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Ch%206s_mod - Operations Management MGT 3200 Dr Samia Siha...

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Unformatted text preview: Operations Management MGT 3200 Dr. Samia Siha Statistical Process Control Supplement 6 Chapter Map Statistical Process Statistical Process Control Control Process Process Variations Variations Process Process Capability Capability SPC Charts SPC Charts Common Common Variations Variations Assignable Assignable Variations Variations Charts for Charts for Variables Variables Charts for Charts for Attributes Attributes Cp and C Cp and Cpkpk P­Chart P­Chart Xbar ­ Chart Xbar ­ Chart © Copyrighted R ­ Chart R ­ Chart Six Sigma Six Sigma C­ Chart C­ Chart 2 Chapter Objectives Define statistical process control (SPC) Describe the difference between natural and assignable variation Calculate control limits for an X-bar chart and interpret the chart Calculate control limits for an R chart and interpret the chart Calculate and interpret Cpk for a process Calculate control limits for an P chart and interpret the chart Calculate and interpret Cp for a process Relate Cpk and Six Sigma © Copyrighted 3 Statistical Process Control (SPC) Variability is inherent in every process Natural or common causes Special or assignable causes Provides a statistical signal when assignable causes are present Detect and eliminate assignable causes of variation © Copyrighted 4 Natural Variations Also called common causes Affect virtually all production processes Expected amount of variation Output measures follow a probability Output distribution distribution For any distribution there is a measure For of central tendency and dispersion of If the distribution of outputs falls within If acceptable limits, the process is said to be “in control” be © Copyrighted 5 Assignable Variations Also called special causes of variation Generally this is some change in the process Variations that can be traced to a specific Variations reason reason The objective is to discover when The assignable causes are present assignable Eliminate the bad causes Incorporate the good causes © Copyrighted 6 Samples To measure the process, we take samples To and analyze the sample statistics following these steps these Figure S6.1 © Copyrighted Frequency (a) Samples of the Samples product, say five boxes of cereal taken off the filling machine line, vary from each other in weight weight Each of these Each represents one sample of five boxes of cereal boxes ## ### #### ####### # ######### Weight 7 Samples (b) After enough samples are After taken from a stable process, they form a pattern called a distribution distribution Frequency The solid line The represents the distribution distribution Figure S6.1 © Copyrighted Weight 8 Samples (c) There are many types of distributions, including There the normal (bell-shaped) distribution, but distributions do differ in terms of central tendency (mean), standard deviation or variance, and shape variance, Frequency Figure S6.1 Central tendency Weight © Copyrighted Variation Weight Shape Weight 9 Samples Frequency (d) If only natural causes of variation If are present, the output of a process forms a distribution that is stable over time and is predictable predictable Prediction e Tim Weight Figure S6.1 © Copyrighted 10 ...
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