Intended for healthcare professionals


Complex interventions or complex systems? Implications for health economic evaluation

BMJ 2008; 336 doi: (Published 05 June 2008) Cite this as: BMJ 2008;336:1281
  1. Alan Shiell, professor1,
  2. Penelope Hawe, professor1,
  3. Lisa Gold, senior research fellow2
  1. 1Population Health Intervention Research Centre, University of Calgary, Calgary T2N 4N1, Canada
  2. 2Health Economics Unit, School of Health and Social Development, Deakin University, Melbourne, Australia
  1. Correspondence to: A Shiell ashiell{at}
  • Accepted 15 April 2008

Although guidelines exist for evaluating complex interventions, they may be of little help in dealing with the multiple effects of interventions in complex systems such as hospitals. Alan Shiell, Penelope Hawe, and Lisa Gold explain why it is important to distinguish the two types of complexity

Health researchers commonly use the notion of complexity to indicate the problems faced in evaluating the effectiveness of many non-drug interventions.1 2 3 However, although it is rarely delineated, complexity has two meanings. In the first it is a property of the intervention, and in the second it is a property of the system in which the intervention is implemented. We examine the implications of these two views for economic evaluation.

What do we mean by complex?

The first view of complexity, in effect, means complicated. This is the meaning used in the Medical Research Council’s framework for the evaluation of complex interventions.4 5 A complex intervention is “built up from a number of components, which may act both independently and inter-dependently.”4 This makes it hard to define the “active ingredients” and to be sure which component or combinations of components is more important.

The second view makes reference to the insights offered by complexity science.6 7 8 9 Complexity is a property of a system not an intervention. A complex system is one that is adaptive to changes in its local environment, is composed of other complex systems (for example, the human body), and behaves in a non-linear fashion (change in outcome is not proportional to change in input).

Complex systems include primary care, hospitals, and schools. Interventions in these settings may be simple or complicated, but the complex systems approach makes us consider the wider ramifications of intervening and to be aware of the interaction that occurs between components of the intervention …

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