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Tripod Beta

Example of a (simplified)
Tripod Beta diagram

Two main principles of Tripod Beta

Science based

The Tripod Beta methodology is based on the Swiss Cheese model and the Human Behaviour model. Both models are science-based and have their origins in the scientific research of Prof. dr. W.A. Wagenaar & Prof. dr. J.T. Reason for the universities of Leiden & Manchester (1986 - 1990).

Tripod Beta was created to help incident investigators to analyse process disturbances (incidents) in such a way that it would allow them to understand the influences on humans from the operational environment (where the risks are) in which the incident occurred. From this understanding, the organisational shortcomings, that created this environment, can be identified which allows for improvement actions to business processes to be taken.


Tripod Beta Incident Analysis is based on the theory that incidents, if not caused by uncontrollable external factors (e.g. meteorites, war), are the result of underlying causes that are deeply woven into the organization. These underlying causes are responsible for causing incidents of different kinds, at different locations, with different people involved and with different kinds of damage or injury. Sooner or later these underlying causes will cause disruptions of the intended process because they are a hidden part of the organization. However, only a small part of these disruptions will surface as real incidents, with or without damage or injury.

Tripod Beta Incident Analysis uses incidents to find the underlying causes which are part of the organization. The incident investigation report contains the identified underlying causes that played a role in the incident. As a result, management will be able to improve their process control in order to eliminate or minimize these ‘seeds’ in the organization before they lead to other business disruptions.

A Tripod Beta Incident Analysis results in a Tripod Beta diagram that shows how certain ‘agents of change’ form a threat for certain ‘objects’. Agents of change can cause harm to the objects as a result of an unwanted event, called incidents. A Tripod Beta diagram is a representation of the complete process, leading from the reported incident all the way back to the initiation of the intended activity that eventually led to the incident. The element of the diagram at the right-hand side usually describes the incident that people witnessed: a fire, a collision, etc. The unwanted events that took place prior to the final incident are displayed before that (to the left-hand side).

Thus, a Tripod Beta diagram consists of linked agent of change-event-object trios. The diagram is built up from left to right, indicating a timeline. The trio on the left has happened first and going through the diagram towards the right, all elements are described that have led to the final reported incident on the right. The individual agent of change-event-object trios are called AEO trios, the complete series of trios are called the Tripod Beta diagram.

Example of Tripod Beta core diagram (the trios)

Every organizational system is designed to prevent unwanted events by putting control measures in place. In case these control measures fail, there are also protective measures (defensive measures) in place to protect objects from damage or injury when control measures fail and unwanted events occur. These control and defensive measures are barriers to protect a system against incidents and the consequences of the incident.

Adding barriers to the Tripod Beta trios

A system that contains failed barriers also contains certain preconditions that have led to the ineffectiveness of the barriers. These preconditions can develop when there is a lack of control in the organization where the incident took place. These preconditions are therefore the key to the underlying causes that apparently have been present in the system for a longer period. These underlying causes are responsible for creating different hazardous preconditions that can lead to different substandard acts  (immediate causes resulting in failed barriers) and eventually to different types of incidents.

A Tripod Beta Incident Analysis creates a logical structure, clearly showing the relations between barriers, immediate causes, preconditions and underlying causes. In relation to the investigated incident, the Tripod Beta Incident Analysis explains which barrier has failed, which precondition has led to a certain substandard act (called ‘immediate cause’ in Tripod Beta) and which underlying cause has caused this precondition. Finally, the underlying causes are categorized in 11 Basic Risk Factors (BRF). Through identification of the Basic Risk Factors that have played a role in this and other incidents, the organization will have a good indication of how to allocate resources to the most vulnerable management areas.


What does a Tripod Beta diagram
intend to tell you?

  • The sequence of undesired events (process disturbances) in the intended process;

  • The measures (barriers) that could have controlled the process and mitigated the consequences;

  • The human behaviour involved at operational level;

  • The context, circumstances, perceptions and beliefs that promoted this behaviour;

  • The shortcomings at system-level that influenced the creation of these contexts. In other words ... 'where did the system fail';

  • The actions needed to fix (or at least to improve) the system.