Diseases with high incidence, we choose for this study and the study needs large group of people. The follow-up of large group (usual in cohort study) over a longer period of time is difficult.

Several problems, like resource crunch, time scarcity, paucity of trained personnel can occur or even the investigator may die. Hence, unless these requirements are adequately met, the cohort study should not be chosen.

Moreover, the attrition, loss on follow up (due to deaths, change in addresses, non-cooperation and withdrawal) is another major draw back with this design. If this loss on follow-up is significant, the internal validity suffers.