Statistical Process Control http:www'managementsupport'com

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Statistical Process Controlhttp//www.managemen
tsupport.com
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Outline

• What is Statistical Process Control
• Variability
• Control Charts
• Process Capability Cp
• Measuring Instruments
• Repeatability Reproducibility

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Statistical Process Control (SPC)

• Invented by Walter Shewhart at Western Electric
• Distinguishes between
• common cause variability (random)
• special cause variability (assignable)
• Based on repeated samples from a process

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Variability
The world tends to be bell-shaped
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Variability
Even outcomes that are equally likely (like
dice), when you add them up, become bell shaped
Here is why
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Normal bell shaped curve
Add up about 30 of most things and you start to
be normal Normal distributions are divide up
into 3 standard deviations on each side of the
mean Once your that, you know a lot about what
is going on
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Causes of Variability

• Common Causes
• Random variation (usual)
• No pattern
• Inherent in process
• adjusting the process increases its variation
• Special Causes
• Non-random variation (unusual)
• May exhibit a pattern
• Assignable, explainable, controllable
• adjusting the process decreases its variation

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Control Limits
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Control Chart
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Process vs. control limits
Distribution of averages
Control limits
Specification limits

• Variance of averages lt variance of individual
  items

Distribution of individuals
Process limits
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Shift in Process Average
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Process Capability

• The ratio of process variability to design
  specifications

Natural data spread
The natural spread of the data is 6s
-1s
-2s
-3s
2s
1s
3s
µ
Upper Spec
Lower Spec
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Repeatability and Reproducibility Studies

• Quantify and evaluate the capability of a
  measurement system
• Select m operators and n parts
• Calibrate the measuring instrument
• Randomly measure each part by each operator for r
  trials
• Compute key statistics to quantify repeatability
  and reproducibility