Unfortunately I cannot copy a question here since I do not have an electronic copy of ActEd (apologies for this). But this is related to Question 8.18 (page 36 of chapter 8).
It is a question about 12 patients who have an operation and time 0 is the start of the period of investigation. t is the time since having the operation.
Some patients only have the operation after the investigation starts (say the 4th week, for example). My issue is this:
Why are these individuals (ones who have operation only at time 4) included in Nj when calculating the estimates of the hazard function (for time periods before time 4)?