why

This approach is used at Toyota where they have established a rule that an investigator needs to ask "why" at least five times to get to a finer level of detail about the event than might otherwise be achieved.

Consequent Event

The consequent/discovery event is located at the top of the causal tree.

The consequent or discovery event is described in terms of the event's consequences:

• harm that either did happen (misadventure)
• or could have happened (no harm event or near miss)

Recovery Side

Below the consequent event, the tree is divided into two sides the failure side and the recovery side.

The recovery side of the causal tree only applies in the case of a near miss event. If the event was a near miss, the recovery side of the causal tree provides information on how the event was stopped from developing into a misadventure or no harm event. If the event actually occurred (misadventure or no harm event), the recovery side is left blank.

Recoveries can be planned (e.g., check point in work process) or unplanned (e.g., accidental discovery of an error).

Antecedent Events

Antecedent events are those conditions, decisions, and actions that led up to the consequent event.

By asking why of the consequent event, all antecedent events and conditions are discovered. They should be listed, chronologically, from left to right on one line across the tree.

After all relevant antecedents have been identified for the consequent event, the investigatior should ask why of each antecedent event. The information in this step may or may not yield several additional layers in the tree.

Root Causes

Root causes are factors that contribute to the generation of an event. These factors fall into one of the following categories:

• Human
• Technical
• Organizational
• Other

The information listed below the antecedent events in the bottom row of boxes are the root causes. A code is assigned to each root cause.

Failure Side

Below the consequent (discovery) event, the tree is divided into two sides the failure side and the recovery side.

The failure side describes the antecedent actions and decisions leading up to the event. As there are usually several (things) leading up to the consequent (discovery) event, the failure side should in most cases have several branches. Each branch is made up of one action or decision.