1. The Certified Quality Engineer Handbook Ch
The Certified Quality Engineer Handbook Ch. 37: Statistical Process Control (SPC)
Dr. Joan Burtner
Certified Lean Six Sigma Black Belt Professional
Associate Professor and
Chair, Department of
Industrial Engineering and Industrial Management

2. ISE 468 ETM 568 JMB CQE Handbook CH 37
Chapter 37 Topics
Introduction to SPC
Objectives
Theory of Process Variation
Rational Subgroups
Types of Control Charts
Construction of Control Charts
Control Charts for Attributes
Control Charts for Variables
Interpretation of Control Charts
Manual Application of Tests
Statistical Software Application of Tests
Other Process Charts
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3. Characterizing Causes of Variation
The intent of process monitoring is to distinguish between random and non-random variation.
Random Cause vs.
Non-random Cause
Common vs.
Special
Chance vs.
Assignable
Chronic vs.
Sporadic
System fault vs.
Local fault
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4. Theory of Process Variation: Statistical Control
The common variations in process variability that are
caused by natural incidences are in general not repetitive,
but various factors due to chance and are called
random variation. All processes are subject to random variation.
If the cause of variation is systematic (not natural) the process variation is called non-random variation. When non-random variation is present, the quality engineer should identify and eliminate the source of the variation.
When a process is subject to non-random variation the process is described as out-of-control.
If only random variation is present, the process is described as in-control.
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5. ISE 468 ETM 568 JMB CQE Handbook CH 37
Control Limits, Random and Nonrandom Sample Observations
Upper
Control
Limit
(UCL)
Non-random
Non-random
+3σ
Process
Mean
99.7%
Lower
Control
Limit
(LCL)
-3σ 1 2 3 4 5 6 7 8 9 10 11 12
Sample number
Source: Ozcan Figure (Modified for Three Sigma Limits)
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6. ISE 468 ETM 568 JMB CQE Handbook CH 37
Statistical Control Chart Types
Attributes
Variables(Subgroups)
c-chart
p-chart
u-chart
Mean Charts
(X-bar Charts)
Variation Charts
σ Method
Range Method
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7. ISE 468 ETM 568 JMB CQE Handbook CH 37
Variables Control Charts (Continuous Data)
When process characteristics can be measured, variables control charts are the appropriate way to display the process monitoring.
The Xbar-chart and the Range chart are displayed and interpreted together.
When the Range chart exhibits out-of-control status, the rules for evaluating the Xbar-chart should not be used.
The Xbar chart is appropriately evaluated after the Range chart indicates that the process is in-control. Use caution when statistical software evaluates both charts simultaneously.
See examples on pages (Handout for ISE 468)
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8. ISE 468 ETM 568 JMB CQE Handbook CH 37
Variables Control Chart for n = 1
Variables(Subgroups)
Variables (Individuals)
Individual observation
Moving Range
Mean Charts
(X-bar Charts)
Variation Charts
Note that the tests that apply to the subgroup charts do not apply to the Individuals Charts.
σ Method
Range Method
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9. ISE 468 ETM 568 JMB CQE Handbook CH 37
Attribute Control Charts (Discrete Data)
When process characteristics can be counted, attribute-based control charts are the appropriate way to display the process monitoring.
The p-chart is the appropriate control chart for a process with only two outcomes (defective or not defective) when the proportion defective is calculated.
The c-chart is the appropriate tool to display monitoring if the number of occurrences per sampling period is recorded.
The u-chart is the appropriate control chart if the number of occurrences and the number of items per sampling period is recorded. The average number of occurrences per sample is calculated.
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10. ISE 468 ETM 568 JMB CQE Handbook CH 37
Attribute Control Charts (Discrete Data)
See text for examples of p-chart.
See text for examples of c-chart.
We will discuss how to use Minitab to build p-charts and c-charts in class.
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11. ISE 468 ETM 568 JMB CQE Handbook CH 37
Other Charts (not discussed in ISE 468 ETM 568)
Cumulative Sum Charts
EWMA Charts
Moving Average Charts
*******Pre-control Charts *******
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12. ISE 468 ETM 568 JMB CQE Handbook CH 37
Contact Information
Dr. Joan Burtner
ISE 428 ETM 528
Quality Engineering
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