Statistical Question

How to read a forest plot

BMJ 2012; 345 doi: http://dx.doi.org/10.1136/bmj.e8335 (Published 7 December 2012)
Cite this as: BMJ 2012;345:e8335

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  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}sgul.ac.uk

Researchers undertook a meta-analysis of the safety and efficacy of antibiotic treatment compared with appendicectomy for the primary treatment of uncomplicated acute appendicitis. Randomised controlled trials were included if they investigated adult patients presenting with uncomplicated acute appendicitis, diagnosed by haematological and radiological investigations. The primary outcome measure was the presence of complications, including wound infection, perforated appendicitis, or peritonitis. Four randomised controlled trials were identified.1 The results of the meta-analysis for complications were presented in a forest plot.

Antibiotic treatment versus appendicectomy for uncomplicated acute appendicitis: forest plot for complications

Which of the following statements, if any, are true?

  • a) Not one of the four trials showed a significant difference between antibiotic treatment and appendicectomy in the risk of complications.

  • b) The forest plot is drawn on a linear scale.

  • c) A relative risk less than 1.0 represents a reduced risk of complications for antibiotic treatment compared with appendicectomy.

  • d) The meta-analysis of complications showed a relative risk reduction of 31% for antibiotic treatment compared with appendicectomy.

  • e) No significant heterogeneity existed between the sample estimates of the population relative risk.

Answers

Statements a, c, d, and e are all true, whereas b is false.

Four randomised controlled trials were identified that compared antibiotic treatment with appendicectomy for the treatment of uncomplicated acute appendicitis. The primary outcome in each trial was complications. For each trial a relative risk was derived that compared the risk of complications for antibiotic treatment relative to appendicectomy. Relative risk has been described in previous questions.2 3 Each sample relative risk was an estimate of the population parameter—that is, the relative risk that would be observed if antibiotic treatment was compared with appendicectomy for the entire population of all adults with uncomplicated acute appendicitis. The purpose of the meta-analysis was …

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