Root Cause Analysis Methods for Workplace Incidents: 5-Why, Fault Tree, and PEEPO Explained

Root Cause Analysis Methods for Workplace Incidents: 5-Why, Fault Tree, and PEEPO Explained

If you run a small business: "Root cause analysis" sounds technical but the concept is simple — it means finding out why an incident really happened, not just what happened on the surface. This article explains the main methods used. For most small business incidents, the 5-Why technique is all you need. Read that section first.

When a workplace incident occurs, the instinct is to identify what went wrong and fix it. But "what went wrong" is rarely a single, obvious answer — and the quality of the root cause analysis conducted in the aftermath determines whether the corrective actions that follow will actually prevent recurrence, or simply address the visible symptom while leaving the underlying failure intact. This article examines the three root cause analysis methods most widely used in workplace incident investigation — the 5-Why technique, fault tree analysis, and the PEEPO framework — comparing their strengths, limitations, and appropriate use cases.

Why root cause analysis method selection matters

Most workplace injuries and incidents have multiple contributing causes. A worker is struck by a forklift: the immediate cause is a vehicle in a pedestrian zone. But why was the vehicle in that zone? Because the designated route was blocked. Why was it blocked? Because a delivery had been staged there temporarily — a practice that had become normalized over time. Why had it become normalized? Because there was no effective supervision of loading dock procedures during shift changeovers. The unsafe act at the end of this chain is real, but addressing only that act — retraining the forklift operator — leaves every other link in the chain intact.

This is the core failure of surface-level investigation: it produces findings that are factually accurate but analytically incomplete. The method used to conduct root cause analysis directly shapes how deep the investigation goes and how many contributing factors it surfaces.

For context on how AI is changing the rigor with which these methods can be applied in practice, see our overview of AI in workplace incident investigations.

The 5-Why technique

The 5-Why technique is the most widely used root cause analysis method in workplace safety — and the most appropriate starting point for most small and medium business investigations. Developed as part of the Toyota Production System, it involves asking "why" repeatedly in response to each answer until the root cause is reached. The number five is a guideline rather than a rule: some causal chains require three iterations, others require seven.

The method's strength is its simplicity. It requires no specialist training, no software, and minimal preparation. It can be conducted at the worksite within hours of an incident while evidence is fresh and witnesses are available. For straightforward incidents with a relatively linear causal chain — a slip, a manual handling injury, a chemical exposure from a single procedural failure — 5-Why analysis conducted rigorously will typically reach the systemic cause.

The 5-Why technique does not guarantee depth — it enables it. An investigator who stops at the first plausible answer will produce a shallow analysis regardless of the method they are using.

The method's limitations become apparent in complex incidents. When multiple causal chains converge — when the incident results from the simultaneous presence of several independent failures — linear 5-Why analysis will follow one chain and miss the others. It also depends heavily on the quality of each "why" question — inexperienced investigators frequently accept procedural or human error answers that should prompt a further question about why the procedure failed or why the human error was possible.

Fault tree analysis

Fault tree analysis (FTA) takes a different approach. Rather than following a single causal chain forward from the incident, it works backward from the undesired event — mapping all the combinations of failures that could have contributed to it. The result is a visual tree diagram in which branches represent causal pathways and nodes represent logical relationships.

FTA is the appropriate method for complex incidents where multiple independent failure paths contributed to the outcome. It is widely used in high-hazard industries — oil and gas, mining, chemical processing, aviation — where the consequences of inadequate investigation are severe and the incident scenarios involve multiple interacting systems.

For most small and medium businesses investigating typical workplace incidents, FTA is disproportionate to the task. Its value is highest in critical incident investigations where the investment in analytical rigor is justified by the severity of the outcome or the complexity of the scenario — typically fatalities or serious multi-party incidents where a specialist consultant is involved.

The PEEPO framework

PEEPO — People, Environment, Equipment, Procedures, Organisation — is a structured categorization framework that ensures investigators consider all possible contributing factor domains before concluding their analysis. Used alongside a causal analysis method, it functions as a completeness check: have we examined contributing factors across all five categories, or have we focused on People and ignored Organisation?

• People — Training, competency, fatigue, health, and behavior at the time of the incident. This category is frequently over-weighted, leading to findings that emphasize human error while underweighting the systemic factors that made that error possible.
• Environment — Lighting, temperature, noise, housekeeping, layout, access, and any environmental conditions that differed from normal at the time of the incident.
• Equipment — Tools, machinery, vehicles, and materials: their condition, maintenance status, design adequacy, and whether they were being used as intended.
• Procedures — Written safe work procedures, permits, and instructions: whether they existed, whether they were adequate, whether they were accessible, and whether they reflected actual practice.
• Organisation — Management systems, supervision, culture, resourcing, scheduling, and the organizational decisions that shaped the conditions in which the incident occurred. This is the category most frequently underdeveloped in workplace investigations — and the one most likely to contain the systemic root cause.

PEEPO's value is precisely in prompting investigators to examine the Organisation category. Incidents that are recorded as equipment failures or procedural non-compliances frequently have organizational contributing factors — inadequate maintenance budgets, production pressure that normalized shortcuts — that a People-Environment-Equipment analysis alone would miss entirely.

Choosing the right method

In practice, the most thorough workplace incident investigations use a combination of these approaches. A typical approach for a serious workplace injury might use PEEPO as an initial structuring tool to ensure all contributing factor domains are examined, and 5-Why analysis within each relevant PEEPO category to trace the causal chain to its origin.

The appropriate level of analytical complexity should be proportionate to the severity and complexity of the incident. A minor strain injury with a straightforward causal chain does not require fault tree analysis. A critical incident involving multiple people, equipment systems, and organizational factors warrants specialist involvement. OSHA's incident investigation guidance emphasizes that the depth of investigation should reflect the actual or potential severity of the incident.

Applying root cause analysis consistently

One of the most common quality problems in investigation programs is inconsistency: different people applying different methods with different levels of rigor, producing findings that cannot be meaningfully compared across incidents. Establishing a standardised approach — applied consistently across all investigations above a defined severity threshold — is one of the highest-value improvements any organization can make.

This is an area where AI-assisted investigation tools add particular value. By structuring the root cause analysis process within a defined methodological framework and prompting investigators to examine all contributing factor domains, they reduce the variation in analytical quality that comes from experience differences and time pressure. MyInvestigationCoach applies established root cause analysis methodology consistently across every investigation, producing analyses that are traceable, defensible, and aligned with the frameworks regulators and legal teams expect to see.