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

[Luke Martinez]

Thanks, Michael. But my question is wouldn't it be ***incongruent*** if I
use the same variable (`count_of_exposure`) as continuous in the fixed part
but as categorical in the random part (at least that congruence is needed
in conventional multilevel models)?

Kind regards,
Luke

On Sat, Aug 14, 2021 at 6:35 AM Michael Dewey <lists using dewey.myzen.co.uk>
wrote:

> Comments in-line
>
> On 14/08/2021 05:53, Luke Martinez wrote:
> > 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?
> >
>
>
> Looking at the different options for the struct parameter it is hard to
> see what sense they make for a continuous variable. What dimensions
> would the variance-covariance matrix have?
>
> > 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?
> >
>
> If converting it into a factor is going to lead to a model which fits
> then it must have relatively few levels otherwise the VCV matrix is
> going to expand uncontrollably. If it has few levels is it useful to fit
> it as continuous rather than as an ordered factor?
>
> Michael
>
> > Thank you for your clarification,
> > Luke
> >
> >       [[alternative HTML version deleted]]
> >
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> >
>
> --
> Michael
> http://www.dewey.myzen.co.uk/home.html
>

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