[R-meta] effect size estimates regardless of direction

Dave Daversa dd@ver@@ @ending from gm@il@com
Mon May 21 13:46:41 CEST 2018

Hi all,

My question regards how to estimate overall magnitudes of effect sizes from
compiled studies regardless of the direction.  I have attached a figure to
illustrate, which I developed using made-up data and the attached code.

In the figure five studies have significantly positive effect sizes, while
5 have significantly negative effect sizes.  Each have equal variances.  So,
the overall estimated mean effect size from a random effects model is
0.   However,
what if we simply want to estimate the mean effect size regardless of
direction (i.e. the average magnitude of effects)?  In this example, that
value would be 9.58 (CI: 6.48, 12.67), correct?

I have heard that taking absolute values of effect sizes generates an
upward bias in estimates of the standardized mean difference.  Also, this
would create a folded normal distribution, which would violate assumptions
of the model and would require an alternative method of estimating
confidence intervals.  What would be your approach to setting up a model
for answering the question of how much the overall magnitude of responses

I suspect this question has come up in this email group in the past.  If
so, my apologies for the redundancy, and please send me any reference that
may be helpful.

Dave Daversa

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