Intended for healthcare professionals

Education And Debate

Ethical issues in the design and conduct of cluster randomised controlled trials

BMJ 1999; 318 doi: (Published 22 May 1999) Cite this as: BMJ 1999;318:1407
  1. Sarah J L Edwards, research associate (,
  2. David A Braunholtz, senior research fellow,
  3. Richard J Lilford, professor of health services research,
  4. Andrew J Stevens, professor of public health.
  1. Department of Public Health Medicine, University of Birmingham, Birmingham B15 2TT
  1. Correspondence to: S J L Edwards

    In most randomised controlled trials, individual patients are randomised to a treatment or control group, but sometimes this is undesirable or even impossible and groups (clusters) of people may be randomised instead. These are called cluster randomised controlled trials, and although they have been around for a long time, the need for them is likely to increase in line with growing concern to evaluate the delivery of health services, public education, and policy on social care.

    Summary points

    Need for cluster trials will increase with concern over health service evaluation, but issues of ethics and guardianship must be addressed

    In some cluster trials the intervention can be targeted at individuals (individual-cluster); where this would be too difficult or expensive the intervention is targeted at the whole group (cluster-cluster)

    Autonomy is important in individual-cluster trials, while the utilitarian welfare of the cluster as a whole is of paramount importance in cluster-cluster trials

    In individual-cluster trials the participants should give consent; cluster-cluster trials need procedural safeguards appropriate to the risks carried by the cluster intervention

    Guardians should sign a consent form that sets out their duties before they volunteer a cluster for a trial

    Ethical context

    The ethical aspects of medical practice and medical research are most often discussed in the context of two main moral traditions—utilitarianism and Kantian ethics. Broadly speaking, utilitarianism is concerned with increasing social utility (value), which usually means that the individuals maximise their expected utility and so act in their own best interests. In the long run social utility will not be served by demanding that individuals be self sacrificing for the common good. This leads to matters of distributive justice whereby utility and disutility, benefits and costs, are distributed as fairly and evenly as possible across society. The Kantian tradition shows why we are duty bound to respect a person's …

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