On the Statistic tab in a Survival session, you can choose to calculate survival using either the Actuarial or the Kaplan-Meier method.

The two methods differ in their handling of individuals with identical survival times. If deaths occur at the same time as cases are lost to follow-up, the Kaplan-Meier method assumes that all of the cases that were lost to follow-up were at risk at the time of the deaths. The Actuarial method assumes that only half of those individuals were at risk at the time of the deaths.

The Kaplan-Meier method was developed for applications in which survival time is measured on a continuous scale, where ties (cases being lost to follow-up and deaths occurring at the same time) are rare, although the method is discrete in nature. The Actuarial method was developed for applications in which data is grouped, and ties are more likely. If the Kaplan-Meier method is chosen, the Months Per Interval control on the Survival Parameters tab must be set to 1.

The matrix footnotes will indicate which calculation method was used.