[R-meta] metafor: Categorizing a continuous moderator in random part of rma.mv

[Luke Martinez]

Dear Experts,

In metafor's multivariate specification (which gives control over the
`struct` argument), apparently there is a requirement for the variable
appearing on the left-side of `|` to only be a categorical factor.

First, why is this required conceptually?

Second, suppose I want to use, say, `count_of_exposure` as a
**continuous variable** in the fixed part, then would it be appropriate if
I use `factor( count_of_exposure) | study` in the random part?

Thank you for your clarification,
Luke

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