What is a funnel plot used to assess in research?

A funnel plot is specifically designed to assess bias in meta-analyses. It is a visual representation that plots the effect sizes from individual studies against a measure of study size (usually the standard error). The plot takes the shape of a funnel, where smaller studies are expected to produce more variable results due to their lower statistical power, thus creating a wider spread at the bottom of the funnel.

In the absence of bias, the studies should be evenly distributed around the overall estimate of effect size, resulting in a symmetrical funnel shape. If there is evidence of publication bias or other types of bias, the funnel will tend to be asymmetrical, indicating that certain results are either missing or are systematically skewed. This is crucial in meta-analyses, where the aim is to provide an accurate and unbiased summary of the data available from different studies. Thus, recognizing patterns in funnel plots helps researchers identify potential biases in their findings, guiding them toward more accurate interpretations of the meta-analysis results.