Randomised controlled trialRCT
A study that assigns people to treatment or control at random — the design that can show cause rather than association.
A randomised controlled trial allocates participants to an intervention or a comparison group by chance. Randomisation is the whole point: it distributes the known and, crucially, the unknown differences between people evenly across the groups, so a difference in outcome can be attributed to the intervention rather than to who chose it.
It matters because almost every other design leaves the possibility that the people who did the thing were already different from the people who didn't. That is why a well-run trial sits at the top of most evidence hierarchies, and why "associated with" in a headline is a much weaker statement than "caused".
In practice, trials are judged on more than the label: allocation concealment, blinding where possible, how many participants dropped out, whether the outcome measured is the one that matters, and whether the trial was large and long enough to detect a real effect. A small, short, unblinded trial is still a trial, and still weak evidence.
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Written and reviewed by Mathew Beale, MSc Biotechnology, University of Reading.
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