The 5 Why technique is simple: when a problem occurs, ask "why?" repeatedly until you reach a cause that, if fixed, would prevent the problem from happening again. It's one of the most widely used problem-solving tools in manufacturing because it needs no special training or software. But it is also widely misused — stopping too early, blaming people or guessing. Used properly, it is remarkably effective.
A simple example
Problem: A machine stopped during the shift.
- Why did the machine stop? The motor overloaded and tripped.
- Why did the motor overload? The bearing was seized, increasing load.
- Why was the bearing seized? It wasn't lubricated sufficiently.
- Why wasn't it lubricated? The lubrication point is hard to reach and was skipped in daily checks.
- Why was it skipped? The checklist doesn't include it, and there's no easy access.
Root causes and actions: add the point to the daily checklist and fit an extended grease line so it can be reached easily. Replacing the bearing alone would have fixed the symptom but not prevented recurrence.
Rules for good 5 Why analysis
- Go to the place — analyse at the machine or process, with the people involved.
- Start with a precise problem statement — what, where, when, how much.
- Base each answer on facts — observations and data, not assumptions.
- Stay within your control — causes like "the weather" or "the customer" are not actionable; ask why your process was vulnerable.
- Don't stop at people — "operator made a mistake" is never a root cause; ask why the process allowed the mistake.
- Check backwards — read the chain from bottom to top with "therefore"; it should make logical sense.
Common mistakes
Stopping too early
"The bearing failed" is not a root cause — it's a symptom. Stopping here leads to replacing parts repeatedly without preventing failures.
Blaming people
"The operator didn't follow the procedure" invites the question: why? Was the procedure unclear, unavailable, impractical or untrained? Systems create most errors; fix the system.
Guessing
Answers based on opinion lead to wrong causes. Check the machine, look at the data, test the hypothesis.
Following only one path
Many problems have more than one contributing cause. Branch the analysis where needed.
Two root causes: occurrence and detection
For quality problems, ask two chains of why: why did the defect occur, and why wasn't it detected? Fixing only occurrence leaves you vulnerable to the next cause; fixing only detection means you keep producing defects and catching them. Both chains matter. This is also the approach used in 8D problem solving.
Combining 5 Why with a fishbone diagram
When the cause isn't obvious, start with a fishbone (Ishikawa) diagram to brainstorm possible causes under categories such as machine, method, material, man, measurement and environment. Then use data to narrow down, and 5 Why to drill into the most likely causes.
Recording and following up
A 5 Why that lives on a whiteboard is quickly forgotten. Record the problem, the why chain, the root cause and the actions with owners and due dates. Check later whether the problem has recurred. Recording analyses also builds a library of solved problems that new staff can learn from.
Where 5 Why fits in daily management
Use 5 Why for the top losses from daily or weekly meetings: the biggest downtime reason, the most frequent defect, a safety near miss. Short, regular analyses on real problems build a culture of problem solving far more effectively than occasional large projects.
How software helps
NCR and CAPA software can include a 5 Why template linked to each problem, assign actions with due dates, send reminders and schedule effectiveness checks. Downtime and defect data from the shop floor points to which problems deserve analysis first. See CAPA software.
Talk to us about building problem solving into your daily routine.
A quality example
Problem: 40 parts rejected for an oversize bore on one shift.
- Why oversize? The boring tool was cutting larger than set.
- Why? The tool insert was worn beyond its life.
- Why wasn't it changed? The operator didn't know it had reached its life.
- Why not? Tool life is tracked on a paper sheet that wasn't updated after the previous shift.
- Why? There is no handover of tool counts between shifts and no automatic counter.
Detection chain: why wasn't it caught earlier? In-process checks were every 50 parts, and the drift happened within that interval. Actions: tool life counter in the machine or shop-floor app, tool count added to shift handover, and inspection frequency adjusted for this feature. The result is a process that prevents the defect and catches drift sooner if it occurs.
Teaching 5 Why to supervisors
The best way to build skill is practice on real problems with coaching. Pick one problem per week in the production meeting, walk through the whys together at the machine, and review the logic. Within a few months, supervisors begin doing it naturally — and the quality of root causes improves noticeably.
Frequently asked questions
Do we always need exactly five whys?
No. Five is a guideline. Stop when you reach a cause you can act on that, if fixed, prevents recurrence — sometimes three, sometimes seven.
Can 5 Why be used for machine breakdowns?
Yes. It works well for breakdowns, defects, delays and safety incidents.
What if there are multiple causes?
Branch the analysis: follow each contributing cause separately, or use a fishbone diagram first to identify candidate causes.
Should 5 Why be done alone?
Preferably with people who know the process — operators, technicians, supervisors — at the place where the problem occurred.
How do we know the root cause is right?
Check the logic backwards ('therefore'), verify with data or tests, and confirm that the corrective action stops the problem.
