Intended for healthcare professionals

Endgames Statistical Question

Meta-analyses IV

BMJ 2011; 342 doi: (Published 02 February 2011) Cite this as: BMJ 2011;342:d540
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Section of Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}

Researchers undertook a meta-analysis of the long term efficacy of approved anti-obesity drugs in reducing weight and improving health status.1 Only double blind randomised placebo controlled trials of drugs used in adults (age over 18 years) for one year or longer were included.

The primary outcome was change in weight from baseline. A separate meta-analysis was performed for each drug identified. Fourteen trials of orlistat were found and results of the meta-analysis were presented in a forest plot (figure). Orlistat was more effective than placebo in reducing weight; with a mean difference in weight loss of 2.87 kg (95% confidence interval 2.53 to 3.21) greater than placebo.

Forest plot of the long term effect of orlistat compared with placebo on weight change. For each trial and the total overall effect, the mean difference in weight change was derived as orlistat minus placebo

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

  • a) The line of no effect would be a vertical line going through unity …

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