In cases where event A contributes to the top event but is not necessarily critical, the Fussell-Vesely importance measure can be used. For an event to contribute to the top event, at least one cut set containing event A should occur. The Fussell-Vesely importance measure is the ratio of the probability of occurrence of any cut set containing event A and the probability of the top event. Therefore, Fussell-Vesely importance measures are calculated quite differently than Birnbaum or criticality importance measures.

Fussell-Vesely importance measures are constructed using minimal cut sets. A cut set is a set of basic events whose occurrence causes the top event to occur. A minimal cut set is a cut set that would not remain a cut set if any of its basic events were removed. If the objective is to minimize the individual contributions of basic events, then Fussell-Vesely importance measure should be used to select the basic event to improve.