- A graph of sample size plotted against effect size that is often used to investigate publication bias or the heterogeneity of studies in systematic reviews. Each blob in the scatter plot indicates the results of one study. The name arises from the fact that precision in the estimation of the true treatment effect increases as the sample size of the component studies increases. Because large studies estimate effect size more precisely than small studies, they tend to lie in a narrow band at the top of the scatter plot, while the smaller studies, with more variation in results, fan out over a larger area at the bottom, thus creating the impression of an inverted funnel. In the absence of bias the plot should resemble a symmetrical inverted funnel.