5 Why is a simple questioning method for finding the root cause of a problem. You state the problem, ask why it happened, then ask why again about the answer, and continue until you reach a cause that you can act on and that would stop the problem from recurring. Five is a rule of thumb; sometimes three questions are enough, sometimes seven are needed.
How to use it#
- Start from a precise problem statement, based on facts.
- Ask why and answer with a fact you have checked, not a guess.
- Repeat for each answer.
- Stop when the answer is a process or system cause that you can change and control.
- Check that the chain makes sense backwards: "because of this, that happened".
Example 1: sewing#
Problem: Skipped stitches on the coverstitch hem of style 7730, line 3.
- Why? The looper is not catching the needle thread loop on some stitches.
- Why? The needle is deflecting when it goes through the side seam crossing.
- Why? The needle is size 70 on a fabric with a thick overlock seam at the crossing.
- Why? The machine was set up for the previous lighter style and not changed.
- Why? The style changeover checklist does not include needle size for each operation.
Action: needle size corrected; needle size per operation added to the changeover checklist and the operation bulletin.
Example 2: cutting#
Problem: Short-cut panels (panels smaller than the pattern) in a lay of 80 plies.
- Why? The bottom plies are smaller than the top plies.
- Why? The straight knife blade leaned during cutting.
- Why? The blade was worn and the cutter pushed harder to get through the lay.
- Why? Blades are changed only when they break.
- Why? There is no blade change schedule or check for blade wear.
Action: blade change schedule by lay height and number of lays; cut panel check from top and bottom plies.
Example 3: finishing#
Problem: Garments packed with the wrong size hangtag.
- Why? The packer took hangtags from the wrong box.
- Why? Hangtag boxes for several sizes were on the same table without labels.
- Why? Trims were issued for the whole order at once and placed wherever there was space.
- Why? The packing section has no layout standard for trims.
Action: labelled trim holders by size at each packing table; trims issued per size batch; packing layout added to the 5S standard.
Common mistakes#
- Ending at a person: "the operator was careless" is not a root cause. Ask why the error was possible or not detected.
- Guessing instead of checking each answer.
- Following only one chain when a problem has several contributing causes. Use a fishbone diagram first when causes are unclear. See root cause analysis.
- Stopping too early at a symptom ("the machine was not set right").
- Going too far to causes you cannot control ("the buyer changed the style").
Recording 5 Why#
Record the problem, each why and its evidence, the root cause, and the actions with owners and dates. The five why worksheet has this layout, and the actions can be tracked on a CAPA form.
Related guides#
- Kaizen: turning the root cause into an improvement.
- DHU: the defect data that tells you which problem to analyse.
- Sewing defects: common defects to practise the method on.