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) 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
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Patterns on Control Charts Systematic Variation/Cycles Systematic variation/cycles , regular "up then back down again" patterns Figure: A Plot of Factory Ambient Temperature vs. Time 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 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. Vardeman and Morris (Iowa State University) IE 361 Module 14 4 / 17
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Instability Instability is a word traditionally applied to patterns on control charts shows two Shewhart ¯ x second shows instability. Figure:
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Module 14C - IE 361 Module 14 Patterns on Control Charts...

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