The 5 Whys method: how to find the real cause of a problem
A method born in the Japanese textile industry in the 1930s, now used by Toyota, NASA, Google and Amazon — and just as useful for understanding why a number in your business has changed.
In 1930 Sakichi Toyoda, a Japanese inventor and the founder of what would become Toyota, was still working in the textile sector, among looms and weaving machines. He had a simple habit: when something broke or went wrong, he did not stop at the first explanation. He asked “why” and then, at the answer, asked “why” again. Usually, after five questions, he reached the real cause.
Twenty years later, Taiichi Ohno — the engineer who built the Toyota production system as we know it today — took this method and made it official, calling it “the basis of Toyota’s scientific approach”. From there the 5 Whys method entered the training of every new Toyota employee, and it is still in use today. In the 1970s it left Toyota and became a tool used everywhere — from NASA in failure investigations to Google and Amazon in post-incident analyses.
How it works, in practice
You start from a problem. You ask “why did it happen?”. At the answer, you ask “why?” again. You continue like this, usually five times, until the answer is no longer a symptom but a cause you can really correct.
Two rules make the method useful and not just a theoretical exercise. First: every answer must rest on a verifiable fact, not a feeling. Second: the aim is to find a process to fix, not a person to blame — if an answer is “because someone made a mistake”, the right next question is “why did the system allow that mistake to go unnoticed?”.
Two examples that apply to almost any business
First scenario — revenue is falling
Revenue dropped 15% this month.
Why? Fewer customers came back to buy a second time.
Why? The experience on the first visit did not meet expectations.
Why? Waiting times to be served got longer.
Why? A key member of the team left and was not replaced in time.
Why? Nobody was monitoring the workload, so the problem was not seen coming.
The real cause is not “fewer customers”. It is a figure nobody was looking at.
Second scenario — costs are rising
Costs rose 10% without a clear explanation.
Why? A supplier raised its prices.
Why? Nobody noticed right away.
Why? Invoices are not compared month on month.
Why? There is no regular check on costs.
Why? That task was never explicitly assigned to anyone.
Here too, the real cause is not “suppliers cost more”. It is the absence of a control.
In both cases, stopping at the first “why” almost always leads to the wrong solution — more marketing in the first case, a different supplier in the second. Neither corrects the real cause.
It is the same principle behind the diagnosis phase we carry out in every Arithmos report: a number that changes is never the answer. It is the first question.
A question for anyone running a small business: the last time something did not work, how many “whys” did you ask before acting?
Sources: Toyota Industries Corporation, ProClipse LLC, Purple Griffon, Lean Enterprise Institute.