 This is the algorithm to decide the type of study design in research. If the investigator assigns an intervention, then it becomes an interventional study or experimental study. If we are providing a randomization for dividing the study groups and the control groups, then it is a randomized control trial. If we are not following any randomization, then it becomes a non-randomized control trial. Quasi-experimental field trials and community trials will come under this non-randomized control trial. Now if the investigator does not assign any exposure or intervention, then it becomes an observational study. If we don't have a comparison group, then it becomes a descriptive study. All your case reports, case reviews, ecological studies come under this descriptive study. If we have a comparison group, we start from the exposure, finish at the outcome, then that becomes a cohort study. If we start from outcome to exposure, to study the exposure, then that becomes a case control study. And if we measure both the exposure and the outcome at the same time, then that becomes a cross-sectional study. The descriptive studies generate hypotheses. Other studies test the hypotheses.