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Poka-yoke (ポカヨケ) [poka yoke] is a Japanese term that means "mistake-proofing". A poka-yoke is any mechanism in a lean manufacturing process that helps an equipment operator avoid (yokeru) mistakes (poka). Its purpose is to eliminate product defects by preventing, correcting, or drawing attention to human errors as they occur. The concept was formalised, and the term adopted, by Shigeo Shingo as part of the Toyota Production System. It was originally described as baka-yoke, but as this means "fool-proofing" (or "idiot-proofing") the name was changed to the milder poka-yoke.
More broadly, the term can refer to any behavior-shaping constraint designed into a process to prevent incorrect operation by the user. Similarly, a constraint that is part of the product (or service) design is considered Design for Manufacturability or Design for X. A modern Poka-Yoke application is when a driver must press on the brake pedal (a process step, therefore a poka-yoke)) prior to starting an automobile. The interlock serves to prevent unintended movement of the car. An additional poka-yoke would be the switch in the car's gear shift that requires the car to be in Park or Neutral before the car can be started. These serve as behavior-shaping constraints as the sequence of "car in park (or neutral)" and/or "Foot on brake" must be performed before the car is allowed to start. Over time, the driver's behavior is conformed with the requirements by repetition and habit.
The term poka-yoke was applied by Shigeo Shingo in the 1960s to industrial processes designed to prevent human errors. Shingo redesigned a process in which factory workers, while assembling a small switch, would often forget to insert the required spring under one of the switch buttons. In the redesigned process, the worker would perform the task in two steps, first preparing the two required springs and placing them in a placeholder, then inserting the springs from the placeholder into the switch. When a spring remained in the placeholder, the workers knew that they had forgotten to insert it and could correct the mistake effortlessly. 
Shingo distinguished between the concepts of inevitable human mistakes and defects in the production. Defects occur when the mistakes are allowed to reach the customer. The aim of poka-yoke is to design the process so that mistakes can be detected and corrected immediately, eliminating defects at the source.
Poka-yoke can be implemented at any step of a manufacturing process where something can go wrong or an error can be made. For example, a jig that holds pieces for processing might be modified to only allow pieces to be held in the correct orientation, or a digital counter might track the number of spot welds on each piece to ensure that the worker executes the correct number of welds.
Either the operator is alerted when a mistake is about to be made, or the poka-yoke device actually prevents the mistake from being made. In Shingo's lexicon, the former implementation would be called a warning poka-yoke, while the latter would be referred to as a control poka-yoke.
Shingo argued that errors are inevitable in any manufacturing process, but that if appropriate poka-yokes are implemented, then mistakes can be caught quickly and prevented from resulting in defects. By eliminating defects at the source, the cost of mistakes within a company is reduced.