What is the most important difference between an experiment and an observational study? An experiment is conducted in a laboratory and an observational study is conducted outside. An experiment has a treated group and a control group while an observational study does not. In an experiment, the researcher applies a treatment to a randomly assigned group and studies the results, while an observational study just looks at things that have happened without intervention. In an experiment, the researcher studies the effects of medical treatments or other scientific results, while an observational study looks at subjects in the social science arena.