Intended for healthcare professionals

Education And Debate

Characteristic and incidental (placebo) effects in complex interventions such as acupuncture

BMJ 2005; 330 doi: (Published 19 May 2005) Cite this as: BMJ 2005;330:1202
  1. Charlotte Paterson (, special training fellow in health services research1,
  2. Paul Dieppe, director1
  1. 1MRC Health Services Research Collaboration, Department of Social Medicine, University of Bristol, Bristol BS8 2PR
  1. Correspondence to: C Paterson
  • Accepted 24 February 2005

The specific effects of non-pharmaceutical treatments are not always divisible from placebo effects and may be missed in randomised trials


The randomised double blind controlled trial has proved an invaluable tool for testing the efficacy of new drugs. However, it is now used to evaluate complex non-pharmaceutical interventions, many of which are based on different therapeutic theories. For example, randomised controlled trials are used to test physiotherapy, a complex intervention with a basis in biomedical theory, and acupuncture, which is often based on Chinese medicine. In order to use a placebo or sham controlled design, an intervention has to be divided into characteristic (specific) and incidental (placebo, non-specific) elements. However, recent research suggests that it is not meaningful to split complex interventions into characteristic and incidental elements. Elements that are categorised as incidental in drug trials may be integral to non-pharmaceutical interventions. If this is true, the use of placebo or sham controlled trial designs in evaluating complex non-pharmaceutical interventions may generate false negative results.

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The characteristic effects of acupuncture extend beyond needling


Characteristic and incidental effects

A treatment, or healthcare delivery encounter,1 contains a spectrum of treatment factors and associated effects for which numerous terms and definitions are used. We will use the terms characteristic effects (specific effects) and incidental effects (placebo, non-specific, context effects) as defined by Grunbaum.2 3 Characteristic factors are therapeutic actions or strategies that are theoretically derived, unique to a specific treatment, and believed to be causally responsible for the outcome—for example, a drug. Incidental factors are the many other factors that have also been shown to affect outcome, such as the credibility of the intervention, patient expectations, the manner and consultation style of the practitioner, and the therapeutic setting.1 4 In randomised controlled designs, these incidental factors also include a dummy …

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