How then would one count chasity? What counts as an occurance of being chaste?
Well, the article says they interviewed 800-and-some-odd kids, and 573 of them reported relationships, so the difference between 800-and-whatever and 573 will tell you how many were chaste for at least the 18 month period the researchers asked about. Those kids don't appear on the diagram, but I guess you could put them on as a set of disconnected dots. Anyway, chastity isn't an event so much as a state - if you had even one relation in that 18 month period, you are defined as not chaste, and if you didn't, you are chaste.