Grouping variables technically suitable for modeling

    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,
K?ry 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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