[R-meta] Standard error of the weighted mean in Metafor
James Pustejovsky
jepusto at gmail.com
Thu Sep 7 04:21:07 CEST 2017
Loren,
You are correct that the default standard error returned by rma.uni is
equivalent to Equation (6) of Hedges, Gurevitch, & Curtis (1999). As far as
I know, Equation (7) is not implemented in metafor. However, it is fairly
straight-forward to carry out the calculations in R. The code below gives
an example. I don't know whether Equation (7) really provides much
improvement in accuracy of SEs or CIs, but the difference is trivial in the
example given below. I'd be curious to hear if it makes any difference with
your data.
As an aside, Equation (7) appears to be generic, in the sense that it could
be applied to any effect size metric, rather than only to response ratios.
The example I give uses standardized mean differences.
James
# use first 8 studies from Raudenbush (1985)
data("dat.raudenbush1985")
dat <- subset(dat.raudenbush1985, study <= 10)
# random-effects model
reml_fit <- rma(yi, vi, data=dat, method="REML")
reml_fit
# calculate FE weights
w <- 1 / dat$vi
# calculate RE weights
w_star <- 1 / (dat$vi + reml_fit$tau2)
# Default metafor SE
reml_fit$se
# Formula (6) from HGC 1999 gives the same result
V_6 <- 1 / sum(w_star)
sqrt(V_6)
# Formula (7) from HGC 1999
df <- with(dat, n1i + n2i - 2)
theta <- w_star / sum(w_star)
V_7 <- (1 + 4 * sum(theta * (1 - theta) * (w_star / w)^2 / df)) /
sum(w_star)
sqrt(V_7)
# SE based on (7) compared to SE based on (6)
sqrt(V_7 / V_6)
On Wed, Sep 6, 2017 at 5:35 PM, Loren Albert <lalbert at email.arizona.edu>
wrote:
> Dear all,
>
> I am using the Metafor package (version 1.9-9) for a meta-analysis of log
> response ratios following Hedges et al. 1999 (citation at bottom). With
> escalc I use measure = "ROM" then I use rma to fit a fixed effect model (so
> method = "FE").
>
> Hedges et al give two different equations for the standard error of the
> weighted mean, one (equation 6) useful when the number of studies is large,
> and one (equation 7) useful when the sample size is smaller.
>
> In Metafor, I think that the standard error of the weighted mean produced
> from the 'rma' command is from equation 6, but I would like to verify this
> and also learn whether there is some option to use equation 7 instead.
> Overall I have found Metafor to be a great package with good documentation,
> so I apologize if I missed something about this in the documentation (so
> far I have looked at help(rma), the 'Metafor' document and Viechtbauer
> 2010).
>
> ---Reference---
> Hedges, L. V., Gurevitch, J., & Curtis, P. S. (1999). The Meta-Analysis of
> Response Ratios in Experimental Ecology. Ecology, 80(4), 1150–1156.
> http://doi.org/10.2307/177062?ref=search-gateway:
> d0e369adb64dd470ff732512b25b3dfd
>
> ----
> Thanks in advance!
>
> Loren
>
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>
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