# How is weight of a study calculated/determined in a Forest Plot (e.g., by using metafor package in R)?

I was wondering how the weight of each study is determined in a Forest Plot?

Here is a theoretical example of weights appearing in a Forest Plot using the metafor package.

q  <- escalc(measure="OR", ai=ai, bi=bi, ci=ci, di=di, data=q)
q1 <- rma(yi, vi, data=q, slab=paste(study, year, citation, sep=", "), method="FE")

forest(q1, showweights=TRUE,...)


The inverse of the variance is the standard weighting, but the metafor package has a ton of options. Introduction to Meta-Analysis by Borenstein, Hedges, Higgins, and Rothstein is a standard textbook on this that I think explains inverse weighting in an accessible way. The metafor package has fantastic documentation, and you can find more information on the options for analyzing your meta-analysis with ?metafor::rma() or by going to the website: http://www.metafor-project.org/doku.php/features#models_and_analysis_approaches It says there that: "The package provides a variety of models and analysis approaches, including: fixed-, random-, and mixed-effects models using the inverse-variance method (rma() function)" (emphasis mine).

If the effect size in Study 1 has a variance of 10, the weight will be 1/10. If the effect size in Study 2 has a variance of 2, the weight will be 1/2, etc.

• Perhaps worth adding that your last sentence is only true in general for fixed effects models (which to be fair is what the OP fitted). – mdewey Aug 14 '18 at 12:32