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1-23-07 Statistical Process Control

1-23-07 Statistical Process Control - Statistical Process...

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1 Statistical Process Control Prof. Robert C. Leachman IEOR 130, Methods of Manufacturing Improvement Spring, 2006 1. Introduction Quality control is about controlling manufacturing or service operations such that the output of those operations conforms to specifications of acceptable quality. Statistical process control (SPC), also known as statistical quality control (SQC), dates back to the early 1930s and is primarily the invention of one man. The chief developer was Walter Shewhart, a scientist employed by Bell Telephone Laboratories. Control charts (discussed below) are sometimes termed Shewhart Control Charts in recognition of his contributions. W. Edwards Deming, the man credited with exporting statistical quality control methodology to the Japanese and popularizing it, was an assistant to Shewhart. Deming stressed in his teachings that understanding the statistical variation of manufacturing processes is a precursor to designing an effective quality control system. That is, one needs to quantitatively characterize process variation in order to know how to produce products that conform to specifications. Briefly, a control chart is a graphical method for detecting if the underlying distribution of variation of some measurable characteristic of the product seems to have undergone a shift. Such a shift likely reflects a subtle drift or change to the desired manufacturing process that needs to be corrected in order to maintain good quality output. As will be shown, control charts are very practical and easy to use, yet they are grounded in rigorous statistical theory. In short, they are a fine example of excellent industrial engineering. Despite their great value, their initial use in the late 1930s and early 1940s was mostly confined to Western Electric factories making telephony equipment. (Western Electric was a manufacturing subsidiary of AT&T. As noted above, the invention was made in AT&T’s research arm, Bell Laboratories.) Evidently, the notion of using formal statistics to manage manufacturing was too much to accept for many American manufacturing managers at the time. Following World War II, Japanese industry was decimated and in urgent need of rebuilding. It may be hard to imagine today, but in the 1950s, Japanese products had a low-quality reputation in America. W. Edwards Deming went to Japan in the 1950s, and his SPC teachings were quickly embraced by Japanese manufacturing management. Through the 1960s, 1970s and 1980s, many Japanese-made products were improved dramatically and eventually surpassed competing American-made products in terms of quality, cost and consumer favor. Many American industries lost substantial domestic market share or were driven completely out of business. This led to a “quality revolution” in US industries during the 1980s and 1990s featuring widespread implementation and
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