Answer: Natural experiment.
Explanation:
A natural experiment is a study in which the experiments carried out on individuals are determined by the forces of nature. It is an experiment that cannot be influenced by the investigators.
Natural experiments are used when there is difficulty in using controlled experimentation. An example of natural experiment is the impact of smoking ban in public on hospital admittance. The question is an example of a natural experiment because it is a natural occurring event without anyone's influence.