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

[Viechtbauer, Wolfgang (SP)]

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