Module 10C

# Module 10C - IE 361 Module 10 Introduction to Shewhart...

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IE 361 Module 10 Introduction to Shewhart Control Charting (Statistical Process Control, or More Helpfully: Statistical Process Monitoring) Reading: Section 3.1 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 10 1 / 18

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Generalities About Shewhart Control Charting SPC (SPM) is process watching for purposes of change detection. Figure: SPC (SPM) is About "Process Watching" One famous statistician has called it "organized attention to process data." Vardeman and Morris (Iowa State University) IE 361 Module 10 2 / 18
Shewhart Charting-Generalities reasoned that while some variation is inevitable in any real process, the variation seen in data taken on a process can be decomposed as overall observed variation = baseline variation + variation that can be eliminated baseline that which can be eliminated inherent to a system con±guration due to system/common/universal causes due to special/assignable causes random nonrandom short term long term Vardeman and Morris (Iowa State University) IE 361 Module 10 3 / 18

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Shewhart Charting-Generalities ... how is one to detect the presence of the second kind of variation so that appropriate steps can be taken to eliminate it? The hope is to leave behind a process that might be termed physically stable (not without variation, but consistent in its pattern of variation). The point of Shewhart control charting is to provide a detection tool. and
Shewhart Charting-Generalities For a generic plotted statistic, Q (di/erent kinds of Shewhart charts correspond to various choices of Q ), this looks like Figure: A Generic Shewhart Control Chart Points plotting "out of control" indicate process change, i.e. the presence of variation of the 2nd kind, and signal the need for intervention (of some assignable cause. Vardeman and Morris (Iowa State University) IE 361 Module 10 5 ± 18

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Shewhart Charting-Setting Limits A basic question is how to set the control limits, UCL Q and LCL Q . If one models process output (individual measurements from the distribution, then probability theory can often be invoked to produce a distribution for Q and corresponding mean μ Q and standard deviation σ Q . For many distributions, most of the probability is within three standard deviations of the mean. So, if
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Module 10C - IE 361 Module 10 Introduction to Shewhart...

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