It is certain. We can also express this certainty as a decimal probability. Expressed in this format, the probability of a unitary passage is equal to 1. A person who passes away before time t participates in the passage. A person who passes away after time t does not.
Time t is the sole criterion upon which the determination depends.
The new, third life will be random in duration. This person may pass away at any time, either before or after time t , unpredictably. The addition of a third person opens up the possibility of a passage type which was not possible in Figure 13.4. To do this we'll add a fourth person. Again, this additional person may pass away at any time, either before or after time t .