Event tree analysis

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The event tree analysis (English Event Tree Analysis ) is an inductive process , which is to determine the possible consequences of a failure occurring. It is a type of system analysis and is described as a quality method in DIN EN 62502 .


In the event tree analysis, an event that can occur in a system is viewed in the event tree as a start event (initial event) and its possible effects on the overall system are examined. In the event tree, the effects of the start event on the system (the reactions of the system components to the event) are graphically represented in the form of branches (function or failure) ( binary tree ).

The event tree is usually drawn from left to right, with branches for two alternatives . An upper branch for the successful behavior of the event and a lower branch for its failure. The individual paths from the start event to a defined end state then represent the possible accident sequences.

Each branch is associated with a certain failure / failure probability . If no empirical values ​​are available for the fault probabilities, these are determined with the help of the fault tree analysis.

By multiplying the probabilities of the start event and those of the branches that are on the path, the probability of this specific accident sequence is obtained.

The accident probability of the system is obtained - provided there are no dependencies between the components of the various paths - by adding up all path probabilities that lead to an accident (see example in).

In the probabilistic safety analysis (PSA), the event tree analysis is used in conjunction with the fault tree analysis (see description of the method in PSA ).


Since the number of branches doubles with each step, very large trees can arise in complex systems. The creation and evaluation of such large trees is then only possible with the help of computer programs (see PSA ).


The event tree analysis can only be used meaningfully when the system requirements are known. It is mainly used in probabilistic safety analyzes (PSA).

See also

  • FTA , Fault Tree Analysis (fault tree analysis)
  • FMEA , Failure Mode and Effects Analysis
  • PSA , Probabilistic Safety Analysis

Sources and individual references

  1. ^ R. Apthorpe: "A Probabilistic Approach to Estimating Computer System Reliability" .

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