Risk Assessment
What is Event Tree Analysis? Modelling Outcomes After an Initiating Event
Vinit Pandey · Published 4 August 2026
In short: Event Tree Analysis (ETA) is an inductive, forward-looking technique that models the range of possible outcomes following an initiating event, based on whether each subsequent safeguard succeeds or fails.
Key takeaways
- ETA works forward from an initiating event, branching at each safeguard's success or failure
- Outcome probability along a branch is the product of the probabilities of each preceding safeguard
- ETA suits scenarios with multiple sequential protection layers affecting the range of outcomes
- LOPA can be understood as a simplified event tree focused specifically on independent protection layers
Where Fault Tree Analysis works backward from a top event to its causes, Event Tree Analysis works forward from a defined initiating event, branching at each subsequent safeguard depending on whether it succeeds or fails, until each branch terminates in a distinct final outcome.
Each safeguard in the tree — a relief valve, an operator response, a fire suppression system — is assigned a probability of success or failure, and the probability of reaching any given final outcome is the product of the probabilities along that branch's path.
ETA is particularly suited to scenarios with multiple sequential layers of protection where the outcome depends heavily on which layers succeed, such as tracing the range of outcomes from a detected gas leak through detection, isolation, and suppression systems.
Event trees and LOPA are closely related in structure — LOPA can be understood as a simplified event tree analysis focused specifically on independent protection layers, using generic rather than fully detailed branch probabilities.