[R-sig-ME] Grouping variables technically suitable for modeling

[Ben Bolker]

    This is a bit of a "how long is a piece of string" question ...


   The "5-6 levels of a grouping variable" rule of thumb is quoted in 
various places: a variety of those references (Gelman and Hill 2006, 
Kéry and Royle 2015, Harrison et al 2018, Arnqvist 2020) are collected 
by Gomes 
(https://www.biorxiv.org/content/10.1101/2021.04.11.439357v2.full).

   I sort of see what you mean by your second paragraph, but can you 
give an example?


On 11/7/21 5:20 PM, Timothy MacKenzie wrote:
> Dear Experts,
> 
> Apologies if this question has come up before. But I'm looking for
> published references that provide guidance on when one or more grouping
> variables that theoretically need to be random factors can also
> "technically" be used as random factors?
> 
> For example, I have heard for a grouping variable to be technically taken
> as a random factor, it needs to have at least 10 or so unique categories?
> (Any reference to confirm or disconfirm this?)
> 
> For example, I have heard for two grouping variables to be technically
> taken as random factors, they each need to have a sufficiently different
> number of unique categories relative to the other one. Otherwise, their
> variance components can't be distinguished from one another and thus only
> one of them can be taken as random, not both (Any reference to confirm or
> disconfirm this?)
> 
> Thanks,
> Tim M
> 
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