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

[Michael Dewey]

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
> 
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-- 
Michael
http://www.dewey.myzen.co.uk/home.html