The tripod method is a way of conducting incident analysis. It is mostly used for high risk, complex incidents, since it is a very extensive and detailed method. Training is highly recommended when using the tripod method.
A Tripod Beta tree is built in three steps. The first step is to ask the question: ‘what happened?’. All the events that happened in the incident are listed as a chain of events. The next step is to identify the barriers that failed to stop this chain of events. The question that is asked in this step is: ‘How did it happen?’. When all the events and the failed barriers in between are identified, the reason for failure of these barriers is analyzed. The last question for this step is: ‘Why did it happen?’. For each of the failed barriers a causation path is identified.
All the items that appear in the Tripod Beta method are explained in more detail below.
First it needs to be identified what happened during the incident; what events occurred. This is the core of the tripod diagram and is represented with three shapes, the head ‘trio’. These three elements are:
The trio can be explained as an AND gate, both the Hazard and the Object need to be present for the Event to occur. The Hazard acts on the Object to change its state or condition that is described as the Event. In a tripod tree there can be multiple trios. Hazards and Objects can form new Events.
In the tripod theory an Event is a happening, a ‘change of state’, whereby an object is affected by a Hazard. All events may cause potential injury, damage or loss. Examples of events are:
- Cut in a finger
- Car collision
- A failed money investment
A Hazard is an entity with the potential to change, harm or damage an object upon which it is acting. Hazards can be a physical energy source or can have a more abstract nature. Examples of a Hazard are:
- Working at height
- Explosive material
- Economic crisis
The Object is the item that is changed by the Hazard. The Object can be someone or something that is harmed, changed or damaged. Examples of Objects are:
- IT system
How did it happen?
The second step in the tripod analysis is to analyze how the incident could have happened by identifying the failed barriers. The barriers can be placed between the Hazard and the Event and between the Object and the Event. To identify these two types of barriers two questions are asked:
- What Barriers should have prevented the exposure to the Hazard?
- What Barriers should have protected the Object from the Hazard?
A Barrier is something that should prevented the meeting of an Object and a Hazard. It protects people, assets, environment from the negative consequences of a Hazard. Barriers can have their effect on the Hazard (e.g. insulation) or the Object (e.g. PPE). In a Tripod analysis a Barrier can be qualified as failed, missing or effective.
Why did it happen?
The last step is to identify why the incident happened; what caused the Barriers to fail. To analyze this we follow a certain pathway, called the ‘Causation Path’. The causation path consists of three items:
- Immediate Cause
- Underlying Cause
The Immediate Cause explains the human act that directly caused the Barrier to break. The Tripod method is based on the Human Error theory. This theory states that incidents happen when people make errors and fail to keep the barriers functional or in place. These errors are Immediate Causes. Examples of Immediate causes are:
- Neglecting to wear PPE
- Wrong design decision
- Inappropriate use of tools
The Precondition is the environmental, situational or psychological ‘state’ in which the Active Failure takes place. It explains the context of the human error and it provides the control breaching capacity of the Active Failure. This can be related to supervision, training, instructions, procedures, etc. Examples of Preconditions are:
- Bad sight
- Budget squeeze
- Poor ergonomics of tools
Underlying Causes are the organizational or systemic deficiencies that create Preconditions. The Underlying Cause acts on a system level, it always involves the organization. A Underlying Cause is not incidental, but is present for a longer time; it is an underlying failure. Examples of Underlying Causes are:
- Inadequate training
- Failure to identify hazards
- Imbalanced budgets
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