Cohort studies are the design of choice when randomization is not practical or ethical [6, 18].
They are also useful in the study of infections  and for hypothesis generation .
In the second we focus on the design of cohort studies, mainly its prospective aspect, and the distinguishing features from the retrospective type.
The section also covers the essential characteristics of a cohort study design and its varied applications in medical research.
Cohort studies can either be prospective (concurrent) or retrospective (non-concurrent) .
Researchers agree that cohort studies, as related to the hierarchy of evidence, rank below meta-analysis, systematic review and randomized controlled trial, but rank higher than case–control studies, cross sectional studies, case series/reports [13, 14, 15, 16].
Many valuable case-control studies, such as Lane and Claypon's 1926 investigation of risk factors for breast cancer, were retrospective investigations.
Most sources of error due to confounding and bias are more common in retrospective studies than in prospective studies.
Prospective studies usually have fewer potential sources of bias and confounding than retrospective studies.
A retrospective study looks backwards and examines exposures to suspected risk or protection factors in relation to an outcome that is established at the start of the study.