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What is a randomised controlled trial?

A randomised controlled trial (RCT) is an experimental study design used to evaluate the effect of an intervention by comparing outcomes between groups that are randomly assigned. In an RCT, a well-defined population is divided into at least two groups:

  • An intervention group that receives the treatment, programme or exposure
  • A control or comparator group that receives either no intervention, standard care or an alternative treatment


Outcomes are measured before and after the intervention, and differences between the groups are attributed to the intervention, assuming randomisation was successful. RCTs are widely regarded as the gold standard for determining the effectiveness of interventions.


How a randomised controlled trial works

In an RCT, participants are first assessed at baseline and then randomly allocated to one of the study groups. After the intervention period, outcomes are measured again to determine whether the intervention caused any changes relative to the control group.


The key distinguishing feature of an RCT is randomisation, which helps ensure that the groups are similar at baseline and that any differences observed after the intervention are due to the intervention itself.


Key features of randomised controlled trials

  • Experimental: Involves an active intervention delivered by the researcher
  • Random allocation: Participants are assigned to groups using a random method
  • Control or comparison group: Provides a reference against which outcomes are assessed
  • Baseline and follow-up measurements: Outcomes measured before and after the intervention
  • Causal inference: Strong evidence for determining intervention effects


Benefits of randomised controlled trials

  • Control for most biases because of randomisation
  • Confounders are more equally distributed between groups
  • Blinding is more feasible than in observational designs
  • Provides the strongest evidence for determining intervention effectiveness


Limitations of randomised controlled trials

  • Often costly in terms of time, funding and resources
  • Volunteer bias may influence who agrees to participate
  • Ethical challenges are common when assigning interventions or withholding treatment
  • Follow-up requirements can be demanding


When should you use a randomised controlled trial?

Use an RCT when you want to:

  • Determine whether an intervention causes a specific effect
  • Compare a new treatment, programme or approach to standard practice
  • Minimise bias and confounding as much as possible
  • Provide high-level evidence for clinical, behavioural or educational interventions
  • Demonstrate causality in a controlled and rigorous way
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