= Solution
The proposed hybrid recording rule updates the event times of students who leave but does not update the follow-up of those who remain. Thus the recorded end of observation depends on the subsequent outcome: this is <outcome-dependent updating of survival follow-up>.
In the four-student illustration, all four are known to be event-free at month six. Under the proposed coding, the three students without event reports are censored at month six, while the student whose event is subsequently reported remains in the <risk set> until month seven. The recorded <risk set> at that event therefore has size one, although the other three may still be studying. The <Kaplan–Meier estimator> factor would be $1-1/1=0$ and the <Nelson–Aalen estimator> jump would be $1/1=1$. This creates an artificial complete loss of estimated survival at that event.
If event notification is complete through the common eight-month cutoff, the absence of a notification establishes that the other three have not had the event by that cutoff. They should remain in the <risk set> at month seven. With only these four subjects, the corresponding factors are instead $1-1/4=3/4$ and $1/4$. \b[Updating only failures while backdating nonfailures' <censoring> creates a severely biased <risk set>.] If notifications are incomplete, absence of a report does not establish continued study; positive follow-up confirmation is then required.
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