Clinical research & EBM · Reference
What is a randomized controlled trial?
A randomized controlled trial is a study design in which participants are allocated to groups by chance to compare an intervention against a control. Random allocation balances known and unknown differences between groups, making the RCT the strongest single study design for causal inference.
Why randomisation matters
Random allocation is the defining feature of the RCT. By assigning participants to groups by chance — using methods such as computer-generated sequences with concealed allocation — researchers expect the groups to be similar at baseline in every respect, both measured and unmeasured. This is what distinguishes the RCT from observational designs, which can balance only the factors investigators happen to measure and adjust for. Because randomisation tackles confounding at its root, a well-conducted RCT supports stronger conclusions about cause and effect than designs that rely on statistical adjustment alone.
Control groups and blinding
The "controlled" element means the intervention group is compared against a control group that receives a placebo, no intervention or standard care. Many RCTs add blinding so that participants, those delivering the intervention and those assessing outcomes do not know the allocation, which limits performance and detection bias. Where possible, analysis follows the intention-to-treat principle, keeping participants in their assigned group regardless of what happened afterwards, so the protective effect of randomisation is preserved.
Strengths, limits and reporting
The RCT is widely regarded as the gold standard for evaluating interventions and sits near the top of the evidence hierarchy, below only systematic reviews of multiple RCTs. Its strength is internal validity — confidence that the intervention caused the observed difference.
RCTs also have limits: they can be expensive, may enrol selected populations that limit generalisability, and are sometimes unethical or impractical, in which case observational designs are used instead. Reporting follows the CONSORT statement, which standardises how the design, conduct and results of a randomized trial are described.
Key facts
At a glance
- Defining feature: Allocation to groups by chance
- Controls for: Known and unknown confounders
- Compared with: Placebo, no intervention or standard care
- Strengthened by: Blinding and intention-to-treat analysis
- Evidence rank: Below systematic reviews; above cohort studies
- Reported via: CONSORT statement
Common questions
FAQ
Why is an RCT considered the gold standard?+
Random allocation balances both measured and unmeasured characteristics across groups, which addresses confounding at its source rather than through statistical adjustment. This gives a well-conducted RCT strong internal validity, so it supports more confident causal conclusions than observational designs.
What is the control group in an RCT?+
The control group is the comparison group that does not receive the intervention being tested; it may receive a placebo, no intervention or standard care. Comparing the intervention group against the control isolates the effect attributable to the intervention.
Are randomized controlled trials always the best choice?+
No. RCTs can be costly, may use selected populations that limit generalisability, and are sometimes unethical or impractical. In those situations observational designs such as cohort and case-control studies are used, accepting weaker control of confounding.
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