Question 6

Explain the meaning of the components of a Forest plot.

PPI vs H2A bleeding

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College Answer

To score full marks candidates needed to describe each feature of the forest plot provided. This included: odds ratio on the x axis; line of no effect; individual studies on the y axis; point estimate for each study (box position); weighting of the study (box size); pooled effect estimate (diamond position); size of the diamond; and the 95% confidence intervals and their interpretation.


The image used above was borrowed from the Fellowship exam, where the Forest Plot makes several appearances (Question 8 from the first paper of 2015 and Question 10 from the first paper of 2009). Chris Nickson from LITFL actually managed to track down the exact PPI vs H2A study where this plot came from. In case you want to read it, there's a reference below. 

In summary:

  • 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


Alhazzani, Waleed, et al. "Proton pump inhibitors versus histamine 2 receptor antagonists for stress ulcer prophylaxis in critically ill patients: a systematic review and meta-analysis*." Critical care medicine 41.3 (2013): 693-705.

Methodological Expectations of Cochrane Intervention Reviews

Schriger, David L., et al. "Forest plots in reports of systematic reviews: a cross-sectional study reviewing current practice." International journal of epidemiology39.2 (2010): 421-429.

Lewis, Steff, and Mike Clarke. "Forest plots: trying to see the wood and the trees." Bmj 322.7300 (2001): 1479-1480.

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 
figures in Cochrane reviews: graphs of statistical data"
 4 December 2003 (updated 27 February 2008)

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.

DerSimonian, Rebecca, and Nan Laird. "Meta-analysis in clinical trials."Controlled clinical trials 7.3 (1986): 177-188.

Biggerstaff, B. J., and R. L. Tweedie. "Incorporating variability in estimates of heterogeneity in the random effects model in meta-analysis." Statistics in medicine 16.7 (1997): 753-768.

The Cochrane Handbook: 9.5.4 "Incorporating heterogeneity into random-effects model"