The following is a Forest plot representing the results of a meta-analysis.
Explain the meaning of all the components of the Forest plot
Candidates struggled to identify, and differentiate the various symbols (e.g. significance of
diamond compared to the square symbol, as well as their relative sizes). Essentially
candidates were expected to include in their answer that the X axis is the Odds ratio, the
vertical line from 1 on the x axis is the line of no effect, the results of the individual trials are
shown as boxes with the size of the box relating to the size of the trial, the position of the
box relates to the result of the trial, the horizontal lines are the 95% confidence intervals,
the diamond at the bottom of the diagram represents the combined result of the trial, the
size of the diamond represents the combined numbers from all the trials and that the
results can be considered statistically significant if the confidence intervals of the combined
result do not cross the line of no effect.
- The x-axis: odds ratio
- The y-axis: a list of studies. How you list them is a lawless free-for-all; Anzures-Cabrera reports that "no general recommendation is appropriate for the order in which the studies should be presented in a forest plot. Many authors default to an alphabetical ordering for ease of cross-referencing with tables and reference lists"
- The vertical line: line of "no effect", OR=1.0
- The horizontal lines: the confidence interval of the individual study
- The position of the square: a point estimate of the odds ratio (OR)
- The size of the square: the weight of the study according to the weighing rules of the meta-analysis, likely representing the sample size and statistical power.
- The diamond at the bottom: the combined result of the trial
- Results can be considered statistically significant if the confidence intervals of the combined result do not cross the line of no effect
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Anzures‐Cabrera, Judith, and Julian Higgins. "Graphical displays for meta‐analysis: An overview with suggestions for practice." Research Synthesis Methods 1.1 (2010): 66-80.
Cochrane: "Considerations and recommendations for
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Reade, Michael C., et al. "Bench-to-bedside review: Avoiding pitfalls in critical care meta-analysis–funnel plots, risk estimates, types of heterogeneity, baseline risk and the ecologic fallacy." Critical Care 12.4 (2008): 220.
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The Cochrane Handbook: 9.5.4 "Incorporating heterogeneity into random-effects model"