Evidence-appraisal glossary
Natural experiment
A natural experiment is a study that exploits a naturally occurring event or policy that assigns exposure in a way that approximates randomization, letting researchers estimate causal effects without a planned trial. Its credibility rests on the assignment being unrelated to the outcome's other causes.
Also called: quasi-experiment.
Examples include policy changes, geographic boundaries, or lotteries that split people into exposed and unexposed groups the investigators did not control. The key question is whether the natural process truly mimics randomization or whether the groups still differ in hidden ways. When the as-if-random assumption fails, a natural experiment can be as confounded as any other observational study.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.