Poka-yoke (pronounced "POH-kah YOH-kay") was invented by Shigeo Shingo in the 1960s. The term "poka-yoke" comes from the Japanese words "poka" (inadvertent mistake) and "yoke" (prevent). The essential idea of poka-yoke is to design your process so that mistakes are impossible or at least easily detected and corrected.Shigeo Shingo was a leading proponent of statistical process control in Japanese manufacturing in the 1950s, but became frustrated with the statistical approach as he realized that it would never reduce product defects to zero. Statistical sampling implies that some products to go untested, with the result that some rate of defects would always reach the customer.
While visiting the Yamada Electric plant in 1961, Shingo was told of a problem that the factory had with one of its products. Part of the product was a small switch with two push-buttons supported by two springs. Occasionally, the worker assembling the switch would forget to insert a spring under each push-button. Sometimes the error would not be discovered until the unit reached a customer, and the factory would have to dispatch an engineer to the customer site to disassemble the switch, insert the missing spring, and re-assemble the switch. This problem of the missing spring was both costly and embarrassing. Management at the factory would warn the employees to pay more attention to their work, but despite everyone's best intentions, the missing spring problem would eventually re-appear.
Shingo suggested a solution that became the first poka-yoke device:
In the old method, a worker began by taking two springs out of a large parts box and then assembled a switch.
In the new approach, a small dish is placed in front of the parts box and the worker's first task is to take two springs out of the box and place them on the dish. Then the worker assembles the switch. If any spring remains on the dish, then the worker knows that he or she has forgotten to insert it.
The new procedure completely eliminated the problem of the missing springs. Shingo went on to develop this mistake-proofing concept for the next three decades. One crucial distinction he made was between a mistake and a defect. Mistakes are inevitable; people are human and cannot be expected to concentrate all the time on the work in front of them or to understand completely the instructions they are given. Defects result from allowing a mistake to reach the customer, and defects are entirely avoidable. The goal of poka-yoke is to engineer the process so that mistakes can be prevented or immediately detected and corrected. Poka-yoke devices proliferated in Japanese plants over the next three decades, causing one observer to note that:
It is not one device, but the application of hundreds and thousands of these very simple "fail-safing" mechanisms that day after day has brought the quality miracle to Japan. Each one is relatively simple -- something you easily could do on your own. It is the totality, the hundreds of devices, that is almost frightening to behold.