DobbinChapter5QualityControl

DobbinChapter5QualityControl - Statistical Process Control...

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Statistical Process Control Process: A process is a series of activities (one or more) that turns inputs into outputs. Examples: Extruding, heat treating, drawing, and machining an aluminum billet into an baseball bat. Reviewing an employee’s expense report for a business trip and issuing a reimbursement check. Making a salad for dinner. Forging an aluminum billet into a aircraft engine mount. Processes are like populations, today’s output is like a sample. If you think of a population as containing all the outputs that would be produced by the process if it ran forever in its present state, the outputs produced today or this week are a sample from this population. There are two ways of organizing a process: Cause-and-effect diagram : Organizes the logical relationships between the inputs and stages of a process and an output. Lecture 5, Statistical Process Control Page 1 1
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Flowchart : A picture of the stages of a process. Lecture 5, Statistical Process Control Page 2 2
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Goal : Make a process stable over time and then keep it stable unless planned changes are made. All process have variation. Statistical stability means the pattern of variation remains stable, not there is no variation in the variable measured. A common cause of variation is the inherent variability of the system, due to many small causes that are always present. When the normal functioning of the process is disturbed by some unpredictable event, special cause variation is added to the common cause variation. We want to eliminate the cause of this type of variation and restore stability to the process. A variable that continues to be described by the same distribution when observed over time is said to be in statistical control . Control charts : are statistical tools that monitor a process and alert us when the process has been disturbed so that it is now out of control. This is a signal to find and correct the cause of the disturbance. Example:
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DobbinChapter5QualityControl - Statistical Process Control...

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