Funnel Plot

In systematic reviews and meta-analyses, a graph (scatter plot) of the distribution of reported treatment effects of individual studies (along the horizontal axis) against the sample sizes of the studies (along the vertical axis). Because studies with larger sample sizes should generate more precise estimates of treatment effect, they are likely to be grouped more narrowly around an average along the horizontal axis; while the studies with smaller sample sizes are likely to be scattered more widely on both sides of the average along the horizontal axis. As such, in the absence of bias (e.g., publication bias), the scatter plot will be narrower at the top (large sample sizes, small variation) and wider at the bottom (small sample sizes, large variation), resembling an inverted funnel.