[R-meta] Transitivity in a three-level contrast-based network meta-analysis

Natan Gosmann n@t@n@go@m@nn @end|ng |rom gm@||@com
Tue May 5 09:45:09 CEST 2020

Dear Wolfgang,

Thank you very much for your really helpful explanation! It certainly
clarified our doubts.


Em ter, 5 de mai de 2020 04:17, Viechtbauer, Wolfgang (SP) <
wolfgang.viechtbauer using maastrichtuniversity.nl> escreveu:

> Dear Natan,
> Based on your description, the network plot should have lines between at
> least some of the medication nodes. It's not a matter of what effects you
> actually computed within the studies (e.g., in a three-arm trial, you
> computed med1 vs placebo and med2 vs placebo) but what arms are actually
> present in trials (so there should be a line between med1 and med2). In
> fact, the information about the head-to-head comparison of med1 vs med2 is
> contained in the two effects comparing med1 vs placebo and med2 vs placebo
> (also computing med1 vs med2 would be redundant). I hope this clarifies
> things.
> Best,
> Wolfgang
> >-----Original Message-----
> >From: R-sig-meta-analysis [mailto:
> r-sig-meta-analysis-bounces using r-project.org]
> >On Behalf Of Natan Gosmann
> >Sent: Tuesday, 05 May, 2020 9:02
> >To: r-sig-meta-analysis using r-project.org
> >Subject: [R-meta] Transitivity in a three-level contrast-based network
> meta-
> >analysis
> >
> >Hi everyone,
> >
> >We are performing three-level models assessing both placebo and medication
> >effects on continuous outcomes, estimating effect sizes with differences
> in
> >standardized mean changes (i.e. we estimate pre-post standardized mean
> >change of the placebo group and of the medication group. After that, we
> >calculate the difference among them). These models are being used to
> >perform a contrast-based (all effect sizes are estimated considering the
> >placebo group) network meta-analysis and we are including both two-arms
> >(placebo vs medication) and three-arms (placebo vs medication 1 vs
> >medication 2) trials.
> >
> >I have two question concerning the network plot of our study:
> >
> >1) Considering that all effect sizes are estimated based on placebo groups
> >and pairwise comparisons among are contrast-based, our network plot should
> >be star-shaped centered on placebo without any lines between medication
> >nodes? If so, how should we assess the transitivity assumption of our
> >network?
> >
> >2) For three-arms trials, even if we are not estimating the effect sizes
> >through the head-to-head comparisons, we consider that some degree of
> >variance is related to the fact that these medication groups are in the
> >same study. These comparisons should be considered direct effects
> >(therefore being presented as direct comparisons in the network plot)?
> >
> >Thank you very much!
> >
> >Best,
> >Natan Gosmann

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