[R-meta] Random effects in MA

[Emily Russell]

Dear All

I was wondering about the definition of 'random effects' in the contexts of a mixed-effects model.  The example in the metafor manual using the Konstantopoulos (2011) data  has a fixed number of districts each of which have a number of studies conducted within them.  The random = ~ 1 | district/study fits a random effect at the study and district level:

res.ml <- rma.mv<http://finzi.psych.upenn.edu/library/metafor/html/rma.mv.html>(yi, vi, random = ~ 1 | district/study, data<http://stat.ethz.ch/R-manual/R-devel/library/utils/html/data.html>=dat)


What I don't understand is how districts can be a random variable, as presumably we have a fixed and known number of them and don't want to make more general inferences about districts; does that not make districts a fixed-effect?


I fear I may be missing something obvious so apologies if it is!

Emily


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