” In 1961 I visited Company X. There, the plant manager tols me the following story.

“One of the operations we do involvs the asembly of an extremely simple push-button device that we deliver to our parent company. The device is composed of two buttons, an on button and an off button, under each of which we have to enclose a small spring. Sometimes, though, one of our workers forgets to put in a spring. When our parent company discovers a swicth without a spring, we have to send an inspector all the way to there to check every switch that was delivered”

“This is a real pain in the neck, so whenever it happens, we tell workers to be particularly careful and for a while things improve a bit…”

I inmediately went into the plant to observe the assembly of the switchtes.

The operation was an extremely simple one. A worker would insert two small springs and then install the buttons. As I watched, however, a worker neglected to put in a spring before installing the button. The head of the manufacturing department saw this, too. In a panic, he scolded the worker for forgetting the spring and then ahd the switch reassembled.

I thought about what I had seen for a moment, and then turned to Mr. Y, the manufacturing department chief.

“What,” I asked him, “does it mean for a human being to ‘forget’ something?”

Mr. Y looked puzzled and replie,”To’forget’ means… well… it just means to forget, doesn’t it?”

When I asked him to explain, he was unable to answer and finally fell silent. After a brief pause, I suggested to him that there were really two kinds of forgetting. The first involves simply forgetting something. Since people are not perfect, they will, on rare occasions, inadvertently forget things. It is not that they forget things intentionally; they just happen, inadvertently, to forget now and then.

“Haven’t you ever, in your whole life, forgotten anything?” I asked Mr. Y.

“Sure I have,” he replied. “I forget things now and then. My wife always chews me out about it.”

I observed that, being the case, he was probably in a poor position to complain to his wife that his workers were forgetting things.

The other type of forgetting, I told him, involves forgetting that one has forgotten. We are all familiar with this kind of forgetting. It is the reason, for example, that we make checklists for ourselves.

If people had the omnipotene of gods, they would be able to remember everything and they would not need checklists.

“The same thing applies to this operation. Rather than thinking that workers ought to assemble the switches perfectly every time, you should recognize that, being human, they will, on rare occasions, forget things. To guard against that,” I suggested, “Why not take the idea of a checklist and incorporate it into the operation?”

The next question was how this could be done, so I had them put the following suggestions into effect:

• A small dish was brought and, at the very beginning of the operation, two springs were taken out of a parts box containing hundreds of springs and placed on the dish.
• Switch assembly took place next; then springs were inserted and buttons installed.
• If any spring remained on the dish after assembly, the worker realized that that spring had been left out, and the assembly was then corrected.

This change in the operation completely eliminated the problem of missing springs and the parent company made no more claims on the subject.

Since springs in the earlier operation had been taken out of a parts box containing hundreds of other springs, there had been no way of knowing whether a spring had been removed or not. The new operation made it possible to know that a part had been forgotten and so eliminated the problem of missing springs.

Whenever I hear supervisors warning workers to pay more attention or to be sure not to forget anything, I cannot help thinking that the workers are being asked to carry out operations as if they possessed divine infallibility. Rather than that approach, we should recognize that people are, after all, only human and as such, they will, on rare occasions, inadvertently forget things. It is more effective to incorporate a checklist-i.e.m a poka-yoke– into the operation so that if a worker forgets something, the device will signal that fact, thereby preventing defects from occurring. This, I think, is the quickest road leading to attainment of zero defects.

In terms of management functions, this sort of poka-yoke device fulfills a control function that supplements the execution function.

This poka-yoke concept is actually based on the same idea as “foolproofing”, an approach devised mainly for preserving the safety of operations. In the early days I used the term “foolproofing” (in Japanese, bakayoke).

…. It was clear to me that “foolproofing” was a poorly chosen term. But what name would ibe suitable? After some thought, I gave the name poka-yoke (mistake-proofing) to these devices because they serve to prevent (or ‘proof;’ in Japanese, yoke) the sort of inadvertent mistakes (poka in Japanese) that anyone can make”

Shigeo Shingo (1986). Zero Quality Control: Source Inspection and the Poka-yoke System. Productivity Press, Pp 43-45