Event Tree Analysis (ETA)
Also known as: ETA, Event Sequence Diagram Analysis, Initiating Event Analysis, Olay Ağacı Analizi
Event Tree Analysis (ETA) is a forward inductive technique used in reliability and risk engineering to model the possible outcomes that follow an initiating event. Starting from a single undesired event, ETA traces all subsequent event sequences through a binary branching tree representing the success or failure of safety barriers and protective systems. Introduced formally in reliability and risk literature by Andrews and Moss (2002), it is widely applied in nuclear, chemical, and aerospace industries to quantify accident sequence probabilities and guide safety decision-making.
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When to use it
ETA is appropriate when a well-defined initiating event can be identified and the subsequent safety responses are sequential and conditionally independent or can be modeled as such. It assumes that each barrier either succeeds or fails in a binary fashion and that sequence order matters. ETA is most effective when used in conjunction with Fault Tree Analysis to quantify barrier failure probabilities. It is less suited for systems with highly dependent barriers, complex feedback loops, or where the order of barrier activation is ambiguous. Alternative approaches include Bow-Tie Analysis, Bayesian networks, or dynamic event trees for time-dependent systems.
Strengths & limitations
- Provides a clear visual representation of accident sequences and their probabilities, making communication to non-specialists straightforward.
- Naturally integrates with Fault Tree Analysis, allowing barrier failure probabilities to be derived from detailed component-level models.
- Identifies dominant accident sequences, enabling prioritization of risk reduction measures where they matter most.
- Supports both qualitative (sequence identification) and quantitative (probability calculation) risk assessment within a single framework.
- Assumes binary (success/failure) barrier outcomes, which may oversimplify systems with partial or degraded performance states.
- The conditional independence assumption between barriers can be violated in real systems with shared components or common-cause failures.
- Tree size grows exponentially with the number of barriers, making manual analysis impractical for complex systems without software support.
- Does not inherently capture time-dependent phenomena, dynamic system responses, or feedback between safety barriers.
Frequently asked
What is the difference between Event Tree Analysis and Fault Tree Analysis?
Fault Tree Analysis (FTA) is a top-down deductive method that begins with an undesired top event and traces backward to identify root causes. ETA is a forward inductive method that begins with an initiating event and traces forward through safety barriers to possible outcomes. The two methods are complementary: FTA is commonly used to quantify the failure probabilities of individual barriers modeled within an ETA.
How many safety barriers should an event tree include?
There is no fixed rule; the tree should include all barriers that are physically or operationally capable of influencing the outcome following the initiating event. In practice, industrial event trees typically model between three and ten barriers. Including too few risks missing important mitigation pathways, while including too many can make the tree unwieldy without meaningful improvement in accuracy.
Can ETA handle dependent safety barriers?
Standard ETA assumes conditional independence between barriers, which simplifies probability calculations. When barriers share components or are subject to common-cause failures, additional modeling is required — such as incorporating dependency factors, using Bayesian networks, or applying dynamic event tree methods. Ignoring dependence in such cases can significantly underestimate risk by treating correlated failures as independent events.
Sources
- Andrews, J. D., & Moss, T. R. (2002). Reliability and Risk Assessment (2nd ed.). Professional Engineering Publishing. ISBN: 978-1-86058-290-5
How to cite this page
ScholarGate. (2026, June 2). Event Tree Analysis (ETA). ScholarGate. https://scholargate.app/en/reliability/event-tree-analysis
Which method?
Set this method beside its closest kin and read them side by side — the library lays the books on the table; the choice is yours.
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