Extending the use of funnel plots into network meta-analysis
needs to account for the fact that studies estimate effects for
different comparisons.
As a result, there is not a single reference line against which symmetry can be judged.
To account for the fact that each set of studies estimates a different summary effect we suggest the ‘comparison-adjusted’ funnel plot.
Before using this plot, investigators should order the treatments in a meaningful way and make assumptions about how small studies differ from large ones.
For example, if they anticipate that newer treatments are
favored in small trials, then they could name the treatments from
oldest to newest so that all comparisons refer to ‘old versus new
intervention’.
Other possibilities include defining the comparisons
so that all refer to an active treatment versus placebo or sponsored
versus non-sponsored intervention.