1. A method of calculating the probability of an event being observed during the occurrence of one or more simultaneous events in a system, said method comprising the step of calculating said probability according to the equation: P event i &equals;∫ 0 ∞ h i ( t )* R sys ( t ) dt&plus;P event0 i, i is the probability that a particular event will be observed; h i (t) is the instantaneous rate of occurrence of event i; R sys (t) is the reliability function of the system in which said events may occur; and Pevent0 i is the probability that an event will be observed when said event occurs simultaneously with other events; and i represents a particular event.