3. Importance measures
A classification and identification of the events that are more
important for the fault of the top event is necessary. The Importance
Measures (IMs) can be used to rank basic events with respect
to their contribution to the probability of the top event. IMs are
calculated by the Birnbaum, Criticality, Structural and Fussell-
Vesely heuristic methods considering the same probability of
fault (0.01) for each event.
Birnbaum introduces a measure of importance of a FTA based on
the probability caused to the fault of the system by each
component k [2].
The Criticality importance measure considers the fault probability
of an event [22].
A new index based on the theoretical development completed
by Birnbaum is defined by Lambert [22] in order to define the
Structural method.
The IM of Fussell-Vesely of any event is given by the conditional
probability that at least one minimal CeS that contains
component i, considering that the system is failed [23]. This