Root Cause Analysis: A Practical How-To Guide to the Four Main Tools
Root cause analysis (RCA) is the structured process of identifying the underlying cause of a problem -- the cause that, if eliminated, would prevent the problem from recurring. RCA addresses causes, not symptoms. Treating symptoms provides temporary relief; eliminating root causes provides permanent improvement. Here is a practical guide to the four most widely used RCA tools: the 5 Whys, the Fishbone Diagram, Fault Tree Analysis, and Failure Mode and Effects Analysis.
Tool 1: The 5 Whys
The 5 Whys is the simplest and most widely used RCA tool. It works by repeatedly asking 'Why?' in response to each answer until the root cause is reached. The name comes from the observation that five rounds of 'Why?' is often enough to reach the root cause, though it may take more or fewer depending on the problem.
When to use it. The 5 Whys works best for problems that have a single root cause. It is fast and requires no special tools. It is appropriate for quality problems, process failures, and incident investigation.
How to apply it. State the problem clearly. Ask 'Why does this problem occur?' and record the answer. Ask 'Why?' of that answer. Repeat until you reach an answer that is a root cause -- typically a systemic or process factor, not a human error. Then ask 'Why did the process allow this human error to occur?'
Common mistake. Stopping at symptoms or human error. 'The operator made a mistake' is not a root cause. 'The process has no error-proofing for this step and relies entirely on operator attention' is closer to a root cause.
Tool 2: The Fishbone Diagram (Ishikawa Diagram)
The Fishbone Diagram is a visual tool for structuring root cause analysis. It is shaped like a fish skeleton, with the problem stated at the head, major cause categories as the main bones, and specific contributing causes as smaller bones. The standard cause categories are the 6 Ms: Man, Machine, Method, Material, Measurement, and Mother Nature (Environment).
When to use it. The Fishbone Diagram works well for complex problems with multiple potential causes. It is useful for facilitating team brainstorming in a structured way and for ensuring that all major cause categories are considered.
How to apply it. Draw the fish spine, label the head with the problem statement. Add the main bones (cause categories). Conduct a team brainstorm to identify potential causes within each category. Evaluate the potential causes to identify the most likely root cause(s) for further investigation.
Tool 3: Fault Tree Analysis
Fault Tree Analysis (FTA) is a top-down deductive analysis that begins with an undesired event (the top event) and works backward to identify the combinations of failures that could cause it. FTA uses a tree diagram with logic gates (AND gates and OR gates) to show how multiple failures can combine to produce the top event.
FTA is more complex than the 5 Whys or Fishbone but is appropriate for complex systems where multiple simultaneous failures can cause a top event. It is widely used in safety engineering, aerospace, and nuclear power.
Tool 4: Failure Mode and Effects Analysis (FMEA)
FMEA is a proactive tool used to identify potential failure modes before they occur, assess their likelihood and impact, and prioritise improvements. For each component or process step, the team identifies: What could go wrong (failure mode)? What is the effect of this failure? What is the likelihood that this failure will occur? What is the likelihood that it will be detected before causing harm?
XNM applies Lean Six Sigma root cause analysis tools to process improvement in public-sector and capital-project environments. Reach out to XNM's strategic advisory team to discuss problem-solving and process improvement for your organisation.