Observational study
An observational study is one where the researchers observe people in their “natural habitat” without interacting with them. Typically this refers to any type of non-experimental research, and is often used when it would be unethical to do an experiment. For example, if we wanted to study the effects of smoking on lung cancer risk, it would be unethical to select a group of people and ask them to start smoking to identify the effects on their health.
See also: case-control study, participant observation