Event Tree Analysis
Acronym
ETA
Description
An event tree analysis (ETA) examines the different paths that can be taken in a given scenario. Each step consists of a node and corresponding exit lines for each possible outcome at that step. The tool was originally developed for use in system reliability analysis, but event trees are also useful for analyzing possible actions in a scenario and their associated consequences.
Uses
For team-based task modeling.
To examine the potential for errors within a system.
To show the sequences of tasks and their potential outcomes.
How do I use this tool?
1. DEFINE SCENARIO(S) UNDER ANALYSIS. Ensure that the scenarios as well as the task being analyzed are both clearly defined. Note that the event tree can analyze systems that are already in place or can evaluate the proposed design of new systems.
2. DATA COLLECTION PHASE. Collect the necessary data to construct the event tree diagram. The type of data you need will depend on the system that you are analyzing. Methods such as observation, interview, and questionnaires will normally suffice for data collection. Storyboards can be used when conducting an event tree analysis of a proposed system design.
3. DRAW UP TASK LIST. The task list should be comprehensive, and all steps that are necessary for task completion should be laid out in order. This first task list should describe the normal performance of the scenario, free from any errors. Subject-matter experts (SMEs) can help to effectively complete this step.
4. DETERMINE POSSIBLE ACTIONS FOR EACH TASK STEP. Following the creation of the task list, all possible outcomes from each task step should be defined and written down. Make note of which parts of the step are performed by humans versus the system in addition to what controls or other interface elements are utilized. SMEs may be useful during this step as well.
5. DETERMINE CONSEQUENCES ASSOCIATED WITH EACH POSSIBLE ACTION. From each action in step 4, find all the possible associated consequences and make note of them.
6. CONSTRUCT EVENT TREE. After completing steps 4 and 5, the event tree diagram can be constructed. It should portray all the possible actions that can be taken and all the consequences of the action.
Expertise Required
Users of this tool usually have some training or experience in its use.
Advantages
When complete, may be used to show any potential error within a system.
Can draw attention to a specific series of tasks and the consequences of choosing that path of task steps.
Disadvantages
Output frequently lacks explanation of task steps.
Using this tool can take a significant amount of time.
For very intricate tasks, the event tree diagram can become extreme in size and complexity.
Where can I go to learn more?
Stanton N, Salmon P, Walker G, et al. Process charting methods. Human factors methods: a practical guide for engineering and design. Great Britain: Ashgate; 2005. p. 109-37.