A root cause analysis engine uses event durations and gradual deletion of events to improve analysis accuracy and reduce the number of required calculations. Matching ratios 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 valid duration, and when the duration 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 matching ratio of each rule by only performing the re-calculation with respect to affected rules related to 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.