5 Whys drills from symptom to root cause by asking “why?” repeatedly, until the answer points to something in the system that can be changed — so the problem doesn’t return.
The problem is concrete and observed — not a generalisation (“line 2 stopped at 9:40”, not “the machine often stops”)
Those who know the process take part — the chain of whys is built on knowledge, not guesses
What happened, where, when, how big? A precise problem statement is half the analysis — “line 2 stopped 40 minutes Tuesday due to a motor fault” can be analysed; “we have too many stops” cannot.
Every answer must be verifiable, not a supposition. When uncertain: go and see, ask, measure — and continue the analysis afterwards. A chain of guesses gives a root cause of guesses.
When the chain hits a person (“the operator skipped the step”), you are about halfway. Ask on: why could the step be skipped? Why didn’t the process detect it? The answer ends in a missing standard, training or mistake-proofing — something that can be changed.
Read the chain from the bottom up: “Because there is no lubrication standard, lubrication was skipped, THEREFORE the bearing ran hot, THEREFORE the fuse blew…” If the logic stutters, a link is missing — or you have jumped into the wrong branch.
The action must change the system (standard, layout, mistake-proofing) — “employee instructed” is symptom treatment with documentation. Follow the problem for a period: if it doesn’t recur, the analysis held.
How it goes wrong in practice — and this is where most implementations part ways.
Some chains reach the target after three whys, others need seven. The stopping criterion is not the number, but that the answer is something you can change yourselves, which prevents recurrence.
Many problems have two or three contributing causes. If the chain splits (“why did X happen? — both A and B”), follow both branches.
Why-answers invented in the meeting room give a plausible story — not a root cause. Every uncertain link is verified at the gemba.
The method only works when it becomes daily routines and leadership behaviour. The operations check shows in 4 minutes where your operations stand — and what should come first.