What Is Event Tree Analysis?

Event Tree Analysis (ETA) is a widely used root cause analysis (RCA) technique, particularly in areas such as manufacturing, energy production, and transportation. RCA is an essential component of an effective maintenance program and is used to identify the root causes of broader systemic failures.

Why is Event Tree Analysis important?

Many industries rely on complex systems made up of interdependent components and processes. In these settings, systemwide failures can often be traced to specific subsystems, machines, and parts. Event Tree Analysis, sometimes also referred to as Fault Tree Analysis (FTA), is designed to help maintenance programs identify the components at the root of a given failure event. Tracing errors and breakdowns to their route helps prioritize corrective actions and enhance system reliability cost-effectively. 

What is event tree analysis (ETA)? 

Event Tree Analysis is a graphically represented analytical technique that traces systemwide failures to their root causes. The ETA process involves assessing subsystems, pieces of equipment, and individual components included within assets. Event Tree Analysis is typically triggered by a failure event, sometimes called an initiating event

The ETA process provides maintenance programs with a framework for understanding, anticipating, and preempting failure events. The information gathered from a comprehensive root cause analysis can also be used to identify the potential symptoms of future breakdowns to prevent recurrence, improve reliability, and optimize performance. 

The primary goals of Event Tree Analysis include: 

  • Conducting a comprehensive risk analysis of possible root causes for a failure event
  • Assigning failure probabilities to specific outcomes
  • Mapping out accident sequences
  • Prioritizing actions based on the probabilistic risk assessment of each potential outcome
  • Limit the escalation of systemic failures
  • Identify critical points in system design that demand improvements
  • Inform future risk mitigation strategies. 

Event Tree Analysis (ETA) and Failure Tree Analysis (FTA)

Event Tree Analysis (ETA) and Fault Tree Analysis (FTA) are often used interchangeably to describe the root cause analysis process. It would be more accurate to say that these techniques are complementary to one another. ETA and FTA refer to two closely related but fundamentally different ways of evaluating failure. 

Overview of Fault Tree Analysis

Fault Tree Analysis (FTA) is a top-down approach that focuses on identifying the root causes leading to a specific event. This differs from the Event Tree Analysis (EFT) in that ETA is forward-looking, beginning with a triggering event and mapping out the potential undesired outcomes.

FTA is based on the core understanding that a failure event is often traceable to multiple causes. This mode of analysis places the undesired event at the top of a graphic representation and uses descending branches to illustrate the numerous potential causes contributing to this event.

The resulting graphic provides a logical map of the various combinations of component failure and human error that have the probability of leading to the top event.

Event Tree Analysis (ETA) vs. Fault Tree Analysis (FTA)

As noted in the section above, ETA is a forward-looking approach to root cause analysis. ETA begins at the root cause event and traces forward.

This is distinct from FTA, which begins with the top-bound failure event and traces downward to all potential contributing events. As a result, ETA is also graphically represented distinctly from FTA, with branches illustrating different potential event pathways (as opposed to the event combinations illustrated by the branches of FTA). 

Using ETA and FTA together

ETA and FTA are focused on evaluating two distinct aspects of a failure. While FTA focuses on the logical relationships between causes and their probabilities, ETA fuses on sequences of events and their probabilities. 

Because of these differences, ETA and FTA can be useful in conjunction with one another. These two modes of analysis can help to provide your maintenance team with more complete and well-rounded risk assessment and risk management strategies. Applying both risk analysis techniques together is sometimes referred to as the “bow-tiemethodology because of the resulting visual representation.

How is Event Tree Analysis performed? 

Event Tree Analysis is a systematic process that can be implemented by following the steps outlined below:  

  1. Define the system being analyzed as well as the specific parts or components of your system that will be included in the analysis. 
  2. Identify the initiating event, such as a process inefficiency, malfunction, breakdown, or system-wide failure. 
  3. Develop an Event Tree, beginning with the initiating event at the left-most point of your Event Tree diagram.
  4. Identify key events that could follow this initiating event, and draw branches representing each possible outcome for each key event. 
  5. Map pathways using branches to represent all possible sequences of events and both successful and unsuccessful potential outcomes.
  6. Assign failure probability to different outcomes, collect relevant historical data, conduct statistical analysis, and incorporate qualitative insights from trained personnel.
  7. Analyze the Event Tree, calculating outcome probabilities along each path to determine the overall probability of each potential outcome, and to prioritize maintenance actions and risk mitigation strategies. 
  8. Document the analysis process, and compile the findings in a clear and comprehensive report that should include graphical representations of Event Trees as well as key findings and recommendations.

Event tree analysis symbols

Event Tree Analysis (ETA) relies on graphic representation to help simplify otherwise complex and overlapping maintenance challenges. Therefore, graphic symbols play an important part in facilitating ETA. These symbols, which are roughly the same as those used in Fault Tree Analysis, help us outline and understand key events and outcomes with greater clarity. 

Below, we identify the symbols that you will need to recognize to conduct and use ETA.

  • Initiating Event:    ⃞  A rectangle or box is used to represent the event that has triggered the analysis.
  • Branching Event:   ━━━  Lines are used to represent the different possible outcomes branching out from the initiating event
  • Intermediate Event: ◯  A circle or oval is used to represent key events occurring between the initiating event and outcomes. 
  • Outcome: A diamond is used to represent the outcome or end state for each path in the Event Tree. 
  • AND Gate:  ⩍  A half oval is used to indicate that multiple events must all occur in combination to yield a subsequent outcome. 
  • OR Gate:  A triangle with curved sides and a curved bottom is used to indicate that at least one of several events, but not necessarily all events, must take place for a subsequent outcome to occur. 

Who uses event tree analysis?

Event Tree Analysis (ETA) applies across a wide range of industries. Professionals who engage in risk assessment, reliability engineering, compliance auditing, safety analysis, and preventive maintenance can benefit from the clarity and insight this mode of analysis provides. Maintenance personnel, in particular, must have a clear understanding of the various interconnecting components and relationships that require upkeep for full systems to remain operational. 

Below, we take a quick look at some of the industries where maintenance personnel must typically leverage the ETA technique to improve outcomes:  

  • Aerospace and aviation professionals use ETA to assess potential failures in aircraft systems, evaluate flight safety protocols, and ensure compliance with stringent regulatory conditions. 
  • Nuclear engineers use ETA to evaluate the safety of reactors, assess the consequences of potential initiating events, and design emergency response strategies at nuclear power plants.
  • Manufacturing quality assurance teams use ETA to evaluate the reliability of physical assets, assess the impact of equipment failures, and plan preventive maintenance schedules.
  • Transportation planners use ETA to assess risks in railways, public transportation systems, and highways to prevent accidents and ensure passenger safety.
  • Military and defense contractors use ETA to evaluate the safety and reliability of military systems, assess potential operational risks, and plan for contingencies.

What are the benefits of event tree analysis

Event Tree Analysis (ETA) offers several benefits, particularly when integrated into a strategic, preventive maintenance program:

  • Comprehensive risk assessment, including a structured and systematic approach to evaluating the potential outcomes of multiple initiating events and accident scenarios
  • Quantitative analysis, with the assignment of probabilities to different events and outcomes allowing for a numerical prioritization of actions
  • Improved safety conditions, with the identification of critical events and failure points enabling refined safety protocols and risk mitigation strategies
  • Strategic preventive maintenance, with ETA helping to pinpoint and prioritize the components and subsystems most likely to fail
  • Cost savings, with proactive maintenance and risk mitigation reducing the likelihood of unplanned downtime and lowering the related expenses of emergency repairs and lost productivity
  • Informed decision-making, with ETA providing valuable data and insights in support of maintenance strategies, safety measures, and risk management
  • Improved regulatory compliance, with ETA helping to identify safety issues, conduct due diligence, and create a documentable trail of relevant maintenance activities
  • Continuous improvement, with insights gained from ETA informing maintenance strategies, safety protocols, and operational procedures

Event tree analysis vs. failure mode and effects analysis

Both Event Tree Analysis (ETA) and Failure Mode and Effects Analysis (FMEA) are commonly used analytical techniques. Though both can provide valuable insights for risk assessment, safety inspections, and reliability engineering, these techniques differ in their approach, focus, and application. 


  • ETA takes a forward-looking approach to analysis, beginning with a specific initiating event and mapping out all possible subsequent events and outcomes.
  • FMEA takes a bottom-up approach to analysis, beginning by identifying potential failure modes within individual components or processes and assessing their probable impact on the broader system.


  • ETA is focused on outlining the sequences of events following an initiating event and calculating their probabilities. 
  • FMEA is focused on identifying and prioritizing potential failure modes based on their Severity, Occurrence, and Detectability.


  • ETA is commonly used in industries that rely on complex systems for operational continuity such as aerospace engineering, power production, and chemical processing.
  • FMEA is widely used in industries that rely on process efficiency such as design, development, manufacturing, and production.

Learn more about root cause analysis and failure modes

To learn more about root cause analysis and how you can more effectively pinpoint and mitigate the risks contributing to various operational failure modes, check out our comprehensive look at Fault Tree Analysis.

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