To further illustrate, if one seeks to identify the etiologic factors (e.g. causal factors) behind an outcome (e.g. an MI), then each step in the epidemiologic framework provides new and important information.

Descriptive studies are useful for identifying hypotheses to test in analytic studies.  Case-control studies are then usually applied to evaluate if the hypothesized factor is related to the outcome of interest.  Subsequently, cohort or longitudinal studies are applied to further define the importance of exposure to the causal agent for the development of the outcome.