In the framework of dependability analysis, it is important to know how much the occurrence of a basic event contributes to the occurrence of the top-event. In order to improve the probability of the top-event's occurrence, if it is a desirable event or to reduce it if not, we need to identify basic events having the highest influence on this probability. In this case it can be effective and optimal to improve (or reduce) one of the most important basic events to influence top-event probability.