The advantage of an experimental study over a descriptive one is that the researcher has so much more control over the outcomes. But even so if the patient�s participation is necessary (as in taking medicines), then close observation of compliance is necessary. Also, a complete follow up of all individuals is necessary. And in addition, there may be a problem with people�s willingness to be randomized to one treatment or the other with the result that people opt out of the study and the study becomes invalid if this is a severe problem.