A random sample of middle age sedentary males was selected from four census tracts, and each man was examined for coronary artery disease. All those having the disease were excluded from the study. All others were randomly assigned to either an exercise group, which followed for a two-year program of systematic exercise or to a control group, which had no exercise program. Both groups were observed semiannually for any difference in the incidence of coronary heart disease.
What type of study (study design) you would assign to this information? Why?