Funnel plots

The above graphs represent funnel plots. In each figure, the x-axis represents the magnitude of the effect and the y-axis the "precision." The dotted line on the x-axis represents "no effect"; to the right of this line, the effect favors the control group while to the left the effect favors the intervention. Circles represent individual studies. Generally, larger studies have higher precision.
The funnel plot on the left shows several smaller studies symmetrically clustered around the "no effect" line. In contrast, the funnel plot on the right appears to be missing smaller studies that favored the control arm, suggesting that there may have been publication bias in favor of studies where the intervention succeeded.
Unfortunately, the graphical depiction of studies using this approach is subject to variable interpretation among raters, particularly when there are relatively few studies. Thus, funnel plots are not always a reliable method to clarify whether there is publication bias.