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The thinking doctor’s guide to placebos

BMJ 2008; 336 doi: (Published 01 May 2008) Cite this as: BMJ 2008;336:1020

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Placebo response, temporal trends and regression towards the mean

For patients who appear to improve when being given a placebo treatment there are three alternative explanations. The placebo phenenomenon, regression to the mean and temporal changes[1]. This article did not consider 2 of the 3 competing explanations for an improvement. A placebo controlled trial of the type the authors mention in erectile dysfunction cannot control for regression or temporal change effects and an examination of the cited paper shows that the design cannot exclude these competing explanations. Regression to the mean occurs when you identify patients (in this case men) who are at the extreme of a clinical measure with error [1] (a questionnaire on sexual functioning will be error prone). Intervening and then remeasuring will appear to show an effect irrespective of any other therapetic effects - placebo or otherwise. Similarly there may be a temporal effect. Both of these need to estimated and eliminated before we can ascribe a placebo effect. The trial by Kaptchuk and colleagues is good evidence to suggest there is a placebo effect in IBS with acupunture - the evidence cited in this article is not so robust and therefore should be treated with caution.

1 Torgerson DJ, Torgerson CJ. Designing randomised trials in health, education and the social sciences. Palgrave MacMillan, 2008. Basingstoke.

2 Kaptchuk et al. Components of placebo effect: randomised controlled trial in patients with irritable bowel syndrome BMJ 2008;336:999-1003

Competing interests: None declared

Competing interests: No competing interests

06 May 2008
David J Torgerson
Director, York Trials Unit
University of York YO10 5DD