Question

I'm currently studying Epidemiology. This question is related to person-years in context to number of events of death. It is proposed, that the numbers of person-years can be added together and the number of events such as deaths can be calculated per number of person-years observed. I do not understand the meaning behind this concept. can someone please elaborate.

Let me further elaborate, that I know how to calculate person-years. That is not what I am asking you to explain. However, I don't understant how person-years and the events of death are calculated to find the observed onset of a given disease.

I got this information from the textbook titled: Epidemiology, 5th edition, by Leon Gordis