[R] researcher with highly skewed data set seeks help finding practical GLMM tutorial
ben.kenward at gmail.com
Tue Nov 30 10:16:47 CET 2010
I am a psychologist who suspects that the only sensible way to analyse
a particular data set is to use generalised linear mixed models. I am
hoping that someone might be able to point me in the right direction
to find some very practical hands on documentation that might be able
to talk me through actually doing such an analysis?
So far in my searches the most useful document I have turned up is
Bolker et al. (2008, TREE) Generalized linear mixed models: a
practical guide for ecology and evolution. As a general guide it
doesn't give enough practical information about how to get the job
done. The R documentation is obviously practical, but doesn't help to
decide what kind of analysis is appropriate. Apart from those sources
I am mainly finding quite theoretical treatments going over my head,
for example: http://www.cmm.bristol.ac.uk/learning-training/multilevel-m-software/reviewr.pdf.
I am moderately competent programming in R, having coded custom
permutation tests before (which in contrast to GLMM I find intutive).
In case anyone is kind enough to give me any specific pointers, here
is the nature of my data set. With an N of 42 subjects, I have a
highly left skewed (about half the data points are zero) frequency
variable as dependent variable. This variable is measured in each
subject in three different task types. There is furthermore a context
variable with two levels. Each task was administered in each context,
but not for every single subject.
So the design is quite simple - two fixed factors (task and context),
one random factor (subject), and an untransformably skewed dependent
variable. I might want to add some additional fixed factors (age
group) in future but for now I would like to keep it simple. I guess
this is straightforward for those in the know. Any help at all much
Dr. Ben Kenward
Department of Psychology, Uppsala University, Sweden
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