Dear Jack,
See below for my responses.
Best,
Wolfgang
-----Original Message-----
From: R-sig-meta-analysis [mailto:r-sig-meta-analysis-bounces at r-project.org] On
Behalf Of Jack Solomon
Sent: Wednesday, 17 March, 2021 22:01
To: r-sig-meta-analysis at r-project.org
Subject: [R-meta] random part in meta-regression vs. that in multilevel models
Hello List Members,
**First, I have always thought it is illegitimate to add random-effects for
something that has not been estimated in the fixed part of the model. For
example:
`lme4::lmer(math ~ female*minority + (ses | sch.id), data = data)` is
illegitimate because `ses` has not been estimated in the fixed part.
It's perfectly fine to do this if one is willing to assume that the mean slope of ses is 0. Often, this is not an appropriate assumption but I would not say this is illegitimate.
This reminds me of the common trope that one should never add an interaction to a model without also adding the corresponding main effects. One can come up with perfectly valid arguments where this is not necessary under certain circumstances. For example, suppose I have conducted a randomized study where I measured people in two groups twice, once pre and once post treatment. Assume the data are in this format:
person group post y
1 T 0 .
1 T 1 .
2 C 0 .
2 C 1 .
...
I assume the meaning of these variables is self-evident. Then
y ~ post + post:group
is a perfectly valid model as far as I am concerned. It assumes that there is no pre-treatment group difference (since the model does not include a group 'main effect'), but for a randomized study, any pre-treatment group difference would be due to chance anyway, so why estimate a pre-treatment group difference that must be in reality, by definition, 0?
So this is a model that includes the post:group interaction, but not all corresponding main effects. Is this wrong? Not to me at least. I might not use this model for other reasons - for example, to avoid discussions with reviewers who will claim that one MUST ALWAYS include all main effects corresponding to an interaction <sigh> - but that's a different issue.
But I frequently see multilevel meta-regression models where intercept is
dropped (~0+...) from the fixed part but at the same time it is added to
the random part. For example:
metafor::rma.mv(es ~ 0+outcome, V, random= ~1|id/outcome, data = data)
Assuming 'outcome' is a factor/character variable, es ~ 0+outcome is just a reparameterization of es ~ outcome and ultimately those are two identical models.
Coincidentally, since you brought this up: I run roughly weekly live streams where I discuss R and stats and the session tonight will cover this in detail (not in the context of meta-analysis or multilevel modeling, but the same idea applies). If you are interested, see:
https://www.wvbauer.com/doku.php/live_streams
These live streams are completely free, no registration required, just click on the link and start watching at 5pm CET.
So, why is this ok in meta-regression?
Yes, in this particular case for sure.
**Second, I have always thought that `outcome` is treated as a categorical
predictor and thus appearing only to the **left** of `|`. For example:
lme4::lmer(es ~0+outcome + (0 + outcome | id), data = data)
But I frequently see multilevel meta-regression models where outcome is
treated as a categorical predictor AND a **grouping variable** thus
appearing only to the **right** of `|`. For example:
metafor::rma.mv(es ~ 0+outcome, V, random= ~1|id/outcome, data = data)
So, why is this ok in meta-regression?
See: https://www.metafor-project.org/doku.php/analyses:konstantopoulos2011 which discusses how these two may be completely analogous parameterizations of the same model, if a certain var-cov structure is assumed for 'outcome | id' (that's not the case above for lmer(), since it automatically will use an unstructured var-cov matrix for 'outcome | id', but rma.mv() or nlme::lme() allow specification of different structures, struct="CS" being the default in for the former).
I don't know how many times I have posted the konstantopoulos2011 link above on this mailing list, but I can highly recommend it for anybody who is doing a multilevel meta-analysis (I got curious; a google search with 'site:https://stat.ethz.ch/pipermail/r-sig-meta-analysis/ analyses:konstantopoulos2011' suggests 101 times ... plus a few more times today!).
Anyway, I hope this helps to clarify some things.