Zero Quality Control: Source Inspection and the Poka-Yoke System (Hardcover)

This book concentrates on practical ways to reduce defects and non-conformances in manufacturing. Specifically it shows Shingo's thinking on how to achieve zero defects.

Shingo never lost touch with the shop floor. He found that foremen did not find statistical quality control user-friendly. Sometimes, merely hearing the words "quality control" gave them headaches, he recalled. He realised that control charts were only mirrors which reflected prevailing conditions and were therefore useless unless used for corrective action.

Shingo realised that there was a need for 100% inspection, as human-dominated tasks inevitably produce defects, because of factors like forgetfulness, distraction, boredom or pressure of work. Defects will always crop up, people told him at regular intervals, in any task performed by humans. His solution was to devise mechanisms which would both carry out 100% inspections and respond immediately. He called this system poka-yoke – mistake-proofing – and a substantial proportion of this book is devoted to showing examples.

The advantages of poka-yokes are that they tend to be simple, ingenious and inexpensive. To take one example, a problem was found in a process for casting engine valves. It was found that a piece would occasionally be mis-hit. To remedy this, a worker would orient the work piece visually before placing it in the hopper. As only one end of the component was magnetic, it was possible to install a magnetic sensor on the conveyer belt so that if a piece was wrongly positioned, the conveyor belt would be stopped. This meant that the problem disappeared. The cost of installing the sensor was about $50, the resulting saving massively more.

Shingo forcibly makes the point that defects should not be seen as the starting point of investigation – as they are when control charts are used – because defects are results, not causes. Defects are produced by errors and the whole aim of zero quality control is to detect and treat the causal error before it has the chance to turn into a defect. Shingo doesn't specify that mistake-proofing should be built into the design process: poka-yokes are add-ons to solve problems as they are discovered. Again, this derives from his intensely practical experience of the shop floor.

The usual objection made to poka-yokes is that they only apply to manufacturing. It's certainly true that switches, sensors, buzzers, meters and gauges have a variety of uses in error detection. But poka-yokes can be very simple. Another example comes from the manufacture of a switch, composed of an on and an off button with a small spring under each. The problem was that occasionally a worker would forget one of the springs, an inspector would then have to come from another town to check the entire output and the parent company would then complain about defects made in such a simple process. The remedy was to incorporate a checklist into the process, so that the operator put two springs at a time into a small dish. If there was a spring left in the dish afterwards, then it was obvious that it had been omitted and the defective assembly could be easily identified. Shingo acknowledges that workers do not have divine infallibility (something which supervisors tend to take for granted) and may forget things.

Shingo seems to assume that he was writing this book for the general public. It's not exactly holiday reading, and the cover doesn't appear designed to attract the casual reader. On the other hand, it is an excellent book for quality engineers or even engineers in general. Shingo's message is simple and clearly expressed so that the book will appeal to anyone involved with business improvement. Another attraction is that it is a helpful book for people put off quality control by fear of statistics. Shingo proposes that the combination of source inspection and poka-yoke may lead to the happy situation where "inductive statistics has been rendered unnecessary in the area of control".

Still, it must be noted that although poka-yokes are powerful and effective ways of reducing defects, they are techniques rather than objectives. The success rate of a process can be vastly improved if self-checks, successive checks and poka-yokes all work together and that will need – or engender – an atmosphere of continuous improvement which will ultimately lead to reliable products, satisfied customers and empowered workers.

As for the people who still – even after reading all this – think that poka-yokes can only work in a manufacturing context, all they need to do is to consider the modern electric kettle, which switches off when the water boils. Remember the dark ages when the kitchen was filled with steam or the domestic disaster when a kettle was left to boil dry? Poke-yoke can be applied in any situation where a process converts an input into an output. All that is needed is some "out of the box" thinking and a desire to improve the process.