Within each of the intervals, an individual can have one of three different states: (1) the individual has not started to use yet and is still at risk, (2) the individual has started to use, and (3) has reported use previously and is not at risk of initiation anymore. The hazard rate is computed simply by considering the subjects who report use as a fraction of all subjects who are still at risk. For example, if N = 100, and in the third interval 60 subjects have already previously reported use, then only 40 subjects are still at risk. If in the third interval ten subjects report first use, the hazard rate would be 1/4 = 0.25.
