Qualitative analysis of an event tree is based on determining the cut sets for selected consequences. A cut set is a set of events that causes the consequence to occur. The events that belong to a cut set provide information such as single point failures and the relative contribution of each cut set. Generally, the cut sets that have the highest probability of occurrence are the ones that have the fewest number of events. If detailed reliability and maintainability information is available for the events, a quantitative analysis may also be performed to provide the numeric probability of occurrence of each event.