~ gender + age.group
+education+residence+sound_before+sound_after+ part.of.speech
+ grammatical.gender + style + (1+gender|speaker) + (1+education|item),
data = qaaf,  family='binomial', control = glmerControl(optimizer =
"bobyqa"), nAGQ = 1)

modle3<- glmer(convergence ~ gender + age.group
+education+residence+sound_before+sound_after+ part.of.speech +
grammatical.gender + style + (1+residence|speaker) + (1+residence|item),
data = qaaf,  family='binomial', control = glmerControl(optimizer =
"bobyqa"), nAGQ = 1)

Model2 has gender and education included in the random slope and model3 has
residence alone.
Q2: Should the fixed factor used in a random slope model be the same (as in
model3)?
Q3: If  it is okay for such a model to have  two fixed factors (as in model2)
or just one (as inmodel3), which is better?
Q4: What should be done if the model does not converge with a random slope,
is there a way to make it work? Or can i trus and report the results of the
model even if there is a warning message?

Best regards


-- 
Saudi Sadiq,
Assistant Lecturer, English Department,
Faculty of Al-Alsun,Minia University,
Minia City, Egypt &
PhD Student, Language and Linguistic Science Department,
University of York, York, North Yorkshire, UK,
YO10 5DD
http://york.academia.edu/SaudiSadiq
https://www.researchgate.net/profile/Saudi_Sadiq
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