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
Serial interval is a measure of ? 
Select and explain. 
A. Sensitivity, B. Specificity, C. Incubation period, D. Positive predictive value
C: Incubation period
Answer: C: Incubation period 
Explanation: Ans. is 'c' i.e., Incubation period "In actual pracatice we seldom know precisely the incubation period of a disease. But we know, when an outbreak of a disease occur, say in a family which is the smallest group and also a closed group, there is an initial primary case. The primary case is followed by two or three secondary cases with in a sho time. The gap in time between the onset primary case and the secondary case is called "serial interval". By collecting information about the whole series of such onset, we get a distribution of secondary case from which we can guess the incubation period of disease"