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Process Control Charts

Process Control Charts. Any process has a certain amount of natural variability. But how can we tell if the process’s variability has gone “out of control”? Example: An automated process whose intent is to fill a bag with 200 pounds of cement.

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Process Control Charts

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  1. Process Control Charts Any process has a certain amount of natural variability. But how can we tell if the process’s variability has gone “out of control”? Example: An automated process whose intent is to fill a bag with 200 pounds of cement. Process Control is a technique for inferring that an unplanned change has taken place in a process measured by a process variable X. Example: X is the exact weight of a bag of cement intended to weigh 200 pounds.

  2. Alternative Meanings forthe Process Variable X • The salt content, thickness, or crispness of a bag of potato chips. • The number of chocolate chips in a container of chocolate-chip ice cream. • The diameter of a bearing, or the center of a gear. • The waiting time at a fast-food restaurant or at an airport check-in counter. • The internal temperature of a “rare” steak when it leaves a restaurant’s kitchen.

  3. Sampling Over some period of time, take N samples with each sample having n observations. Example: During each of N=10 consecutive hours, remove n=4 bags of cements from the filling process and weigh them.

  4. Two Ways a Process Can be Out-of-Control Both of the processes below are out-of-control. But in different ways! Can you see the difference?

  5. Two Ways to be Out-of-Control (continued)

  6. Establishing theControl Chart’s UCL & LCL Go to Excel Workbook

  7. Establishing theControl Chart’s UCL & LCL (continued)

  8. The Mean is out-of-control! Out-of-control

  9. The Range is out-of-control! Out-of-control

  10. Patterns to InvestigateCase #1 Why might this process be out-of-control?

  11. Patterns to InvestigateCase #2 Why might this process be out-of-control?

  12. Patterns to InvestigateCase #3 Why might this process be out-of-control?

  13. Patterns to InvestigateCase #4 Why might this process be out-of-control?

  14. The Process Control “Cycle” Initialization. Take an initial set of N samples with n observations, and use these to compute the initial lower and upper control limits. Step 1. Continue with periodic samples until the process goes out-of-control. Look for an assignable cause. Step 2. If possible, improve the process in a manner that decreases the chance that the same assignable cause will reoccur. Step 3. After a process improvement, recalibrate the lower and upper control limits by taking another set of N samples with n observations.Return to Step 1.

  15. Another Type of Control Chart • We have discussed control charts in the context of a process whose performance is measured by a continuous variable X. • For some processes, performance is measured by an binary attribute – an attribute that is either present or not present. • Examples: • A product is either defective or non-defective. • A invoice either contains an error or is error-free. • A customer is either satisfied or unsatisfied. • To control a process measured by an binary attribute, you need to use another type of control chart known as a p-chart.

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