704 CHAPTER 14 Statistical Process Control Out of Control Criteria ■ Increasing Variation: As the run chart proceeds from left to right, the vertical variation of the points is increasing, so the corresponding data values are experiencing an increase in variation. This is a common problem in quality control. The net effect is that products vary more and more until almost all of them are considered defective. ■ Upward Trend: The points are rising from left to right, (as in Figure 14-1), so the corresponding values are increasing over time. ■ Downward Trend: The points are falling from left to right, so the corresponding values are decreasing over time. ■ Upward Shift: The points near the beginning are noticeably lower than those near the end, so the corresponding values have shifted upward. ■ Downward Shift: The points near the beginning are noticeably higher than those near the end, so the corresponding values have shifted downward. ■ Exceptional Value: There is a single point that is exceptionally high or low. ■ Cyclical Pattern: There is a repeating cycle. Causes of Variation Many different methods of quality control attempt to reduce variation in the product or service. Variation in a process can result from two types of causes as defined below. DEFINITIONS Random variation is due to chance; it is the type of variation inherent in any process that is not capable of producing every good or service exactly the same way every time. Assignable variation results from causes that can be identified (such as defective machinery or untrained employees). Later in the chapter we will consider ways to distinguish between assignable variation and random variation. The run chart is one tool for monitoring the stability of a process. We will now consider control charts, which are also useful for monitoring the stability of a process. Control Charts Because control charts were first introduced by Walter Shewhart in 1924, they are sometimes called Shewhart charts. We begin with a basic definition. DEFINITION A control chart (or Shewhart chart or process-behavior chart) of a process characteristic (such as mean or variation) consists of values plotted sequentially over time, and it includes a centerline as well as a lower control limit (LCL) and an upper control limit (UCL). The centerline represents a central value of the characteristic measurements, whereas the control limits are boundaries used to separate and identify any points considered to be significantly high or significantly low. O Improving Quality in Cars by Reducing Variation Ford and Mazda were producing similar transmissions that were supposed to be made with the same specifications, but it soon became apparent that the Ford transmissions required many more warranty repairs than the Japanese-made Mazda transmissions. Ford researchers investigated this and found that their transmissions were meeting the required specifications, but the variation in the Ford transmissions was much greater than those from Mazda. Mazda was using a better and more expensive grinder, but the increased cost was offset through fewer warranty repairs. Armed with these important results, Ford made changes and proceeded not only to meet the required specifications but also to improve quality by reducing variation. (See Taguchi Techniques for Quality Engineering by Phillip J. Ross.) F M p la s s b th ifi
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