Develop practical investigation skills, find the true causes of incidents, and create safer workplaces through effective corrective actions.
Five Whys is best for exploring a relatively simple causal chain, Fishbone Analysis helps teams organise multiple contributing factors, and Fault Tree Analysis is suited to complex events involving interconnected technical or operational failures.
Construction incidents rarely result from one unsafe act. Supervisors may identify the immediate event but miss planning, equipment, communication or management failures. Choosing the wrong analysis method can produce weak corrective actions. Understanding root cause analysis in construction means selecting the tool that matches the incident complexity.
Root cause analysis in construction requires understanding four distinct causal levels:
Immediate Cause: What happened directly (e.g., scaffold moved unexpectedly)
Contributing Factor: What increased the likelihood or severity (e.g., load imbalance, floor unevenness)
Underlying Cause: The organisational or system weakness (e.g., inspection procedure not followed)
Root Cause: A fundamental failure that must be addressed to prevent recurrence (e.g., inspection and handover process not adequately defined)
Investigations should support learning, not worker blame. "The worker failed to follow the procedure" is not normally a complete analysis. Investigators should ask why the behaviour, condition or decision was possible.
OSHA's guidance supports looking beyond surface-level events to equipment, procedures, training and safety-program deficiencies. Professionals responsible for investigations can strengthen these skills through the Incident Investigation / Root Cause Analysis course.
|
Method |
Best Suited To |
Main Strength |
Main Limitation |
|
Five Whys |
Simple or moderately complex events |
Fast causal questioning |
Can oversimplify multi-causal incidents |
|
Fishbone Analysis |
Incidents involving several categories of causes |
Broad team-based exploration |
Generates possible causes that still require verification |
|
Fault Tree Analysis |
Serious or technically complex events |
Maps logical combinations and failure pathways |
Requires more expertise, evidence and time |
Use the simplest method capable of explaining the incident without excluding important causal pathways.
AHRQ describes Five Whys and Fishbone as root cause analysis tools, while NASA describes Fault Tree Analysis as a top-down method for identifying process hazards.
Five Whys is a questioning technique that repeatedly asks why an event occurred until the investigation reaches a controllable system or organisational cause. Remember that five is a guide, not a mandatory number.
Use the scaffold example to demonstrate:
Why did the worker fall? The scaffold moved.
Why did it move? One or more wheels were not effectively secured.
Why was this not identified? The pre-use inspection was incomplete.
Why was the inspection incomplete? Responsibilities were unclear during a shift change.
Why were responsibilities unclear? The site's scaffold inspection and handover process was not adequately defined.
This final answer points to a controllable organisational failure—the process itself requires redesign.
Five Whys is suitable for:
Five Whys can:
Fishbone Analysis is a cause-and-effect method that organises possible contributing factors into categories before evidence is used to confirm or reject them. Alternative names include Fishbone diagram, Ishikawa diagram and Cause-and-effect diagram.
Use construction-specific categories rather than generic manufacturing categories:
For the scaffold event, the team might identify several credible factors instead of one linear explanation. A wheel brake may be defective (equipment), the pre-use inspection may have been rushed (method), the inspector may lack competency (people), and site supervision may be inadequate (management). Fishbone organises these possibilities visually.
Fishbone Analysis works best when:
A Fishbone diagram identifies possible causes; it does not prove them. Every branch must be tested against photographs, inspection records, interviews, training records, equipment condition and site procedures. AHRQ describes the method as a visual way to organise suspected causes, while HSE guidance stresses the need to investigate underlying and organisational factors.
Fault Tree Analysis is a top-down method that begins with an unwanted event and works backwards through the combinations of failures that could produce it.
Key definitions:
Top Event: The unwanted outcome (e.g., Worker falls from mobile scaffold)
Basic Events: Individual failures or conditions
OR Gate: Any listed event could produce the outcome
AND Gate: Several events must occur together
Fault Tree Analysis works best for:
NASA describes FTA as a top-down approach used to identify process hazards and analyse complex safety and reliability pathways.
Likely conclusion: The scaffold inspection and shift-handover process failed to ensure that wheel security was verified.
This produces a focused, actionable chain but may miss parallel factors. It answers the "why" chain efficiently but does not explore whether other failures contributed simultaneously.
Potential findings:
Fishbone creates a broader view but requires evidence validation. It surfaces possibilities that Five Whys might overlook but still needs investigation to confirm which factors actually contributed.
Possible outcome: The fall required one of several initiation pathways, while the resulting injury severity depended on whether preventive and protective barriers were available and effective.
Fault Tree maps the logical combinations of failures and how multiple safeguards (fall-arrest systems, training, competent supervision) either prevented or failed to prevent harm.
The three methods do not necessarily compete. They examine the same event at different levels of depth and complexity. A simple incident might need only Five Whys, while a fatality might benefit from all three applied in sequence.
Important note: Incident severity alone should not determine the method; causal complexity, uncertainty and available evidence also matter. A serious incident with a clear single cause may need only Five Whys. A minor incident with multiple contributing factors may warrant Fishbone.
Yes. A layered approach often produces stronger analysis:
A strong investigation should lead to:
Contrast weak and strong actions:
Weak: Remind workers to be careful
Better: Introduce a documented scaffold handover and inspection process
Stronger: Improve planning, inspection, supervision and physical controls together
Link actions to the hierarchy of controls where appropriate. HSE presents investigation as a process for learning what went wrong and taking action to prevent recurrence.
Five Whys provides speed, Fishbone provides breadth and Fault Tree Analysis provides logical depth. Effective root cause analysis in construction depends less on choosing the most sophisticated tool and more on selecting and applying the right method objectively to the evidence.
Build the skills to investigate incidents beyond the obvious cause. The Incident Investigation / Root Cause Analysis course helps safety professionals examine evidence, identify contributing factors and develop corrective actions that support stronger workplace safety performance.
