[R] metafor rma.mv weights questions

[Viechtbauer Wolfgang (SP)]

Hi Laura,

As far as I am concerned, you account for multiple effects sizes from the same study by setting up and fitting an appropriate model, not by fiddling with the weights. Several examples are described here:

http://www.metafor-project.org/doku.php/analyses#multivariate_multilevel_meta-analysis_models

The model implies the structure of the variance-covariance matrix of the estimates. The inverse of this var-cov matrix then implies the weights (and for multilevel/multivariate data, this is no longer a diagonal matrix, so there are not just weights, but an entire weight matrix). See also help(rma.mv) in metafor.

So, if you fit an appropriate model to the data at hand, the 'default weights' used by rma.mv() will be just fine.

Best,
Wolfgang

-- 
Wolfgang Viechtbauer, Ph.D., Statistician | Department of Psychiatry and    
Neuropsychology | Maastricht University | P.O. Box 616 (VIJV1) | 6200 MD    
Maastricht, The Netherlands | +31 (43) 388-4170 | http://www.wvbauer.com    

>-----Original Message-----
>From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Duncan,
>Laura
>Sent: Monday, January 30, 2017 18:18
>To: r-help at r-project.org
>Subject: [R] metafor rma.mv weights questions
>
>Hi there,
>
>Question:
>Does the rma.mv command in metafor automically adjust the weights to
>account for multiple effect sizes within study or do these weights need to
>manually calculated and applied?
>
>Details:
>In setting up a multilevel meta-analysis and meta-regression with multiple
>effect sizes within studies, I have seen the suggestion to adjust the
>weights according to Hedges, Tipton, & Johnson (suggestion made at
>24.33mins in this tutorial https://www.youtube.com/watch?v=rJjeRRf23L8)
>using:
> Wij=1/Kj*mean V.j
>I am wondering if the data is structured hierarchically and the random
>effects for each level specified correctly, does the rma.mv command
>automatically adjust the weights in this way? Another way to ask this
>question is - what are the default weights used by rma.mv when there are
>multiple estimates within studies?
>
>Thanks
>Laura
>
>Laura Duncan, M.A.
>Research Coordinator
>Offord Centre for Child Studies
>McMaster University
>
>Tel: 905 525 9140 x21504
>Fax: 905 574 6665
>duncanlj at mcmaster.ca
>ontariochildhealthstudy.ca
>offordcentre.com
>
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