Randomised trials

The randomised controlled trial (RCT) is a simple and powerful tool of research

The RCT aims to measure and compare the outcomes of two or more clinical interventions. One of the interventions is regarded as a standard of comparison or control, and the group of participants who receive it is called the control group. The key feature is random allocation to the groups, and apart from this random allocation to the comparison groups, the elements of a RCT are no different from the components of any other type of prospective, comparative, quantitative study.

Random allocation means that all participants have the same chance of being assigned to each of the study groups. However despite it’s simplicity the principle of randomisation is often misunderstood, so it is important that an appropriate method of randomisation is used (see list of resources below). Random allocation means that the characteristics of the participants are likely to be similar across the groups at the start of the comparison. Proper randomisation reduces the risk of serious imbalance in unknown but important factors that could influence the clinical course of the participants.

The most frequent unit of allocation in RCTs is the individual people, either patients (the most common) or caregivers, but groups can also be randomised.


  • unbiased distribution of confounders;
  • blinding more likely;
  • randomisation facilitates statistical analysis.


  • expensive in terms of time and money;
  • volunteer bias;
  • ethical issues at times.



  • CONSORT – checklist and flow diagram to improve the quality of reports of randomized controlled trials
  • SPIRIT – Standard protocol items: recommendations for interventional trials


Journal Articles

Why randomised controlled trials fail but needn’t – Series from the Canadian Medical Association Journal

Sackett DL, Hoey J. Why randomized controlled trials fail but needn’t: a new series is launched

Articles on Randomised Controlled Trials from the British Medical Journal

( In order to access these BMJ articles you will need to register with the BMJ )

Other Resources