The criticality importance measure of event A is the probability that component A is critical for the system and has occurred given that the top event has occurred. While the Birnbaum importance measure considers only the conditional probability that event A is critical, the criticality importance measure also considers the overall probability of the top event occurrence due to event A.

Alternatively, the criticality importance measure modifies the Birnbaum importance measure by adjusting for the relative probability of basic event A to reflect how likely the event is to occur and how feasible it is to improve the event. These modifications enable the criticality importance measure to focus on truly important basic events and make it possible to compare basic events between fault trees.

The criticality importance measure is defined as:

While the Birnbaum importance measure considers the maximum possible improvement in system performance with respect to event A while changing the event probability from 1 to 0, the criticality importance measure indicates the actual possible improvement from the current situation. Therefore, it is appropriate to use the criticality importance measure to improve system performance.