Experimental studies are characterized by the investigator assigning an exposure of interest to individuals or populations for the purposes of comparing the effect of the exposure on an outcome of interest (Aschengrau & Seage, p. 142). The active manipulation of the exposure by the investigator is the hallmark that distinguishes experimental designs from observational ones (Aschengrau & Seage, p. 169). The most common type of experimental design used in epidemiology is the Randomized Trial.