TABLE OF CONTENTS An event tree is a visual model describing possible event chains which may develop from a hazardous situation. Initiating events (sometimes called top events) are defined and their frequency or probability of occurrence calculated. Possible outcomes from the initiating event are determined by using a list of questions where each question is answered 'yes' or 'no'. The questions will often correspond to safety barriers in a system such as 'isolation failed?' The method therefore reflects the designer's way of thinking.
The probability of alternative outcomes is calculated for each question which forms a branching point in a logic diagram. These branching points are often called the 'nodes' of the event tree. The probability or frequency of alternative end events (also often called terminal events) is calculated based on the probability or frequency of the initiating event and the conditional probability associated with each branch. End events may be gathered in groups having similar consequences to give on overall risk picture.
The event tree is quite similar to a cause consequence diagram although the latter uses more text and a few more graphical symbols. The cause consequence diagram is somewhat easier to read, but significantly less information can be compressed into one sheet. This may be part of the reason why event trees appear to be preferred. From event trees the following are often performed:
Frequency calculation for consequence classes
Sensitivity analyses (effect of variations of some parameters)
Identification of major contributions...
