This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Natural history of disease is studied with ? 
Select and explain. 
A. Longitudinal studies, B. Cross-sectional studies, C. Both, D. None
A: Longitudinal studies
Answer: A: Longitudinal studies 
Explanation: Ans. is 'a' i.e., Longitudinal studies Longitudinal studies In this type of study observations are repeated in the same population over a prolonged period of time by means of follow up examinations. They are useful to Study the natural history of disease For identifying risk factors of disease For finding out incidence rate or rate of recurrence of new cases of disease. Note : Longitudinal studies are difficult to organize and more time consuming than cross sectional studies.