QC

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Quality Control
COB 300 C - The Operations Dimension Busing
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Causes of Variation

• Random Causes
• difficult or expensive to control
• e.g., outside humidity, line voltage
• Assignable Causes
• easier to correct
• e.g., employee error, new materials

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Comparing Process Variation to Tolerance Limits
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High Process Capability After Technological
Process Change
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Control Charts

• Variables - concentrates on mean for some
  measurable characteristic.
• diameter
• length
• Attribute - data is based on counts or the number
  of times we observe a particular event.
• proportion defective/non-defective
• go/no go
• proportion pass/fail

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Control Limits - Variables Charts

• X-Bar Chart
• R Chart

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Factors for Control Limits(Slide 1 of 2)
Sample of Size
n
A
D
D
2
3
4
2 3 4 5 6 7 8 9 10
1.880 1.023 0.729 0.577 0.483 0.419 0.373 0.337 0.
308
0 0 0 0 0 0.076 0.136 0.184 0.223
3.268 2.574 2.282 2.114 2.004 1.924 1.864 1.816 1.
777
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x-bar Chart Example (contd)
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Range Chart
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Warning Conditions

• Two successive points near limit
• Run of five above or below mean
• Trend
• Erratic behavior

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Process Control for Attributes

• Proportion defective
• Number of defects per unit

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