  The funnel plot is a graphical visualisation of summary data estimates from a
meta-analysis, and is a useful tool for detecting departures from the standard
modelling assumptions. Although perhaps not widely appreciated, a simple
extension of the funnel plot can help to facilitate an intuitive interpretation
of the mathematics underlying a meta-analysis at a more fundamental level, by
equating it to determining the centre of mass of a physical system. We used
this analogy, with some success, to explain the concepts of weighing evidence
and of biased evidence to a young audience at the Cambridge Science Festival,
without recourse to precise definitions or statistical formulae. In this paper
we aim to formalise this analogy at a more technical level using the estimating
equation framework: firstly, to help elucidate some of the basic statistical
models employed in a meta-analysis and secondly, to forge new connections
between bias adjustment in the evidence synthesis and causal inference
literatures.
