Errors Are Normal, Defects Are Not

Every person who does a task many times will sometimes get it wrong. A skilled fitter picks up the wrong bolt. An experienced clerk types 18 instead of 81. A careful picker takes a box from the shelf next to the right one. This is not laziness. It is how human attention and memory work.

Mistake-proofing, often called by its Japanese name poka-yoke, starts from this fact. It does not ask people never to make errors. It changes the process so that an error cannot happen, or so that it is found at once, before it can do harm.

Error and defect are not the same thing

An error (also called a mistake in everyday speech) is something a person does that is different from what was intended: a wrong action, a missed step, a wrong reading. A defect is the result that reaches the next step or the customer: a part assembled the wrong way round, an invoice with the wrong amount, a parcel sent to the wrong address.

The key idea of poka-yoke is that an error only becomes a defect if nothing stops it. Between the error and the defect there is a short moment. A good device works in that moment.

Why this matters

The later a defect is found, the more it costs. An error caught at the workstation costs a few seconds. The same error found at final inspection costs rework and a delay. Found by the customer, it costs a return, a complaint and some trust. Mistake-proofing aims for the cheapest point: the moment of the error itself.

The course moves from ideas to practice. First the history and the kinds of human error. Then the ways of inspecting, the two jobs a device can do, the three methods of detection and how strong each type of solution is. After that come common devices from plants, warehouses, dispensing counters and offices, and finally how to find where a device is needed, design it, test it and check every day that it still works.