Determine whether the study described below is a randomized experiment or an observational study.
A medical researcher wants to determine whether exercising can lower blood pressure. She recruits 100 people with high blood pressure to participate in the study. She assigns a random sample of 50 of them to pursue an exercise program that includes daily swimming and jogging. She assigns the other 50 to refrain from vigorous activity. She measures the blood pressure of each of the 100 individuals both before and after the study.

Randomized experiment is a type experimental study design. It is a study design in which the researcher or investigator is in complete control of the research environment. For example, an investigator may wish to assess the effects of certain treatments on some experimental units. The investigator decides the type of treatment, the type of experimental unit to use, the allocation and assessment procedures of the treatments and effects respectively while in observational study, the researcher or investigator is usually a passive observer as he only observes and analyses facts and events as they occur naturally.

In experimental studies, the researcher is in complete control of the exposure and the result should therefore provide a stronger evidence of an association or lack of association between an exposure and a health problem than would an observational study