A root cause analysis engine uses event survival times and gradual deletion of events to improve analysis accuracy and reduce the number of required calculations. Certainty factors of relevant rules are recalculated every time notification of an event is received. The calculation results are held in a rule memory in the analysis engine. Each event has a survival time, and when the time has expired, that event is deleted from the rule memory. Events held in the rule memory can be deleted without affecting other events held in the rule memory. The analysis engine can then re-calculate the certainty factor of each rule by only performing the re-calculation with respect to affected rules that are related with the deleted event. The calculation cost can be reduced because analysis engine processes events incrementally or decrementally. Analysis engine can determine the most possible conclusion even if one or more condition elements were not true, because analysis engine can calculate the certainty factor of rule even if one or more events were not notified to analysis engine.