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Module 14C

# Module 14C - IE 361 Module 14 Patterns on Control Charts...

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IE 361 Module 14 Patterns on Control Charts and "Special Checks"/Extra Alarm Rules Reading: Section 3.4 of Statistical Quality Assurance Methods for Engineers Prof. Steve Vardeman and Prof. Max Morris Iowa State University Vardeman and Morris (Iowa State University) IE 361 Module 14 1 / 17

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Patterns on Control Charts (and Other Time Plots) To this point, we°ve spoken as if the only use one makes of a Shewhart chart is to compare values of a summary statistic Q to control limits. If that were true, there would be little point in making the plot. Calculations alone would su¢ ce. But, in fact, people make and examine the plots, looking for interpretable patterns. Under stable process conditions, one expects a sequence of plotted values Q that are without obvious pattern or trend, only on rare occasions fall outside control limits, tend to cluster about the center line, about equally often above and below it, but on occasion approach the control limits. When something other than this kind of "random scatter" shows up on a control chart, it can be possible to get clues to what physical causes are acting on the process that can be used in process improvement e/orts. Vardeman and Morris (Iowa State University) IE 361 Module 14 2 / 17
Patterns on Control Charts Systematic Variation/Cycles Systematic variation/cycles , regular "up then back down again" patterns sometimes appear on plots against time. The ±gure below is an example. Figure: A Plot of Factory Ambient Temperature vs. Time Identi±cation of the period of variation can give hints where to start looking for physical causes. Vardeman and Morris (Iowa State University) IE 361 Module 14 3 / 17

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Patterns on Control Charts Systematic Variation/Cycles Examples of the kind of factors that can produce cycles are seasonal and diurnal variables like ambient temperature. And sometimes regular rotation of ±xtures or gages or shift changes in operators running equipment or making measurements can stand behind systematic variation. While systematic variation is variation of the "second kind," it may not always be economically feasible to eliminate it. For example, it may be preferable to live with e/ects of regular cycles in ambient temperature rather than try to control the environment in which a process operates. But recognition of its presence at least allows one to intelligently consider options regarding remedial measures, and to mentally remove that kind of variation from the baseline against which one looks for the e/ects of other special causes.
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Module 14C - IE 361 Module 14 Patterns on Control Charts...

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