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.