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

[Viechtbauer, Wolfgang (SP)]

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