<prompt> Serial interval is a measure ofSensitivitySpecificityIncubation periodPositive predictive value  <response>: The gap in time between the onset of primary case and the secondary case is known as serial interval. By collecting information of the whole series of onset, we get a distribution of secondary cases from which we can guess the incubation period. Ref: 25th edition, Parks Textbook of Preventive and Social Medicine, Page no. 108 