- What the 5 Whys are
- Taiichi Ohno's classic example
- How to do the 5 Whys, step by step
- Worked example: scrap at a drilling operation
- How to run a 30-minute session
- When a question has more than one answer
- 5 Whys template
- 5 Whys vs fishbone, Pareto and A3: when to use each
- Mistakes that lead to false causes
- When the 5 Whys are not enough
- Next step
- Frequently asked questions
What the 5 Whys are
The 5 Whys is the practice of asking "why?" repeatedly when a problem appears, to get past the obvious symptom and reach the root cause. It is one of the simplest tools in the lean problem-solving kit and one of the most misused.
The Lean Enterprise Institute (LEI) stresses that the number five is not the point: keep asking until you reach the cause that, once removed, keeps the problem from coming back. Sometimes three questions are enough, sometimes seven.
The gap between fixing and fixing the cause separates the plant that fights fires from the plant that improves. Replacing a blown fuse solves the day. Finding out why it blew solves the month.
The 5 Whys works for equipment failures, quality defects, safety incidents, late deliveries and even office problems. It needs only a board, a marker and the willingness to look at the process without hunting for someone to blame. That is why it makes a good first response to any deviation, before heavier tools.
Taiichi Ohno's classic example
The LEI reproduces the example Taiichi Ohno used to explain the method, about a machine that stopped:
- Why did the machine stop? There was an overload and the fuse blew.
- Why was there an overload? The bearing was not sufficiently lubricated.
- Why was it not lubricated? The lubrication pump was not pumping enough.
- Why was it not pumping enough? The pump shaft was worn and rattling.
- Why was the shaft worn? There was no strainer, and metal scraps got in.
Without asking repeatedly, a manager would replace the fuse or the pump and the failure would come back. The root cause was the missing strainer.
How to do the 5 Whys, step by step
- Describe the problem with facts: what, where, when and how much. "3.2% scrap at operation 20 this month" beats "poor quality."
- Gather people who know the process: operator, maintenance, quality and the area leader.
- Go to the place. Whys answered in a meeting room turn into opinions. See genchi genbutsu and the practice of the gemba walk.
- Ask why and confirm each answer with evidence before asking the next why.
- Stop at the root cause: the one that, once removed, prevents the problem from returning and that the team can act on.
- Define the countermeasure with an owner and a date, and test it in a PDCA cycle.
A good quality check is to read the chain backward with the word "therefore." If cause 5 exists, then cause 4 happens, and so on up to the problem. If any link does not hold, the chain has an error.
Worked example: scrap at a drilling operation
Illustrative example, with exercise data and not from a real company. At operation 20 (drilling), 640 of 20,000 parts in the month were scrapped for holes out of position: 640 ÷ 20,000 = 3.2%.
| Question | Answer | How it was confirmed |
|---|---|---|
| Problem | 3.2% scrap (640 of 20,000 parts) from holes out of position | Scrap log for the month |
| Why? (1) | The part shifts during drilling | Observing the cycle at the station |
| Why? (2) | The fixture stop is worn and the part does not seat | Measuring the stop against the drawing |
| Why? (3) | Nobody measures stop wear | Interview with the operator and the leader |
| Why? (4) | The fixture is not in the maintenance plan, which covers machines only | Checking the maintenance plan |
| Why? (5) | Fixtures are registered as tooling, with no owner and no inspection routine | Checking the tooling register |
| Countermeasure | Add the fixture to the plan, inspect weekly with a go/no-go gauge and replace the stop | Owner: maintenance. Due: 2 weeks |
Notice the pattern: each answer comes from the previous one and each has evidence. The first three answers describe what happens at the machine. The fourth and fifth change level and talk about the management system, which is where the root cause usually sits. When an analysis ends on something technical and never reaches the system, ask once more.
The action target is to bring scrap from 3.2% down to 1.0% or less, which equals 200 parts in 20,000. The expected reduction is 640 − 200 = 440 parts per month. If the analysis had stopped at the second why, the team would replace the stop and the wear would return. The root cause is the lack of an owner and a routine for fixtures. The blocking action can include a poka-yoke, such as a pin that prevents loading the part out of position.
How to run a 30-minute session
The analysis pays off more when done as a group, at the site of the problem and with simple rules. A session of about 30 minutes is usually enough for most shop floor failures. That time is a practical reference, not a rule.
- Facilitator: one person leads the questions and another records on the board, in plain view.
- Participants: the operator, maintenance, quality and the area leader. Anyone who does not know the process listens and learns.
- Golden rule: focus on the process, not the person. When people feel they will be blamed, the analysis ends at the first answer.
- Closing: the session ends only with a countermeasure, an owner, a due date and a verification date.
When a question has more than one answer
Not every why has a single answer. In the drilling example, the wear on the stop could have a second branch: why was the worn stop not noticed at shift change? The answer may be that the start-of-shift checklist does not ask for a fixture check. Two causes add up, and each produces its own countermeasure.
When more than one answer appears, draw the branches side by side and follow each to the end. Then compare: which branch explains most of the problem? Scrap data split by shift and by fixture helps decide where to start.
5 Whys template
Use a sheet with the columns below. The third is the one most often missing in everyday analyses.
| Field | What to write |
|---|---|
| Problem | What, where, when and how much, with numbers |
| Why? (1 to 5) | One answer per line, based on fact |
| How it was confirmed | Measurement, photo, record or observation on site |
| Root cause | The cause that, once removed, keeps the problem from returning |
| Countermeasure | Action, owner, due date and result indicator |
5 Whys vs fishbone, Pareto and A3: when to use each
The tools do not compete, they fit together. This is a practical way to organize them, and each team builds its own sequence.
| Tool | Best use | Limit |
|---|---|---|
| 5 Whys | One specific problem with a clear causal chain | Follows a single path; depends on who asks |
| Fishbone | Listing many possible causes by category | Shows hypotheses, proves none |
| Pareto | Choosing which problem to attack first | Does not explain the cause |
| A3 | Complex problems with a plan and follow-up | Takes more time and discipline |
A common sequence: Pareto picks the problem, the fishbone gathers hypotheses, the 5 Whys dig into the most likely ones and the A3 records it all.
Mistakes that lead to false causes
- Stopping at a person. "The operator made a mistake" is almost never a root cause. Ask why the mistake was possible.
- Generic answers, such as "lack of training" or "lack of attention," that end the analysis without explaining anything.
- Following a single path when the problem has more than one cause.
- Not confirming each answer on site, with data.
- Stopping too early, at a cause the team can "fix" but that does not keep the problem from returning.
- Using the method to find a culprit, which makes people hide information.
When the 5 Whys are not enough
The method works well for failures with a direct causal chain. For problems with several combined causes, a lot of variation or statistical data, it is not enough alone. In those cases open branches (a set of whys for each cause), or use the A3 report, statistical tools and, for foreseeable failures, FMEA. Two people can reach different causes with the same questions, which is why evidence for each answer is what protects the analysis.
Next step
Pick the problem that repeats most in your area, write it with a number, go to the site with the team and fill in the template above down to the root cause. Define the countermeasure with an owner and a date, then go back to check the result. One complete analysis per week already changes the routine of the area.
Frequently asked questions
What are the 5 Whys?
It is the practice of asking why repeatedly when a problem appears, until you reach the root cause and not just the symptom.
Do you always have to ask five times?
No. Five is a reference. What matters is to keep going until you reach a cause that, once removed, keeps the problem from returning.
How do you do the 5 Whys in practice?
Describe the problem with facts, go to the site, ask why and confirm each answer with evidence, stop at the root cause and define the countermeasure with an owner and a date.
Who created the 5 Whys?
The method is associated with Toyota and Taiichi Ohno, who used the stopped machine example to explain it.
What is the difference between 5 Whys and a fishbone diagram?
The fishbone lists many possible causes by category. The 5 Whys drill down a single causal chain. They are often used together.
When are the 5 Whys not enough?
When a problem has several combined causes or a lot of variation. Then use branches of whys, an A3 report or statistical tools.
How do you know you reached the root cause?
Read the chain backward with the word therefore. If each link holds and removing the last cause keeps the problem from returning, you have it.
Sources
- Lean Enterprise Institute. 5 Whys. Lean Lexicon. https://www.lean.org/lexicon-terms/five-whys/
- Lean Enterprise Institute. A3 Report. Lean Lexicon. https://www.lean.org/lexicon-terms/a3-report/
- OHNO, T. Toyota Production System: Beyond Large-Scale Production. Portland: Productivity Press.
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About the author
Lean Manufacturing & Behavioral Management Specialist
Over 20 years in the automotive and metalworking industries (GM and Dana), Lean Manufacturing practitioner since 2006. SENAI instructor and mentor in Brazil’s Brasil Mais Produtivo program, delivering consulting, training and audits for 50+ companies, combining quality, productivity and people development.