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Lecture 13

ECON 2500 Lecture 13: Chapter 17 Statistics for quantity Control and Capability Lecture 13

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York University
ECON 2500
Andrei Semenov

Chapter 17 Statistics for quantity: Control and Capability Lecture 13 Comments on statistical control • Having seen how x and s (or x and R) charts work, we can turn to some important comments and cautions about statistical control in practice. • Focus on the process rather than on the product This is perhaps the fundamental idea in statistical process control. We might attempt to attain high quality by careful inspection of the finished product or reviewing every outgoing invoice and expense account payment. • Inspection of finished products can ensure good quality, but it is expensive. Perhaps more important, final inspection often comes tool ate: • when something goes wrong early in a process, much bad product may be produced before final inspection discovers the problem. This adds to the expense, because the bad product must then be scrapped or reworked. • The small samples that are the basis of control charts are intended to monitor the process at key points, not to ensure the quality of the particular it eosin the samples. • If the process is kept in control, we know what to expect in the finished product. We want to do it right the first time, not inspect and fix finished product. Choosing the “key points” at which we will measure and monitor the process is important. • The choice requires that you understand the process well enough to know where problems are likely to arise. • Flowcharts and cause-and effect diagrams can help. It should be clear that control charts that monitor only the final output are often not the best choice. Rational subgroups • The interpretation of control charts depends on the distinction between x-type special causes and s-type special causes. This distinction in turn depends on how we choose the samples from which we calculate s (or R). • We want the variation within a sample to reflect only the item-to-item chance variation that (when in control) results from many smal
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