eventtree

An event tree is a diagrammatic method used in risk assessment and reliability engineering to map the possible outcomes that follow a defined initiating event. The analysis begins with a specific initiating event and then branches into subsequent actions, system responses, or failures, each with an associated probability. The branches represent conditional events conditioned on prior steps, such as the success or failure of safety systems, detection, and manual interventions. End states are typically stated in terms of safety outcomes or losses, such as safe operation, minor injury, major injury, or catastrophic loss, along with their cumulative probabilities.

Event trees are commonly employed in industries with high safety or reliability concerns, including nuclear power, oil and gas, chemical processing, aviation, and aerospace. They are often used in conjunction with fault trees, which work in the opposite direction by analyzing the causes of a given undesired consequence. Together, they provide a complementary view of risk: event trees focus on forward progression from a hazard, while fault trees trace back from outcomes to initiating failures.

Process and data: constructing an event tree involves identifying the initiating event, outlining possible subsequent actions or system responses, assigning conditional probabilities to each branch, and propagating these through the tree to determine overall risk. Event trees can be qualitative (structure only) or quantitative (with probabilities). They rely on reliability data, incident histories, and expert judgment, and are frequently analyzed with software tools to perform sensitivity analyses and to estimate expected risk.

Limitations include dependence on a complete and realistic set of initiating events, potential unaccounted dependencies between branches, and model complexity as the tree grows.