What does a "funnel plot" visualize in meta-analysis?

A funnel plot is a graphical tool used in meta-analysis to assess potential publication bias among the studies included in the analysis. The plot typically displays the effect sizes of the studies on the horizontal axis and their standard errors (or another measure of variance) on the vertical axis. In an absence of bias, the plot is expected to resemble a funnel shape, where studies with smaller sample sizes (which usually have greater variability) scatter widely, while studies with larger sample sizes cluster more closely around the average effect size.

When there is publication bias, particularly with smaller studies showing significant or positive outcomes being published more frequently than negative results, the plot will appear asymmetrical. This asymmetry can indicate whether the body of literature is complete or biased toward publishing certain types of results. Thus, the funnel plot is an essential tool in meta-analysis, providing insights into the reliability of the evidence being evaluated.

The other options do not accurately describe the primary function of a funnel plot. While sample sizes are relevant to meta-analysis, the plot itself is not focused on displaying this information, nor does it simply show results of individual studies without context related to bias. Lastly, while data collection is fundamental to meta-analysis, the funnel plot does not illustrate the process of data collection directly but