Event Tree Analysis (ETA) : Event tree analysis is a method to illustrate the intermediate and final outcomes which may arise after the occurrence of a selected initial event. This technique is complementary to Fault Tree, but in reversed direction. Whereas a fault tree starts from a final event and works from the top down, an Event Tree begins with an initial event such as a power failure and explores all possible outcomes by working from the bottom up. An illustration is shown below, for an initial event of release of flammable gas. ETA identifies the sequences of events following an initiating event that results in accident. Event tree considers operator response or safety system response to the initiating event in determining the potential sequence.