5 Whys: the method, a drawn diagram, a worked example and when it fails
Short answer
The 5 Whys is a way to find the cause of a problem by asking why it happened, then asking why again of each answer, until you reach a cause you can remove. It needs no software and no statistics, so a team can run one at the machine with a sheet of paper.
Start from one problem you have seen and measured. Ask why it happened and answer with a fact you checked at the machine. Ask why of that answer, and keep going until the answer is something in the process that you can change, such as a missing check or an out-of-date work instruction. Five is a habit, not a rule: some chains end in three steps and some need seven. The method suits a single failure with one line of cause. When several causes act together, a fishbone diagram or a fault tree does the job better.
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A short story from the plant floor
· 0:50
What's a 5 Whys? Explained like I'm five
A 60-second explainer of the 5 Whys. A bike keeps getting a flat tire, so you ask why five times, from the hole to the loose screw on a wobbly shelf, and fix that root cause.
0:04Repeated questioning, about five times, from symptom to root cause.
0:07No. Simpler.
0:09Okay. Picture your bike with a flat tire.
0:11Why is it flat? That's one.
0:13A hole.
0:14Why a hole? That's two.
0:16A nail.
0:17Pull it out! Why a nail? Three.
0:21Pulling helps once. A falling toolbox spilled nails there.
0:25Toolboxes don't fall alone! Why? Four.
0:29The shelf was wobbly. It dropped the box.
0:32Why wobbly? Five!
0:34A loose screw. That's the root cause.
0:37So I just tighten it?
0:38Yes. And you've asked why since age two.
0:42Ask why about five times, find the real cause, fix it.
0:46Fix the shelf and this flat tire won't keep coming back.
What is the 5 Whys method?
The 5 Whys is a root cause method: say what went wrong, ask why it happened, then ask why of each answer until you reach a cause in the process that you can change. The first answer is nearly always a symptom. Replace a blown fuse and stop, and the failure comes back.
Toyota attributes the habit to Taiichi Ohno, whom its Toyota Traditions page calls the pioneer of the Toyota Production System in the 1950s. The page gives his advice in two parts: "Observe the production floor without preconceptions", and then "Ask 'why' five times about every matter." People skip the first part, and a 5 Whys done from memory in a meeting room produces five opinions in a row.
The method suits one kind of problem best: something that used to work and stopped, or a result that has slipped below a standard. The Lean Lexicon calls these gap from standard problems. A loose description like "quality is poor on nights" needs narrowing first with 5W1H or an is/is not sheet, then ask why.
The welding robot: the 5 Whys example Toyota tells
This is the chain as Toyota published it in Toyota Traditions in March 2006, where Ohno "used the example of a welding robot stopping in the middle of its operation". The wording below is Toyota's.
The welding robot chain, quoted from Toyota Traditions (Toyota Motor Corporation, March 2006)
ProblemA welding robot stops in the middle of its operation.
Why 1Why did the robot stop?
The circuit has overloaded, causing a fuse to blow.
Why 2Why is the circuit overloaded?
There was insufficient lubrication on the bearings, so they locked up.
Why 3Why was there insufficient lubrication on the bearings?
The oil pump on the robot is not circulating sufficient oil.
Why 4Why is the pump not circulating sufficient oil?
The pump intake is clogged with metal shavings.
Why 5Why is the intake clogged with metal shavings?
Root cause: the answer you can fix
Because there is no filter on the pump.
Each answer becomes the subject of the next why. The first answer, a blown fuse, is where most repairs would stop. The fifth is something a person can change: a filter on the pump.
Each answer points to a different fix. Stop at the first and you replace a fuse. Stop at the third and you replace a pump. Only the fifth keeps the robot running: fit a filter. The Lean Lexicon makes the same point about an older version of the story: a manager who stops asking replaces the fuse or the pump, and the failure comes back.
That older version is the one in Ohno's book. As the Lean Lexicon cites it (Ohno 1988, page 17), the machine is not named as a robot, the fourth answer is a worn pump shaft, and the last is that no strainer was attached so metal scraps got in. The Lexicon adds: "The specific number five is not the point."
How to run a 5 Whys in six steps
Running a 5 Whys in six steps
1
Write the problem as one measured sentence
2
Go and look
3
Ask why and answer with a fact
4
Ask why of the answer
5
Read the chain backwards
6
Fix it, then confirm
Steps 3 and 4 are where most chains go wrong: one cause per answer, and every answer checked at the machine before the next why.
1
Write the problem as one measured sentence
Say what failed, where, when and how many. "14 of 600 cartons had a crooked label on Line 2" can be investigated. "Label problems" cannot.
2
Go and look
Stand at the machine with the people who run it and keep the failed part. Toyota calls this genchi genbutsu, going to the source.
3
Ask why and answer with a fact
Each answer is something you saw, measured or tested. An answer you cannot check is still a guess.
4
Ask why of the answer
Repeat until the answer is something in the process: a missing step, a standard nobody updated, a check that does not exist. If a why has two true answers, draw two branches.
5
Read the chain backwards
Start from the last answer and join each line to the one above with "therefore". If a link sounds wrong, an answer is missing or false.
6
Fix it, then confirm
Give the countermeasure an owner and a date, then watch the original measure. The cause is confirmed when the problem stops.
Bring the operator or technician who saw the failure and someone who knows how the process should run. A facilitator from outside the area helps by asking the question everyone thinks is too obvious.
Is this answer ready for the next why, or is the chain done?
1
Did you see, measure or test this answer?
No
If no
Go and check it at the machine before the next why. Cross it out if the evidence says no.
Yes
2
Does the answer stop at a person, such as forgot or was careless?
Yes
If yes
Ask why the process let the slip happen and why nothing caught it.
No
3
Is it something the plant can change?
No
If no
Go back one level, to the last thing you control, such as the incoming check or the specification.
Yes
4
Does the chain read correctly backwards, with therefore between the links?
No
If no
An answer is missing or false. Fix that link before you write any action.
Yes
Every answer right
Stop asking. Give the countermeasure an owner and a date, then watch the original measure.
Four checks on each answer. If it was not seen, measured or tested, go and check it. If it stops at a person, ask why the process allowed the slip. If the plant cannot change it, go back one level. If the chain does not read correctly backwards with therefore, an answer is missing or false. When all four pass, stop and write the countermeasure.
The stop rule is these four checks, not the number five.
How to draw a 5 Whys diagram
A 5 Whys diagram is the chain drawn so that anyone can follow it: the problem at the top, one box per answer, an arrow labelled why between them, and the last box marked as the root cause. Three layouts are common.
Three ways to draw a 5 Whys diagram
Single chain
What it looks like
One column of boxes from the problem down to the root cause.
Use it when
Every why has one answer that the evidence supports. The welding robot is a single chain.
Why tree
What it looks like
The chain splits where a why has more than one possible answer. Each branch is followed or crossed out.
Use it when
There are several candidate causes, or two had to be present together. Also called a why-why diagram.
Three chains
What it looks like
Three chains side by side: why the defect was made, why it was not caught, and why the system allowed both.
Use it when
A defect reached a customer and the report must cover detection as well as occurrence. Often called a three-legged 5 Why.
Three small drawings, one above each card. A single chain is one column of boxes from the problem down to a root cause. A why tree splits into branches where a why has more than one answer, with one branch crossed out. Three chains are three columns side by side.
Dark box: the problem. Yellow box: a root cause. Dashed and crossed: a branch that was checked and ruled out, which stays on the page.
Whichever layout you pick, write next to each box how the answer was checked. The 5 Whys canvas builds the chain in the browser and prints it on one page, and the 5 Whys template is the form to fill in by hand.
A worked 5 Whys example from a packing line
Illustrative example. On Tuesday's first shift, Line 2 packed 600 cartons and the end-of-line check rejected 14 for a crooked label. That is 14 / 600 = 2.3 percent rejected, so 586 cartons, or 97.7 percent, passed. Rejects on this line are normally close to zero, so this is a gap from standard.
The labeller chain, with the check behind each answer
Example numbers
Problem14 of 600 cartons on Line 2 rejected for a crooked label (2.3 percent).
Why 1Why were 14 labels crooked?
The label head had moved sideways during the run.
Checked: The gap between the head and the guide rail was 3 mm wider than the setup mark.
Other branch, ruled out
The label roll was wound unevenly.
Checked: The roll was measured and found square, so this branch was crossed out.
Why 2Why did the head move?
The clamp bolt on the head bracket was loose.
Checked: The technician turned it by hand at the machine.
Why 3Why was the bolt loose?
It was tightened by hand at the changeover, without a torque wrench.
Checked: The setter who did the changeover said so, and no wrench was at the line.
Why 4Why was it tightened by hand?
The changeover sheet says "tighten clamp" and gives no torque value or tool.
Checked: The sheet at the line was read.
Why 5Why does the sheet give no value?
Root cause: the answer you can fix
The sheet was written for the previous label head, and nothing in the equipment change procedure asks anyone to review work instructions.
Checked: The sheet's revision date is older than the new head, and the change checklist has no such line.
The chain ends at a procedure, not a person: the equipment change checklist never asks anyone to review the work instructions.
Read it backwards as a test. The change procedure does not ask for a review of work instructions, therefore the sheet has no torque value, therefore the bolt was tightened by hand, therefore it worked loose, therefore the head moved, therefore 14 labels were crooked.
Two countermeasures follow. Add the torque value and the tool to the changeover sheet, which fixes this labeller. Add a line to the equipment change checklist that asks for affected work instructions to be reviewed, which fixes the next machine too. More worked chains are in the 5 Whys examples.
When 5 Whys fails, and what to use instead
The method has limits and published critics: see Alan J. Card, "The problem with '5 whys'" (BMJ Quality & Safety, 2017). On the shop floor the failures fall into five patterns.
Five ways a 5 Whys goes wrong and what to do about each
Where it fails
The problem has several causes
What happens: A single chain follows one path and drops the rest, so the fix removes one cause and the problem returns.
Do this instead
Lay out all the candidates on a fishbone diagram first, or draw a why tree and check every branch.
Where it fails
The chain stops at a person
What happens: The last answer is "operator error" or "did not follow the procedure", and the action is retraining.
Do this instead
Ask why the process let the error happen and why nothing caught it. A human error is a link in the chain, not the end of it.
Where it fails
The answers are guesses
What happens: The chain is built in a meeting from what people believe, and nobody has tested it.
Do this instead
Go to the machine. Write the check beside every answer and cross out what the evidence does not support.
Where it fails
Two teams reach two causes
What happens: The chain depends on what the people in the room know, so another group ends somewhere else.
Do this instead
Bring the people who do the work. Confirm the cause by removing it and watching the problem stop.
Where it fails
The cause is outside your control
What happens: The last why lands on a supplier or the budget, and nobody in the room can act.
Do this instead
Go back one level to the last thing the plant controls, such as the incoming check or the specification.
None of the fixes drops the method. Each one adds what a single unchecked chain lacks: a branch, a check, or a stopping rule.
For the first pattern, switch tools. A fishbone diagram spreads causes out by category. A fault tree shows which causes must combine. For a customer complaint, the chain becomes one section of an 8D report or an A3. The root cause analysis guide covers all of them.
5 Whys vs fishbone diagram, 5W1H and fault tree
5 Whys next to the three tools it gets confused with
5 Whys
Question it answers
Why did this happen?
Shape
A chain, top to bottom
Best for
One failure with one line of cause
Weak spot
Follows one path and can miss the others
Fishbone diagram
Question it answers
What could have caused this?
Shape
A spine with a branch for each category of cause
Best for
Many possible causes and a team that needs to see them all
Weak spot
Lists causes without proving any of them
5W1H
Question it answers
What exactly is the problem?
Shape
Six answers and one sentence
Best for
Pinning the problem down before any cause is discussed
Weak spot
Describes the problem and does not explain it
Fault tree
Question it answers
Which combinations of causes produce this failure?
Shape
A tree with AND and OR gates
Best for
Safety and reliability failures where causes combine
Weak spot
Takes time and someone who knows the method
The four tools do different jobs. 5W1H describes, a fishbone collects candidates, 5 Whys follows one of them down, and a fault tree maps every combination.
Used in sequence, 5W1H states the problem, the fishbone collects candidate causes, and a 5 Whys chain goes deep on the one or two the evidence supports. The problem solving methods guide covers how to choose between them.
Templates and software for 5 Whys
Paper is enough to start. It fails when the same cause turns up on a second line and nobody can find the first analysis.
In LeanSuite, Lean Studio has a 5 Whys board next to the fishbone, A3 and 8D boards, so a team can work on one chain. AI Root Cause Analysis takes a description of the problem, asks its own questions and returns likely root causes with an action plan for the team to check at the machine.
AI Root Cause Analysis
Run a 5 Whys in LeanSuite
AI root cause analysis in every Kaizen and EWO: the RCA Agent drafts a 5 Whys, 4M1D and fishbone from the problem, your attached manuals and SOPs, and your plant’s past problems.
Bring the problem you are working on to a 45-minute demo. Or look around with sample data first, or pilot LeanSuite on your own lines for 90 days, with the setup done by our team.
It is used to find the root cause of a specific problem, usually an equipment failure, a defect or a missed standard. You ask why the problem happened, then ask why of each answer, until you reach a cause in the process that you can change. It is quick, needs no data analysis and fits problems with one line of cause.
Toyota's own example is a welding robot that stopped. The fuse had blown because the circuit overloaded, because the bearings locked up, because the oil pump was not circulating oil, because its intake was clogged with metal shavings, because there was no filter on the pump. The fix is a filter, not a fuse. The page also works a packing line labelling chain, which is illustrative.
No. The Lean Enterprise Institute's Lean Lexicon says the specific number five is not the point. You stop when the answer is something in the process you can fix and when the chain reads correctly backwards with the word therefore. That can take three questions or seven.
Draw two branches and follow both. Check each answer at the machine, and cross out the one the evidence does not support, writing the check beside it. When two causes had to be present together, keep both branches on the diagram and fix both. This is the why tree layout.
Read the chain backwards from the last answer to the problem, joining each link with the word therefore. If a link sounds wrong, an answer is missing or false. Then confirm it in the work: put the countermeasure in and watch the original measure. The cause is confirmed when the problem stops.
It is the chain drawn out: the problem in a box at the top, one box for each answer, and the last box marked as the root cause. When a why has more than one possible answer the diagram branches into a why tree, and each branch is either followed or crossed out with the evidence written beside it.
A fishbone diagram goes wide and a 5 Whys goes deep. The fishbone lists every possible cause under categories such as machine, method and material. A 5 Whys takes one of those causes and follows it down to its root. Teams often use the fishbone first and then run a 5 Whys on the causes that the evidence supports.
Toyota's own Toyota Traditions page attributes the instruction to ask why five times about every matter to Taiichi Ohno, and tells his example of a welding robot that stopped. The same example, with slightly different details, is in his 1988 book on the Toyota Production System.
Do not rely on it alone when several causes act together, when the failure is a safety or reliability event where causes combine, or when the team cannot check its answers at the machine. Use a fishbone diagram or a fault tree for those, and keep the 5 Whys for the single branches inside them.
There is no fixed list of seven. The ones plants use most are the 5 Whys, the fishbone diagram, the fault tree, the Pareto chart, 8D, A3 problem solving and FMEA. They do different jobs: some collect candidate causes, some follow one cause down, and some manage the whole problem from containment to prevention.
It depends on the problem. Use a 5 Whys for a single failure with one line of cause, a fishbone diagram when many causes are possible, a fault tree when causes combine, and an 8D or A3 report when the work must be recorded and reviewed. Pick the lightest tool that fits, and check every answer at the machine whichever you choose.
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