Observational studies are studies that measure the outcome of events without the introduction of interventions. They are useful in the formation of a hypothesis. The experiment described above is an observational study that compares two groups so as to measure the predictors of an outcome. No intervention is introduced to influence the outcome of the research.
An experiment on the other hand is often organized as a randomized control trial where subjects are divided into groups and a placebo or control group is introduced to monitor the outcome of the study.
