A causal tree is a picture of how a series of facts led to an event. It shows what the evidence supports. It is not a finding of blame, and a competent person must judge the result. Check whether the event must be reported under your legal duties (for example RIDDOR in Great Britain).
Link each action to the cause it deals with. An action only closes once someone other than its owner has checked that it worked. Look higher up the hierarchy of controls before settling for training, procedures or PPE.
Sending root causes to the register adds one risk for each root cause, with its linked actions as controls. The scores are provisional. Open the risk register, check each one, score it and sign it off.
What the checks found
The checks look for gaps in the record. They cannot tell whether a fact is true, whether a cause is missing, or whether an action is good enough.
What a causal tree is
A causal tree is a structured diagram that traces an incident back through its immediate, underlying and root causes. It maps the actions, conditions and system failures that led to the event as a hierarchy of causal factors, so an organisation can find every contributing factor and stop the same thing happening again.
Start with the event at the top. Ask what had to be true for it to happen, and write each answer as a fact. For each fact, ask the same question again, until every branch reaches a failure in how the work was planned, resourced, maintained, supervised or checked.
The three levels
- Immediate cause: what directly happened. The acts and the conditions at the moment of the event.
- Underlying cause: why those acts and conditions existed. The job, the workplace, the equipment, the people and the pressures on them.
- Root cause: the management system failure behind it. A gap in planning, rules, training, supervision, maintenance, design, or resources, which would let the same event happen again if it were left.
Three kinds of fact
- Action: something a person did or did not do. Always ask why it made sense to that person at the time.
- Condition: the state of equipment, place or material.
- System failure: a gap in how the work is managed.
How to build one
- Describe the event and its actual outcome, and set the severity.
- Add the facts that directly led to it. One fact per box, written as something seen, measured or recorded.
- For each fact, say how you know it: Verified, Reported or Assumed, and what was checked.
- Ask: would the fact above have happened without this one? If not, it is necessary. Then ask whether the causes you listed are enough together to explain it.
- Add the next level down. Keep asking why until each branch reaches a root cause.
- Link actions to the causes, choose the strongest control you can, and have someone else check each one.
What the checks flag
- A cause with no text, level, type or evidence status. A Verified cause with no note of what was checked.
- A cause marked Assumed. It counts as a gap until it is supported.
- A cause that is Reported only is listed as not yet verified.
- A branch that stops before a root cause. This is the most common failing: stopping at what a person did, instead of asking why.
- A cause not confirmed as necessary, or causes not confirmed as enough together.
- Levels out of order, for example a root cause sitting above an immediate cause.
- Wording that sounds like a judgement ("careless", "failed to", "human error"). Rewrite it as what was seen or recorded.
- No root cause found at all, or a root cause with no action.
- An action with no owner or due date, one that is overdue, or one marked done with no independent check.
- A severity 4 or 5 event with no reviewer, or a reviewer who is the lead investigator.
- An event where every action is training, procedure or PPE.
How it differs from the other tools
- The 5 Whys tool follows one line of cause. A causal tree branches, because most incidents need several things to be true at once.
- The Swiss cheese dashboard looks at layers of defence that failed together. A causal tree looks at the chain of facts that led to one event.
- The bow-tie builder looks forward, at barriers before something happens. A causal tree looks back, after it has.
- Actions can be tracked to closure in the Corrective Action Tracker, and root causes can feed the risk register.
Limits
No gaps found does not mean the investigation is right or complete. A tree can be tidy and still miss a cause. Ask the people who do the work whether it matches what happens, and have a competent person review it.