[R-meta] Choice of moderator for Egger's regression test in rma.mv

[Elizabeth Wade]

Ah, I see my mistake. Thank you for clarifying!

Betsy Wade, MA
Clinical Psychology Doctoral Student
Department of Psychology
University of Pennsylvania


On Fri, May 22, 2020 at 3:43 AM Viechtbauer, Wolfgang (SP) <
wolfgang.viechtbauer using maastrichtuniversity.nl> wrote:

> Take a closer look: It said 'mods = ~ I(1/ni)' where I is a capital i, not
> the vertical bar symbol (that is used in 'random'). When doing a
> transformation on a predictor variable inside of a formula, it needs to be
> wrapped in I() -- see help(I). But yes, one can of course also create the
> transformed variable beforehand.
>
> Best,
> Wolfgang
>
> >-----Original Message-----
> >From: Elizabeth Wade [mailto:elizwade using sas.upenn.edu]
> >Sent: Friday, 22 May, 2020 1:53
> >To: Viechtbauer, Wolfgang (SP)
> >Cc: r-sig-meta-analysis using r-project.org
> >Subject: Re: [R-meta] Choice of moderator for Egger's regression test in
> >rma.mv
> >
> >Dear Dr. Viechtbauer,
> >
> >Thank you so much for your thorough explanation and quick reply.
> >
> >Interestingly, I could not get the model to run properly (with results for
> >the test of moderation) until I created a separate variable representing
> the
> >inverse sample sizes (data <- data %>% mutate(inverse_ni = (1/ni)). I also
> >found I had to drop the "|" from the mods line, so my final model was:
> >
> >model.egger <- rma.mv(ri.c, vi.c, mods = ~ inverse_ni, random = ~ 1 |
> >Study_ID/Effect_ID, data = data)
> >
> >I imagine this is all expected behavior, but I thought I would include
> these
> >details here in case others are following or run into the same issue in
> the
> >future.
> >
> >Thank you to you and this community for your support and guidance.
> >
> >My best wishes,
> >Betsy Wade
> >
> >Betsy Wade, MA
> >Clinical Psychology Doctoral Student
> >Department of Psychology
> >University of Pennsylvania
> >
> >On Thu, May 21, 2020 at 2:23 PM Viechtbauer, Wolfgang (SP)
> ><wolfgang.viechtbauer using maastrichtuniversity.nl> wrote:
> >Dear Betsy,
> >
> >Unless you know exactly what you are doing, I would not mess with the
> >weights in the context of a multilevel model. The (default) weighting
> >structure in such models is more complex than just assigning a particular
> >weight to each estimate. Such a multilevel model also implies a certain
> >degree of covariance between the underlying true effects within studies,
> >which results in a weight matrix that is not just diagonal, but also has
> >non-zero off-diagonal elements. When estimating the overall mean (or fixed
> >effects in general), this ensures that multiple estimates coming from the
> >same study are not treated as if they are independent.
> >
> >So, I would recommend using:
> >
> >model <- rma.mv(ri.c, vi.c, random = ~ 1 | Study_ID/Effect_ID, data =
> data)
> >
> >As for your actual question: Since you are meta-analyzing correlations, I
> >would not use the sampling variances (or some function thereof) for an
> >Egger-type test. There is an inherent correlation between correlations and
> >their sampling variances, which can lead to false positives. I would use
> the
> >inverse sample sizes as the predictor, that is:
> >
> >model <- rma.mv(ri.c, vi.c, mods = ~ I(1/ni), random = ~ 1 |
> >Study_ID/Effect_ID, data = data)
> >
> >where 'ni' is the name of the variable containing the sample sizes.
> >
> >And with respect to 'mods = ~ <moderator>' vs 'mods = <moderator>': This
> is
> >explained under help(rma) and help(rma.mv). But you can essentially
> ignore
> >the latter and always use the formula way of specifying moderators.
> >
> >Best,
> >Wolfgang
> >
> >>-----Original Message-----
> >>From: R-sig-meta-analysis [mailto:r-sig-meta-analysis-bounces using r-
> >project.org]
> >>On Behalf Of Elizabeth Wade
> >>Sent: Thursday, 21 May, 2020 18:34
> >>To: r-sig-meta-analysis using r-project.org
> >>Subject: [R-meta] Choice of moderator for Egger's regression test in
> rma.mv
> >>
> >>Dear all,
> >>
> >>I have fit a three-level model using rma.mv to meta-analyze correlation
> >>coefficients corrected for measurement unreliability. Based on this
> >>documentation (
> >>http://www.metafor-project.org/doku.php/tips:hunter_schmidt_method), I
> have
> >>weighted the model using the inverse of the corrected variance.
> >>
> >>My model is:
> >>model <- rma.mv(ri.c, vi.c, W = 1/vi.c, random = ~ 1 |
> >>Study_ID/Effect_ID, data = data, method = "REML")
> >>
> >>Now I am intending to perform Egger's regression test. Based on this
> >>explanation (
> >>
> https://stats.stackexchange.com/questions/155693/metafor-package-bias-and-
> >>sensitivity-diagnostics),
> >>I selected the inverse of the variance to use as a moderator in the
> >>regression test.
> >>
> >>So I have:
> >><http://www.metafor-
> >>project.org/doku.php/tips:hunter_schmidt_method>egger.model
> >><- rma.mv(ri.c, vi.c, W = 1/vi.c, mods = 1/vi.c, random = ~ 1 |
> >>Study_ID/Effect_ID, data = data, method = "REML")
> >>
> >>As I do this, I wonder whether it is appropriate to use the inverse
> >>variance to both weight the model and to perform Egger's test. Will this
> >>not detect publication bias, given that I am examining potential bias
> using
> >>the same variable with which I weighted my model? Do you recommend a
> >>different approach?
> >>
> >>As an aside, I also wonder why some documentation uses the tilde before
> the
> >>moderators are listed (mods = ~age) and some do not (mods = 1/vi.c).
> >>
> >>Thank you for reading,
> >>Betsy Wade
> >>
> >>Betsy Wade, MA
> >>Clinical Psychology Doctoral Student
> >>Department of Psychology
> >>University of Pennsylvania
>

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