5 Whys
5 Whys
What are the 5 Whys?
- Keep asking “why” about a symptom until you reach the root cause
- Five is a rule of thumb, not a rule
- Stopping at blame guarantees the problem will recur
What 5 Whys really changes is not the list of causes but the level of the countermeasure. Ask once about the same incident and you get “insufficient operator checking”; go down four or five levels and you get “the process has no mechanism that forces the check.” The first answer can only be paired with training and reminders; the second points to things you can actually change, such as equipment, forms and authority. The subject of the countermeasure report shifting from a person’s name to a process name is the real payoff of this technique.
There is a way to check whether you went deep enough. Once everything is written down, read it from the bottom up, inserting “therefore” at each step. If “there is no filter → therefore metal particles got in → therefore the shaft wore down” flows without a hitch, the causal chain holds; if the wording turns awkward somewhere in the middle, you skipped a step. There is also a rule for where to stop. The last step is the point where you move beyond what your team can act on. If you go down to “because the economy is bad” or “because it is industry practice,” the cause may be right, but there is no countermeasure you can carry out. You usually reach that boundary somewhere between the third and seventh why.
You also need to be selective about where to apply it. 5 Whys works well when the cause runs in a single line; used alone on a problem where equipment, people, materials and methods are all tangled together, it follows only the first thread you pick up and misses the rest. If you see two or more branches, it is safer to lay them out with a fishbone first and then go down each branch separately, and a serious accident involving lives or legal liability should be handed to a technique that examines logical structure, such as FTA. The assumption that a single event has only one root cause is the technique’s biggest constraint.
The most common failure on the shop floor is that “why” sounds like an interrogation rather than a question. When a team leader sits an operator down and asks five times, the answer converges around the third round on “I failed to check,” and the meeting ends with an apology. If a person’s name comes up as the answer, treat that branch as a failure and reframe the question: “What should have been in place so that defects would not get out even if that person made a mistake?” Another is filling in all five boxes from memory in a meeting room. If you don’t attach evidence to each step, such as a work log, an equipment log or the physical part, you end up with a story built from five plausible guesses.
John KimAuthor of B2B Marketing Design and CEO of Nanum Consulting
Okgum LeeExpert in corporate analysis, fundraising, and business planningToyota founder Sakichi Toyoda developed the method. It became known worldwide when Taiichi Ohno, who systematized the Toyota Production System, presented it in his book Toyota Production System as a scientific approach to problem solving.
Consider a hypothetical case. Company A, a tier-2 auto parts supplier with 180 employees and annual revenue of KRW 42 (≈ US$1) billion, received four label mis-application claims from its customer in six months. Each time, the countermeasure report read “Retrain operators; strengthen visual checks before shipping,” and about two months later the same problem returned. After the fourth claim, when the customer’s quality manager warned that “next time we will downgrade your supplier rating,” the head of quality assurance gathered the production and logistics staff.
- Manager Kim from quality assurance asked the first question: “Why did the claim happen?” The answer came back that 48 of the 320 units shipped on March 12 carried labels from a discontinued old model.
- At the second why, the line foreman answered, “It was the first batch right after a model changeover,” and CCTV footage confirmed that an old-model label roll left on the workbench on changeover day had been used as is.
- At the third and fourth whys, they opened the work standard. None of the 7 steps in the changeover procedure covered retrieving leftover labels, and when the standard was written in 2019, the model was changed only once a day.
- At the fifth why, Manager Park from production control said, “We now switch 4.2 times a day on average.” The final cause was that the three-year standard revision cycle had not kept pace with the change in how production runs.
- Instead of retraining, the countermeasure became a lock-out step in which the next work order only opens once leftover labels are placed in a return bin, plus a six-month standard revision. Over the following nine months there were zero mis-application claims, and rework hours fell from 11 to 3 per month.