[R-meta] Moderator Analysis

[Jake Downs]

Hello R Friends,
I am a doc student newish to R  and meta-analysis. It's a lot to wrap my
brain around, but I'm eager to learn.

I am conducting a 3-level meta-analysis using rma.mv on student reading
outcomes for various types of related practices. Level one is effect sizes,
level two models covariance between effect sizes within studies, and level
three models covariance between studies.

The meta-analysis is multivariate, so level one outcomes are coded as
either "fluency" or "comprehension."  I have ran the analysis for all
effects (g=0.58; code below), but I am also very interested in producing a
'fluency' effect size and a 'comprehension' effect size. I would like
assistance to figure out the best way to do that.
3 Level Fit:
rq1.fit1 <- tx.cg %>%
  rma.mv(
    yi = tx.cg.yi,  #fit one, 3 level meta-analysis
    V = tx.cg.vi,
    random = ~ 1 | study.number/effect.number,
    level=95,
    digits=2,
    data = .,
    method = "REML"
  )
summary(rq1.fit1)

Option 1:
Moderator analysis. I ran a moderator analysis using this code:
rq2.f.c <- tx.cg %>%
  metafor::rma.mv(
    yi = tx.cg.yi,
    V = tx.cg.vi,
    random = ~ 1 | study.number/effect.number,
    level=95,
    digits=2,
    data = .,
    method = "REML",
    mods = ~ f.c)
summary(rq2.f.c)

The QM test of moderators is approaching statistical significance (p =
0.08), however the intercept (reference group of comprehension) did report
statistically significant results. Does that mean that only comprehension
moderates outcomes? (And that only a comprehension 'effect size' would be
valid?)

Option 2: Single Variable Moderator analysis?
To calculate an effect size for fluency and comprehension, is there a way
to run a single variable as a moderator? For example, rather than running
fluency and comprehension in a combined moderator analysis run
comprehension only in one moderator analysis and fluency only in another?
Is this a viable method?

Option 3: Subset Independent Meta-analysis
I don't think this option is viable, but I could use the subset = function
in metafor to run an analysis using ONLY comprehension, and using ONLY
fluency. This would throw away half my data however, which I think would
limit the validity of my findings.

In short: I just want to be able to say that the effect size for fluency
was X and the effect size for comprehension was Y. What is the best way to
do that?


Thanks very much for your help.

Jake

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