In terms of quality, Shingo's paramount contribution was his development in the 1960s of Poka-Yoke and source inspection systems. These developed gradually as he realised that statistical quality control methods would not automatically reduce defects to zero.
The basic idea is to stop the process whenever a defect occurs, define the cause and prevent the recurring source of the defect. This is the principle of the JIT production system. No statistical sampling is therefore necessary. A key part of this procedure is that source inspection is employed as an active part of production to identify errors before they become defects. Error detection either stops production until the error is corrected, or it carries adjustment to prevent the error from becoming a defect. This occurs at every stage of the process by monitoring potential error sources. Thus defects are detected and corrected at source, rather than at a later stage.
Following a visit to Yamada Electric in 1961, Shingo started to introduce simple, mechanical devices into assembly operations, which prevented parts from being assembled incorrectly and immediately signalled when a worker had forgotten one of the parts. These mistake-proofing or 'Poka-Yoke' devices had the effect of reducing defects to zero.
In 1967 Shingo further refined his work by introducing source inspections and improved Poka-Yoke systems which actually prevented the worker from making errors so that defects could not occur. Associated advantages were that statistical sampling was no longer necessary, and that workers were more free to concentrate on more valuable activities such as identifying potential error sources.
Having learned about and made considerable use of statistical QC in his 40s, it was some 20 years later, in 1977, that Shingo observed that the Shizuoko plant of Matsushita's Washing Machine Division had succeeded continuously for one month with zero defects on a drain pipe assembly line with involvement of 23 workers. He realised that statistical QC is not needed for zero-defect operations. This was achieved principally through the installation of Poka-Yoke devices to correct defects and source inspection to prevent defects occurring. Together these techniques constitute Zero Quality Control, which, Shingo argues, can achieve what may have been impossible using statistical quality control methods.
Shingo advocated the practical application of zero defects by good engineering and process investigation, rather than slogans and exhortations that have been associated with the quality campaigns of many American and Western companies. Shingo, like Deming and Juran, argued that such American approaches of displaying defects statistics were misguiding and demoralising. Instead, the results of improvement should be announced and displayed.
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