You would like to investigate whether smokers are more likely than nonsmokers to get lung cancer. You take the students in your class, select half at random and tell them to smoke a pack of cigarettes each day, and you tell the other half not to ever smoke. Fifty years from now, you will analyze whether more smokers than nonsmokers got lung cancer. Is this an experiment or an observational study?

Experiment

Explanation:

This would be an example of an experiment. This is an experiment because you are manipulating what the behaviour of the students is. A treatment is being applied to your subjects, as you are telling some of them to smoke and others not to. An observational study would require you not to interfere with the behaviour of the subjects, and simply observe what the outcome of their actions is in time.