In simple terms
A randomized controlled trial, usually shortened to RCT, is an experiment on people. Some receive the treatment being tested and others do not, and who gets what is decided by chance.
Chance is the whole point. It is the most reliable way to make the groups comparable before the treatment starts.
How it works
NIH describes randomization as the process by which two or more alternative treatments are assigned to volunteers by chance rather than by choice. A computer usually does the assigning.
The comparison group receives either standard care or a placebo, a dummy that looks like the real treatment but has no active ingredient. Without that group there is nothing to measure the treatment against.
Many trials are blinded. In a single blind trial the participants do not know which group they are in. In a double blind trial the researchers do not know either, so expectations cannot color the findings.
Trials run in phases. NIH describes phase 1 studies in 20 to 80 people to check safety, phase 2 in 100 to 300 to look at effectiveness, and phase 3 in 1,000 to 3,000 to confirm it against standard treatment.
Why it matters
Most of what is known about whether medicines work rests on this design. Regulators rely on it, and so do the treatment guidelines clinicians follow.
It has limits. Trials can be impractical for very rare conditions, and are not considered ethical where a condition moves too fast to withhold treatment.
Where you’ll see it
- News reporting that a drug worked in a large trial
- Drug and vaccine approval decisions
- Clinical guidelines and systematic reviews
- Trials of teaching methods, cash transfers and other policies
Example
To test a new blood pressure pill, researchers randomly give half the volunteers the pill and half an identical looking placebo, then compare readings a year later.
Often confused with
An observational study is not an RCT. It watches what people already do, so any difference between groups may come from the people rather than the treatment.
Key facts
- NIH defines randomization as the process by which two or more alternative treatments are assigned to volunteers by chance rather than by choice.1
- In a double blind study neither the participants nor the research team know which treatment each person is receiving.1
- NIH describes phase 1 trials in 20 to 80 people, phase 2 in 100 to 300, and phase 3 in 1,000 to 3,000.1
- Double blind randomized controlled trials are considered the gold standard for determining the safety and efficacy of a therapy.2
- Where no standard treatment exists, the control group may receive a placebo, a dummy treatment made to look like the real one.3
Related concepts
In the news
Quick checkWhy are participants assigned to groups by chance rather than by choice?Show answer
So the groups are comparable at the start and differences afterward can be credited to the treatment.
Sources
- National Institutes of Health. Glossary of Common Terms, NIH Clinical Research Trials and You. Undated (accessed 22 September 2026)
- National Center for Advancing Translational Sciences, National Institutes of Health. Randomized controlled trial, NCATS Toolkit glossary. Undated (accessed 22 September 2026)
- Cancer Research UK. Randomised trials. 17 July 2025 (accessed 22 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 22, 2026Researched and drafted with AI assistanceReport an issue