What is the difference between an observational study and a randomised trial?
Updated: 2026-09-09 · Safety and evidence
Short answer
An observational study looks at what people already do; a randomised trial assigns at random who gets what. Randomisation also spreads out the differences nobody measured, which is why it supports stronger causal claims. Observational work is still useful for spotting signals and studying large, real-world populations.
In plain English
- Observational: cohorts, case-control, registries. No assigned intervention.
- Randomised: chance decides the group, which reduces selection bias.
- An observational pattern can be driven by a third factor linked to both things.
- The designs complement each other: one generates hypotheses, the other tests them.
What we know
- Randomisation is the standard mechanism for reducing confounding bias.
- Observational studies supply long-term safety data that short trials cannot.
What we don't know yet
- No statistical adjustment fully removes unmeasured confounding.
- A single trial, randomised or not, rarely settles a question on its own.