A funnel plot (Egger et al., 1997) is a graphical tool for detecting bias in meta-analysis.
In a funnel plot treatment effect is plotted on the horizontal axis and SciStat.com plots the standard error on the vertical axis (Sterne & Egger, 2001).
The vertical line represents the summary estimated derived using fixed-effect meta-analysis.
Two diagonal lines represent (pseudo) 95% confidence limits (effect ± 1.96 SE) around the summary effect for each standard error on the vertical axis. These show the expected distribution of studies in the absence of heterogeneity or of selection bias. In the absence of heterogeneity, 95% of the studies should lie within the funnel defined by these diagonal lines.
|Example of symmetrical funnel plot||Example of asymmetrical funnel plot|
Publication bias results in asymmetry of the funnel plot. If publication bias is present, the smaller studies will show the larger effects. See Sterne et al. (2011) for interpreting funnel plot asymmetry. The funnel plot may not always be a reliable tool, in particular when the number of studies included in the analysis is small.
- Egger M, Smith GD, Schneider M, Minder C (1997) Bias in meta-analysis detected by a simple, graphical test. BMJ 315: 629–634.
- Sterne JAC, Egger E (2001) Funnel plots for detecting bias in meta-analysis: guidelines on choice of axis. Journal of Clinical Epidemiology 54:1046–1055.
- Sterne JAC, Sutton AJ, Ioannidis JPA et al. (2011) Recommendations for examining and interpreting funnel plot asymmetry in meta-analyses of randomised controlled trials. BMJ 2011;343:d4002.