Certified Quality Engineer Body of Knowledge

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Chapter 6 Statistical Process Control (SPC)
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Descriptive Statistics

• 1. Measures of Central Tendencies (Location)
• Mean
• Median The middle value
• Mode - The most frequent number
• 2. Measures of Dispersion (Spread)
• Range RMaximum-Minimum
• Standard Deviation
• Variance

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The Standard Deviation
(x-µ)
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River Crossing Problem
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Inferential Statistics
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The Normal (Gaussian) Curve
-3? -2? -1? ? 1? 2? 3?
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Red Bead Experiment
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Types of Control Charts
Quality Characteristic
Attribute
Variable
Type of Attribute
X and MR chart
No
ngt1
Defective
Defect
Yes
Constant sample size?
Constant sampling unit?
Yes
Yes
X-bar and R chart
No
ngt6
Yes
np-chart
c-chart
No
No
X-bar and s chart
p-chart
u-chart
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Data
Stats

• Information
• Central Tendency
• Dispersion
• Shape

No Action
Decision
Action
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The Shape of the Data Distribution

• Box-and-Whisker Plot

mean median mode

• Pearsonian Coefficient of Skewness

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Control Charts
Special Cause (Assignable)
3s Average -3s
Common Cause (Chance or Random)
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Central Limit Theorem
Sample (x-bar) Distribution
Population (individual) Distribution
µ
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X-Bar and R Example
Rational Subgroup
Subgroup Interval
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X-Bar and R Control Chart Limits
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Attribute Control Chart Limits
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p-Chart Example
Note Use n-bar if all ns are within 20 of
n-bar
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The a and ß on Control Charts
a .00135
3s Average -3s
a .00135
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Out of Control Patterns
8 successive points same side of centerline
2 of 3 successive points outside 2 ?
-3?
4 of 5 successive points outside 1 ?
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Control Chart Patterns
Instability
Freaks
Gradual Trend
Sudden Shifts
Cycles
Hugging Centerline
Hugging Control Limits
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Six Sigma Process Capability
USL
LSL
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Cause and Effect Diagrama.k.a. Ishikawa
Diagram, Fishbone Diagram

Person
Procedures
B
C
A
Process
Material
Equipment
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Pareto Charta.k.a. 80/20 Rule

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Taguchi Loss Function
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Response Curves