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

[Timothy MacKenzie]

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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