Transcript Effect of Measurement Error on SPC.pptx

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Effect of Measurement
Error on SPC
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subgroup weight
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-6
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-5
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time
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Consider the following Control Charts
R-chart
X-bar chart
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Suppose that some measurement error
was added to the data?
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Our data should be more “variable”
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Our estimate of Common Cause variation should increase
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It affects the variability of our plotted data
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It affects the upper and lower control limits
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It obscures Special Cause
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As expected it hides Special Cause
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Side by side R-charts
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Side by side X-bar charts
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What about Capability?
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Summary
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Increased Measurement Error will widen Control Limits.

Will tend to hide Special Cause when present.
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Will increase our estimates of Common Cause, σc
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Will reduce our estimates of Capability indices.
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