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
Which among the following is the major practical problem in a coho study? 
Select and explain. 
A. Differential loss of follow up, B. Long duration of study, C. Can be used only for rare conditions, D. None of the above
A: Differential loss of follow up
Answer: A: Differential loss of follow up 
Explanation: Disadvantages of coho studies: Have to follow large numbers of subjects for a long time. They can be very expensive and time consuming. They are not good for rare diseases. They are not good for diseases with a long latency. Differential loss to follow up can introduce bias.