5 Whys Root Cause Analysis
The 5 Whys drives past a symptom to a cause you can actually act on. Its strength is that it needs no data infrastructure; its weakness is that it follows one causal chain and rewards confident guessing. Used with evidence at each step, it is fast and effective. Used from memory in a meeting room, it produces a plausible story.
Worked example
A conveyor stopped mid-shift.
- Why did the conveyor stop? The motor overload tripped.
- Why did the overload trip? The motor drew current above its rating.
- Why did current rise? The bearing seized, increasing load torque.
- Why did the bearing seize? It ran without adequate lubrication.
- Why was lubrication inadequate? The greasing route was rescheduled and this asset was dropped from the new route list.
The actionable cause is a gap in the PM route revision process, not the bearing. Replacing the bearing fixes this occurrence; adding a change-control check on route revisions fixes the class. Notice the last answer names a process, not a person — if a 5 Whys chain terminates on someone’s name, it has stopped one step short.
Common mistakes
- Accepting answers without evidence. Each why should be supported by a record, a measurement or a physical observation.
- Stopping at human error. "Operator did not follow procedure" is a starting point — why was the procedure not followed, not the conclusion.
- Treating five as a rule. Some chains resolve in three, some need eight. Stop when the cause is within your control to change.
- Following only one branch when a failure had several contributing causes — that is when a fishbone or fault tree is the better instrument.
Frequently asked questions
- When should I use a fishbone diagram instead?
- When the problem plausibly has several contributing causes rather than one chain. A fishbone spreads across categories — people, method, machine, material, measurement, environment — and is better for exploring a poorly understood problem. 5 Whys is better for drilling into a specific, well-defined event.
Related tools
About Reliability Pro
5 Whys is part of a suite of 126 reliability, maintenance and quality engineering tools covering life data analysis, accelerated testing, system reliability, FMEA and root cause, SPC, and design for reliability. It runs in the browser and as native iOS and Android apps, so the same calculation is available at a desk or in front of the asset.