Funnel Plots

What Is a Funnel Plot?

A funnel plot is a scatter plot that helps detect publication bias in meta-analyses. Each study is plotted with its effect size on the horizontal axis and a measure of precision (typically standard error) on the vertical axis. In the absence of bias, the plot should resemble a symmetrical inverted funnel.

Generating a Funnel Plot

  1. 1In the Analysis Chat, ask the AI to generate a funnel plot (e.g., 'Create a funnel plot for the depression meta-analysis')
  2. 2The AI extracts the necessary data and generates the plot
  3. 3The funnel plot appears as an interactive card in the chat
  4. 4Ask for customizations in follow-up messages (e.g., 'Add confidence interval contours', 'Label the outlier studies')

Interpreting the Plot

  • Symmetrical funnel: suggests no significant publication bias
  • Asymmetrical funnel: may indicate publication bias, with smaller studies skewing to one side
  • Gaps in the plot: could suggest missing studies, often small studies with non-significant results
  • Outliers outside the funnel: may represent studies with unusually large effects or methodological differences

Customization Options

  • Choose between standard error, variance, or sample size for the vertical axis
  • Add confidence interval contours to the plot
  • Label individual study points for identification
  • Adjust axis ranges and plot dimensions

Limitations

  • Funnel plots are most useful with 10 or more studies — with fewer, asymmetry is difficult to assess
  • Asymmetry can be caused by factors other than publication bias, such as study heterogeneity or methodological differences
  • Visual assessment is subjective — consider supplementing with statistical tests where appropriate

Funnel plots are a recommended tool for assessing publication bias in systematic reviews. While they are not definitive proof of bias, they provide valuable visual evidence that should be discussed alongside your meta-analysis results.