In clinical research, not only understanding the association between disease and exposure is important, but also to know whether the exposure caused the disease holds equal concern. If it can be showed that the exposure caused the outcome, the impact of the outcome can be evaluated by intervening on the exposure. An ideal experiment is unrealistic, given that the same individual is followed for outcome under identical conditions with and without the exposure, by going back in time. To mimic such counterfactual, one possible way is to design a randomized control trial (RCT) where the exposure is assigned by the investigator and followed up till the outcome occurs. In many cases it may not be a feasible option to randomly assign exposure of interest in RCT due to ethical issues or as it is cost prohibitive. The best alternative is conducting observational studies. Observational studies can be either analytical or descriptive. Analytical studies feature a comparison (control) group, whereas descriptive studies do not. Hypotheses about causation from descriptive studies are often tested in rigorous analytical studies. Within analytical studies, cohort studies, case control studies and cross sectional studies are popularly used among epidemiologists based on their research objective.