Event Tree Analysis

• Event Tree Analysis is a bottom-up, deductive, system safety analytical technique.
• It explores system Responses to initiating "Challenges (Adverse Events)".
• It enables Probability Assessment of "Success/Failure".
• Applicable to: physical systems (with or without human operators) and decison-making/management systems.
• Complementary to other techniques; for example, Fault Tree Analysis (FTA). Failure Modes and Effects Analysis (FMEA)

Event Tree Analysis (Bernouilli Model)

• Portray all credible system operating permutations. Trace each path to eventual success or failure.
• Reduce tree to simplified representation of system behavior. Use binary branching. Lead unrecoverable failures and undefeatable successes directly to final outcomes.
• A fault tree or other analysis may be necessary to determine probability of the initiating event or condition. (Unity probability may be assumed).

Assess Risk and Judge Tolerability

• Failure statement express Severity
• Event tree analysis explores Outcomes/Assess Probability
• Probability and Severity establish Risk

Is the Risk Acceptable?

• If not, develop intervenors
• Select intervenor(s) on the basis of
  • Effectiveness
  • Cost
  • Feasibility (including schedule)

Event Tree Shortcomings and Advantages

• Shortcomings
  • Operating pathways must be anticipated
  • Partial successes/failures are not distinguishable
• Advantages
  • End events need not be foreseen
  • Multiple failures can be analyzed
  • Potential single-point failures can be identified
  • System weaknesses can be identified
  • Zero-payoff system elements/options can be discarded

References

1. NEBOSH National Diploma - Unit A | Managing Health and Safety Fault Tree Analysis (FTA) and Event Tree Analysis (ETA): Element A3 | Identifying Hazards, Assessing and Evaluating Risks www.icao.int