Root Cause Analysis in Health and Safety: The 5 Whys

After an accident or near miss, it is tempting to blame the person involved or fix the obvious problem and move on. Root cause analysis means digging deeper to find out why it really happened, so that the same thing does not happen again somewhere else. It is the heart of a good accident investigation.

Three levels of cause

Level	What it means	Example
Immediate cause	The agent of injury or the unsafe act or condition at the time	Worker slipped on oil on the workshop floor
Underlying cause	The less obvious system or organisational reasons	A leaking machine had not been repaired and no one cleaned the spill
Root cause	The initiating management or organisational failure	No planned maintenance system, and no process for reporting and fixing leaks

HSE’s guidance on investigating accidents, HSG245, uses these levels and explains that root causes nearly always come back to failings in planning, organisation, control, monitoring or review.

The 5 whys

The 5 whys is the simplest root cause method. Start with the problem and ask why it happened. Take the answer and ask why again, and keep going until you reach something the organisation can fix. Five is a guide, not a rule.

Why did the worker slip? There was oil on the floor.
Why was there oil on the floor? The press had been leaking hydraulic oil for days.
Why was the leak not repaired? It had not been reported to maintenance.
Why was it not reported? Operators thought someone else had reported it, and there is no simple way to log defects.
Why is there no simple way? The defect reporting process was never set up when the workshop changed hands.

The fix is not “tell people to be careful”. It is a defect reporting system, routine checks of machines for leaks, and clean up kits at the point of use.

Other methods

Fishbone (Ishikawa) diagram: groups possible causes under headings such as people, equipment, methods, materials, environment and management.
Fault tree analysis: works backwards from the event through a logic diagram of contributing failures. Useful for complex or high hazard events.
Barrier analysis: looks at which controls should have prevented the event and why each one failed.
Events and causal factors charting: lays out the sequence of events on a timeline with the conditions behind each one.

Common pitfalls

Stopping at human error. People make mistakes, so ask why the system allowed the mistake to cause harm.
Blaming people, which stops others reporting honestly. See our guide to safety culture.
Jumping to a cause before gathering evidence.
Actions that cannot be measured or closed out, such as “raise awareness”.
Not sharing the lessons across the organisation.

Training that helps

Our accident investigation eLearning is a quick introduction. For managers and supervisors, IOSH Managing Safely covers investigating incidents, and the NEBOSH National General Certificate covers investigation methods and root causes in depth. In construction, SMSTS and SSSTS cover accident investigation for site managers and supervisors.

Frequently asked questions

What is root cause analysis?

A structured way of finding the underlying organisational reasons why an accident or near miss happened, so they can be fixed.

How does the 5 whys method work?

You ask why the problem happened, then ask why again of each answer, until you reach a cause the organisation can fix.

What is the difference between an immediate cause and a root cause?

The immediate cause is what directly led to the event, such as oil on the floor. The root cause is the management failure behind it, such as no maintenance system.

Is human error a root cause?

Rarely. Human error usually has its own causes, such as poor design, training, workload or supervision.

Read the source

HSE HSG245, Investigating accidents and incidents; HSE HSG65, Managing for health and safety; and HSE HSG48, Reducing error and influencing behaviour. Facts checked on 6 October 2026.

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