the question of investigation in most cases cannot be answered with all investigation designs, that is, each question of investigation should be answered with the most appropriate design.bwhen a question of investigation seeks to establish causality, the best model to establish this relation is the clinical trial but sometimes because of ethical or methodological implications this cannot be used and we should resort to observational models. among the observational studies the best one to establish causality and measure risks is the cohort study. the cohort study allows the comparison of two groups of patients free of illness, exposed and non exposed to a particular risk factor, to observe after a period of time which of these two groups presents the expected outcome.
