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Every accident and near miss is a chance to stop something worse happening. A good investigation finds out why it happened, not just who was involved, and leads to changes that prevent it happening again. HSE’s guidance HSG245, Investigating accidents and incidents, sets out a simple four-step method.
Why investigate?
- To prevent a repeat, which is the main reason
- To meet the legal duty to monitor and review health and safety arrangements
- To decide whether the incident must be reported under RIDDOR
- To provide information for insurers and in case of a claim or enforcement
- To show workers that their safety matters and reports are taken seriously
Immediately after an incident
- Make the area safe and give first aid.
- Call the emergency services if needed.
- Preserve the scene as far as possible, without putting anyone at risk.
- Record names of those involved and witnesses.
- Report it to the right people, and decide whether it is reportable under RIDDOR.
HSE’s four steps
| Step | What it involves |
|---|---|
| 1. Gather the information | Photographs, measurements, statements from those involved and witnesses, records such as RAMS, training, inspection and maintenance records. |
| 2. Analyse the information | Work out what happened and why, looking past the immediate causes to the underlying and root causes. |
| 3. Identify risk control measures | Decide what needs to change to stop it happening again, using the hierarchy of controls. |
| 4. Action plan and implementation | Agree actions with owners and dates, carry them out, and check they have worked. |
Immediate, underlying and root causes
| Level | What it means | Example: a fall from a stepladder |
|---|---|---|
| Immediate cause | The direct cause of the harm | The stepladder slipped on a wet floor |
| Underlying causes | Unsafe acts and conditions that allowed it | No check of the floor, ladder used for a task beyond its use, no supervision |
| Root causes | Failures in management, planning or organisation | The task was not planned, RAMS did not cover access, and the programme pushed the work into a wet area |
Stopping at the immediate cause leads to actions like “told the operative to be more careful”, which rarely prevent a repeat. Asking “why” several times usually leads to the root cause, which is often in planning, supervision or resources. The hierarchy of controls helps choose actions that actually work.
Interviewing witnesses
- Interview people as soon as possible, separately, somewhere quiet
- Explain that the aim is to learn, not to blame
- Use open questions: what happened, what were you doing, what did you see
- Listen more than you talk, and record what was said
- Thank them, and tell them what happens next
How deep should the investigation go?
The level of investigation should match the potential, not just the actual, outcome. A near miss that could easily have killed someone deserves a thorough investigation, even though nobody was hurt. HSG245 describes levels from minimal to high, with more senior involvement as the potential seriousness increases.
Common mistakes
- Blaming the injured person and stopping there
- Investigating only injuries, not near misses
- Actions with no owner or date
- Not checking whether actions worked
- Not sharing lessons with other sites and teams
Training that helps
Our accident investigation eLearning teaches a structured method for supervisors and managers. IOSH Managing Safely and the NEBOSH National General Certificate both cover incident investigation in depth.
Where this fits in SMSTS and SSSTS
Accident investigation and the difference between immediate and root causes are tested on both papers. Our free SMSTS mock test and SSSTS mock test let you check your knowledge before the course or a refresher, and you can email yourself the results sheet at the end.
When you are ready, the five day SMSTS course is for site managers and the two day SSSTS course is for supervisors. If your certificate is due, the SMSTS refresher and SSSTS refresher keep it current.
Frequently asked questions
What are the four steps of accident investigation?
Gather the information, analyse it, identify risk control measures, and make and carry out an action plan. They come from HSE guidance HSG245.
What is a root cause?
The underlying failure in management, planning or organisation that allowed the incident to happen, as opposed to the immediate cause of the harm.
Should near misses be investigated?
Yes. Near misses show where controls are failing before anyone is hurt. The depth of investigation should reflect what could have happened.
Who should carry out an investigation?
Someone competent, usually the supervisor or manager with help from health and safety advisers, and with workers involved. Serious incidents need more senior involvement.
Read the source
HSE guidance HSG245, Investigating accidents and incidents; the Management of Health and Safety at Work Regulations 1999, regulation 5; and the Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013. Facts checked on 6 October 2026.
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