It is not exactly clear to me what "start" and "stop" is. If you have divided your survival time into subintervals which "start" and "stop" is the endpoints for the subintervals, then I think you did it right (and you can forget my comment). But if "stop" is the survival time and you used that for create "fortnight", then it does create predictors is introducing dependency on future events: In the cox regression you go through the timeline, and at each timepoint you can legally construct the rate by using only what happens in the past. Using the survival time for creating the rate before the event time will then be to conditioning on the future. Thanks again for the response JacobSimonsen    "Start" and "stop" mark the end of subintervals of time, conditioned only on the date (every two weeks, a new interval is created on a new row). However, intervals are created until either The interval (covering two weeks) contains the survival event (in which case, for this interval, it will show the survival event to have happened; or 26 fortnights are counted and the event has not occurred, and the last row is right-censored. So if somebody left in three fortnights, they would have three rows, but if somebody never left, they'd have 26 rows. If somebody left in the third fortnight, their survival time would be equal to the 'stop' variable on that row only. The previous rows would show that the survival event did not occur in that time period.     ... View more