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Figure 10.2 Control Chart for a Process within Statistical Control

limit limit

Upper control limit Two standard deviations

One standard deviation

Mean

deviations Lower control "

limit

Figure 10.3 Control Chart for a Process Not in Statistical Control

■ A single point falls outside the 3o control limits.

■ Although we won't get too bogged down in statistics, we can look for patterns that suggest that the observed data are not statistically independent. A process may not be in control if we observe:

At least two of three successive values that fail on the same side of and more than two standard deviations from the centerline.

At least four out of five successive values that fall on the same side of and more than one standard deviation away from the centerline. At least eight successive values that fall on the same side of the centerline.

Control charts are a valuable too! for monitoring quality; however, it is important to keep in mind that one can see patterns where patterns may not exist (Florae, Park etal. 1997).

W. Edwards Deming (1900-1993)

While working at the Western Electric Hawthorne plant in Chicago, Illinois, during the 1920s, Deming became aware of the extensive division of labor. Management tended to treat the workers as just another cog in the machinery. Moreover, the workers were not directly responsible for the quality of the products they produced. Final inspection was used as a means to control quality and reductions in the per piece rate reflected scrap and rework.

Deming met Shewhart while working at Bell Laboratories in New Jersey in the 1930s and became interested in Shewhart's application of statistical theory. Deming realized that costly inspections could be eliminated if workers were properly trained and empowered to monitor and control the quality of the items they produced.

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