Randomized Controlled Trials Explained

A randomised controlled trial (RCT) assigns participants to an intervention or a comparator by chance, then compares outcomes. Random allocation is the main reason RCTs give the strongest evidence for treatment effects, provided the trial is well designed and reported.

Key design elements

• Eligibility criteria and recruitment defined in advance.
• Randomisation: a method that produces an unpredictable sequence (for example computer-generated), sometimes stratified or in blocks.
• Allocation concealment: keeping the next assignment hidden from those enrolling participants, to prevent selection bias.
• Blinding: reducing bias by concealing group assignment from participants, clinicians or outcome assessors, where possible.
• Pre-specified primary outcome and analysis plan, with the sample size calculated for it.
• Intention-to-treat analysis: analysing participants in the groups to which they were randomised.
• Trial registration and protocol publication (for example per SPIRIT).

Reporting

The CONSORT statement lists the information that a trial report should contain, including a flow diagram of participants. Use it to plan the trial as well as to write it.

Illustrative example

Invented for teaching. In a trial of a new inhaler-technique programme, 200 adults with asthma are randomised 1:1 to the programme or usual care. The primary outcome is asthma control score at 12 weeks, assessed by a blinded nurse. After 12 weeks, mean scores differ by 2.1 points (95% CI 0.9 to 3.3). Because participants know which programme they received, the trial cannot be double-blind, and it is described accordingly.

Strengths and limitations

Strengths: control of confounding, causal inference. Limitations: cost, time, selected participants (limited generalisability), ethics (equipoise needed), and inability to study rare long-term harms.

Beyond the basic trial

Variants include cluster trials (groups, not individuals, are randomised), crossover trials (participants receive each treatment in sequence), factorial trials and non-inferiority trials. Each has its own design and reporting extensions, and each changes the analysis plan, so involve a statistician early. Trials of drugs and devices may also require regulatory approval, trial registration before enrolment, and independent safety monitoring.

Common pitfalls

• Inadequate allocation concealment.
• Changing the primary outcome after seeing results.
• Excluding participants after randomisation without reporting.

Key takeaways

• Randomisation, concealment and blinding protect against bias.
• Pre-specify outcomes and register the trial.
• Report with CONSORT and appraise with a risk-of-bias tool.

This page is general educational information about research methods. It is not medical, legal or regulatory advice; follow your institution's ethics board and your target journal's instructions.