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Survey of claims of no effect in abstracts of Cochrane reviews

BMJ 2003; 326 doi: https://doi.org/10.1136/bmj.326.7387.475 (Published 01 March 2003) Cite this as: BMJ 2003;326:475
  1. Phil Alderson, associate director (training) (palderson@cochrane.co.uk),
  2. Iain Chalmers, former director
  1. UK Cochrane Centre, NHS Research and Development Programme, Oxford OX2 7LG
  1. Correspondence to: P Alderson
  • Accepted 23 December 2002

It is never correct to claim that treatments have no effect or that there is no difference in the effects of treatments. It is impossible to prove a negative or that two treatments have the same effect. There will always be some uncertainty surrounding estimates of treatment effects, and a small difference can never be excluded.1

Claims of no effect or no difference may mean that patients continue to be denied or exposed to interventions with important effects, either beneficial or harmful. They may also suggest that further research is unnecessary, so delaying satisfactory estimates of treatment effects.

The impossibility of proving no effect or no difference should be distinguished from the concept used for equivalence trials, where bounds are set on the differences that are deemed practically important. An analysis of 45 reports of trials purporting to test equivalence …

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