Intended for healthcare professionals

Education And Debate

Complex interventions: how “out of control” can a randomised controlled trial be?

BMJ 2004; 328 doi: (Published 24 June 2004) Cite this as: BMJ 2004;328:1561
  1. Penelope Hawe, professor (,
  2. Alan Shiell, professor1,
  3. Therese Riley, postdoctoral fellow1
  1. 1Department of Community Health Sciences, University of Calgary, 3330 Hospital Drive NW, Calgary T2N 4N1, Alberta, Canada
  1. Correspondence to: P Hawe
  • Accepted 24 March 2004

Complex interventions are more than the sum of their parts, and interventions need to be better theorised to reflect this


Many people think that standardisation and randomised controlled trials go hand in hand. Having an intervention look the same as possible in different places is thought to be paramount. But this may be why some community interventions have had weak effects. We propose a radical departure from the way large scale interventions are typically conceptualised. This could liberate interventions to be responsive to local context and potentially more effective while still allowing meaningful evaluation in controlled designs. The key lies in looking past the simple elements of a system to embrace complex system functions and processes.

Divergent views

The suitability of cluster randomised trials for evaluating interventions directed at whole communities or organisations remains vexed.1 It need not be.2 Some health promotion advocates (including the WHO European working group on health promotion evaluation) believe randomised controlled trials are inappropriate because of the perceived requirement for interventions in different sites to be standardised or look the same.1 3 4 They have abandoned randomised trials because they think context level adaptation, which is essential for interventions to work, is precluded by trial designs. An example of context level adaptation might be adjusting educational materials to suit various local learning styles and literacy levels.

Lead thinkers in complex interventions, such as the UK's Medical Research Council, also think that trials of complex interventions must “consistently provide as close to the same intervention as possible” by “standardising the content and delivery of the intervention.”5 By contrast, however, they do not see this as a reason to reject randomised controlled trials.

These divergent views have led to problems on two fronts. Firstly, the field of health promotion is being turned away from randomised …

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