Intended for healthcare professionals


Designing and evaluating complex interventions to improve health care

BMJ 2007; 334 doi: (Published 01 March 2007) Cite this as: BMJ 2007;334:455
  1. Neil C Campbell, reader1,
  2. Elizabeth Murray, director, e-health unit2,
  3. Janet Darbyshire, director3,
  4. Jon Emery, professor of general practice4,
  5. Andrew Farmer, university lecturer5,
  6. Frances Griffiths, associate professor6,
  7. Bruce Guthrie, professor of primary care7,
  8. Helen Lester, professor of primary care8,
  9. Phil Wilson, senior clinical research fellow9,
  10. Ann Louise Kinmonth, professor of general practice10
  1. 1Department of General Practice and Primary Care, University of Aberdeen, Foresterhill Health Centre, Aberdeen AB25 2AY
  2. 2Department of Primary Care and Population Sciences, University College London, London N19 5LW
  3. 3MRC Clinical Trials Unit, London NW1 2DA
  4. 4School of Primary, Aboriginal and Rural Health Care, University of Western Australia, Claremont, Australia
  5. 5Department of Primary Health Care, University of Oxford, Oxford OX3 7LF
  6. 6Health Sciences Research Institute, University of Warwick, Coventry CV4 7AL
  7. 7Tayside Centre for General Practice, University of Dundee, Dundee DD2 4BF
  8. 8National Primary Care Research and Development Centre, Manchester M13 9PL
  9. 9Section of General Practice and Primary Care, University of Glasgow, Glasgow G12 9LX
  10. 10General Practice and Primary Care Research Unit, University of Cambridge, Cambridge CB2 2SR
  1. Correspondence to: N Campbell n.campbell{at}
  • Accepted 9 December 2006

Determining the effectiveness of complex interventions can be difficult and time consuming. Neil C Campbell and colleagues explain the importance of ground work in getting usable results

Complex interventions are “built up from a number of components, which may act both independently and interdependently.”1 2 Many health service activities should be considered as complex. Evaluating complex interventions can pose a considerable challenge and requires a substantial investment of time. Unless the trials illuminate processes and mechanisms they often fail to provide useful information. If the result is negative, we are left wondering whether the intervention is inherently ineffective (either because the intervention was inadequately developed or because all similar interventions are ineffective), whether it was inadequately applied or applied in an inappropriate context, or whether the trial used an inappropriate design, comparison groups or outcomes. If there is a positive effect, it can be hard to judge how the results of the trial might be applied to a different context (box 1).

Box 1: Illustration of problems of interpreting randomised controlled trials of complex interventions

Primary care mental health workers

The NHS Plan in 2000 suggested that by 2004, primary care trusts in England should employ 1000 new primary care mental health workers to help deliver better quality mental health care.3 There was little underpinning evidence of the value of the role or time to evaluate whether it would be effective before nationwide implementation.

In 2002, one trust decided to pilot the role. It employed and trained five psychology graduates and assigned them to one or two practices each.4 Their role included direct work with clients, supporting practice teamwork, and work in the wider community. It used a pragmatic inexpensive cluster randomised controlled trial to explore the effect of these workers on patient satisfaction, mental health symptoms, and the cost effectiveness of care. Sixteen practices and 368 patients participated.

At three months, patient satisfaction (the …

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