[R-meta] Meta-analysis Hedges g - comparisons with same placebo

Gerta Ruecker ruecker at imbi.uni-freiburg.de
Wed Oct 18 10:40:35 CEST 2017

Dear Roberto,

it is a network meta-analysis what you are planning? Do you think of 
multi-arm studies including a placebo group? If so, and if it is the 
same placebo that is compared to several antidepressants, then I think 
there is no problem. The different placebo groups in different studies 
provide independent information and network meta-analysis may in fact 
increase precision (provided you have loops in your network), but this 
is exactly what network meta-analysis is intended to do.

To avoid misunderstandings: What you should not do is to use a placebo 
group in a multi-arm study twice in a pairwise meta-analysis without 

For an introduction, see 
http://onlinelibrary.wiley.com/doi/10.1002/jrsm.1259/full .




Dr. rer. nat. Gerta Rücker, Dipl.-Math.

Medical Faculty and Medical Center - University of Freiburg
Institute for Medical Biometry and Statistics

Stefan-Meier-Strasse 26, D-79104 Freiburg, Germany

Phone +49 (0)761 2036673
Fax   +49 (0)761 2036680

Mailruecker at imbi.uni-freiburg.de

Am 18.10.2017 um 08:12 schrieb P. Roberto Bakker:
> Hi,
> We are conducting a meta-analysis of RCTs between antidepressants and placebo and calculate Hedges g.
> Question:
> How can we deal with studies which compare different antidepressants with the same placebo? Our concern is when we include the different antidepressant comparisons with the same placebo in the meta-analysis the placebo size will increase artificially which will lead to more precise spurious results. If our reasoning is correct, how can we deal this?
> Thank you in advance,
> Roberto
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