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Practice Clinical epidemiology notes

What is heterogeneity and is it important?

BMJ 2007; 334 doi: (Published 11 January 2007) Cite this as: BMJ 2007;334:94

This article has a correction. Please see:

  1. John Fletcher, clinical epidemiologist
  1. 1 BMJ, London WC1H 9JR
  1. jfletcher{at}

    This is the first in a series of occasional articles explaining statistical and epidemiological tests used in research papers in the BMJ

    Three systematic reviews published in the BMJ, including one in this issue, have referred to heterogeneity and dealt with it in three different ways.1 2 3 So what is heterogeneity, and how do we assess its importance in a systematic review?

    Clinical heterogeneity

    Sometimes trials are just looking at different concepts. Reviewers might set out to summarise interventions for improving patients' ability to make treatment choices; the trials, however, might have covered diverse interventions, such as information leaflets, CD Roms, counselling sessions with a nurse, and training in consultation techniques for doctors. Although the interventions try to achieve the same end result (to improve patients' ability to make choices), they are different in nature.

    In theory, we could add all the trials in this review together and come up with a number, but would this be useful? Would the averaged number apply to all these diverse interventions? The interventions are so different that combining them does not make clinical sense. This is an example of clinical heterogeneity. Other circumstances that may give rise to clinical heterogeneity include differences in selection of patients, severity of disease, and management. Judgments about clinical heterogeneity …

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