Endgames Statistical Question

How to read a funnel plot in a meta-analysis

BMJ 2015; 351 doi: https://doi.org/10.1136/bmj.h4718 (Published 16 September 2015) Cite this as: BMJ 2015;351:h4718
  1. Philip Sedgwick, reader in medical statistics and medical education1,
  2. Louise Marston, senior research statistician2
  1. 1Institute for Medical and Biomedical Education, St George’s, University of London, London, UK
  2. 2Department of Primary Care and Population Health and Priment Clinical Trials Unit, University College London, London
  1. Correspondence to: P Sedgwick p.sedgwick{at}sgul.ac.uk

Researchers undertook a meta-analysis of the effects of home blood pressure monitoring on blood pressure levels. Randomised controlled trials were included if home or “self” monitoring was compared with standard monitoring in the healthcare system. Participants were patients with essential hypertension, followed for two to 36 months. The main outcomes included measurements of systolic and diastolic blood pressure and the achievement of hypertension targets.1

Eighteen trials were eligible for inclusion. When the results of the trials were combined, home monitoring resulted in significantly lower systolic blood pressure than standard monitoring (mean difference 4.2 mm Hg, 95% confidence interval 1.5 to 6.9) and significantly lower diastolic blood pressure (2.4 mm Hg, 1.2 to 3.5). Home monitoring patients were more likely to achieve predetermined targets (relative risk 1.11, 1.00 to 1.11). The researchers presented funnel plots for the outcomes of systolic and diastolic blood pressure (figure). Egger’s test gave P=0.038 for systolic blood pressure and P=0.095 for diastolic blood pressure.

Funnel plots for the meta-analysis of the effects on blood pressure of home monitoring compared with standard monitoring in the healthcare system

It was concluded that home monitoring results in lower blood pressure than standard monitoring. Although the difference in blood pressure between the two methods was small it may contribute to an important reduction in vascular complications in the hypertensive population.

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

  • a) Failure to include in the meta-analysis all of the relevant trials that have been conducted may have been due to reporting bias

  • b) A funnel plot can suggest whether relevant trials were not included in the meta-analysis only as a result of publication bias

  • c) The funnel plots for systolic and diastolic blood pressure indicate that not all of the relevant trials that have been conducted were identified …

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