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6-1 Is Process Stable ? The Quality Improvement Model Use SPC to Maintain Current Process Collect & Interpret Data Select Measures Define Process Is Process Capable ? Improve Process Capability Is Process Stable ? Investigate & Fix Special Causes No Yes No Yes Is Process Stable? Purpose:Determine the stability of key measures of the product.

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6-2 Is Process Stable ? Types of Variation Common Causes Causes that are inherent in the process over time, and affect all outcomes of the process. l Ever-present l Create small, random fluctuations in the process l Lots of them l The sum of their effects creates the expected variability l Predictable Time Quality Characteristic Run Chart

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6-3 Is Process Stable ? Types of Variation Special Causes Causes that are not present in the process all the time, but arise because of specific circumstances. l Not always present in the process l Can create large process disturbances, or sustained shifts l Relatively few in number l Pull the process beyond the expected level of variability l Unpredictable Control charts help identify the presence of special causes. Time Quality Characteristic Run Chart

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6-4 Is Process Stable ? Control Chart Components l Run chart of the data l Center Line (CL) A line at the average of the data or target of the process l Upper Control Limit (UCL) A line at the upper limit of expected variability l Lower Control Limit (LCL) A line at the lower limit of expected variability 8222424610121416182026283032 Run Order UCL CL LCL Control Chart The control limits are based on data collected from the process.

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6-5 Is Process Stable ? Rules for Separating Common & Special Causes Two commonly used signals of special causes are: Rule 1: Any point above the Upper Control Limit (UCL) or below the Lower Control Limit (LCL) Rule 2: 8 points in a row on the same side of the center line (CL) Note: Additional rules do exist.

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6-6 Is Process Stable ? Process Stability Stable Process A process in which the key measures of the output from the process show no signs of special causes. Variation is a result of common causes only. Unstable Process A process in which the key measures of the output from the process show signs of special causes in addition to common causes. Variation is a result of both common and special causes.

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6-7 Is Process Stable ? Process Stability

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6-8 Is Process Stable ? A Stable Process

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6-9 Is Process Stable ? Look For: Points Outside the Control Limits Shifts Cycles Runs Trends UCL LCL Assignable Cause(s) Are Present In Addition To Common Cause Variation Un-Stable Process

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6-10 Is Process Stable ? Advantages of Stable Processes Are:

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6-11 Is Process Stable ? Polymer Manufacturing Data Note: b* is a measure of yellowness Histogram does not show whether the process is stable! 012345 b* Histogram LS US

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6-12 Is Process Stable ? Histograms & Control Charts Histograms l Plot past data l Cannot tell if process is stable l Only useful for prediction if the process is stable Control Charts l Real-time evaluation l Help identify presence of special causes l Assess past and present stability of process

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6-13 Is Process Stable ? Pump Maintenance Data Number of Failures 0 2 4 6 8 10 12 14 16 18 20 24681012141618202224 Avg=4.8 LCL=None UCL=11.4 Week Are there any signals of special causes? Circle them. Is Process Stable ?

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Forecasting and Statistical Process Control MBA Statistics 51-651-02 COURSE #5.

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