More Information: https://flevy.com/browse/business-document/control-chart-rules-western-electric-rules--5504 BENEFITS OF DOCUMENT Assist management in making good decisions for when and when not to demand investigation and corrective actions to a process. Learn what actions to take if a specific control chart rule is broken. Enhance process performance by reducing overall variation and special cause variation. DOCUMENT DESCRIPTION Control Chart Rules Overview Control Chart Rules, sometimes referred to as Western Electric Rules, help Management, Process Owners and support personnel make informed decisions about process performance. Control Chart Rules have a dual benefit, informing you when to take action and just as importantly when not to take action. Many times Management, reacting to one or two data points on a chart will demand investigation and corrective action when it is not needed?wasting valuable time and unnecessary process tampering. What This Course Provides This 17-page course provides learners with a thorough understanding of the terminology and methodology associated with Control Chart Rules. It is presented in FranklinGood's comprehensive and highly visual learning format. This course will help learners understand the why and how control chart rules work, how to apply them and how to improve process performance. The learner is also provided insights into the causes of process issues, enhancing their ability to make improvements. Learners will understand how probability is the basis for control chart rules, the difference between normal variation and special cause variation. Learning Objectives ? Learn the rationale and logic behind control chart rules, enabling their adoption and support across the organization. ? Learn the value of using control chart rules. ? Learn how "zones" increase insights into process performance. ? Learn what actions to take if a specific control chart rule is broken. ? Learn each rule with a graphic that demonstrates what the rule looks like on a control chart. Got a question about this presentation? Email us at support@flevy.com.

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Control Chart (Western Electric –
Nelson) Rules
Four of the five
most recent points
plot outside and on
the same side as
one of the 1-sigma
control limits
Eight points in a
row outside one
sigma
Rule 8
Rule 7
Rule 6
Rule 5
Rule 1 Rule 3 Rule 4
Rule 2
The most recent
point plots outside
one of the 3-sigma
control limits
Two of the three
most recent points
plot outside and on
the same side as
one of the 2-sigma
control limits
Eight out of the last
eight points plot on
the same side of
the center line, or
target value.
Six points in a row
increasing or
decreasing.
Fifteen points in a
row within one
sigma.
Fourteen points in
a row alternating
direction
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Overview
Likelihood of Occurrence
• The basis for making decisions using the Western
Electric Company and Nelson rules (Control Chart
Rules) are based on the “likelihood this could
happen.”
• In other words, these rules are based on the
science of probability.
• Example: What is the probability that a coin toss
could yield 7 heads in a row?
• Very small…in fact it is 0.78%
• Essentially, if the likelihood is so low, then it
should get our attention…and needs to be
investigated.
Another way of stating this is, “This did not happen by random chance…
therefore, there must be an assignable cause.”
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Overview
Probability Basis for Decision Rules
• When the parameter being measured behaves in
a normally distributed manner then the data, over
time, forms a bell-shaped curve.
• When this is the case, we can easily determine
the likelihood the process is or is not performing
as intended.
• This is the basis for statistical process control
(SPC) and control charts.
For example, it is commonly understood that the
likelihood of a data point falling 3 standard
deviations from the center line (mean) is only
about 0.1%
• There is a 34.1% likelihood the parameter being
measured will be between the center line and 1
sigma, 13.6% chance between 1 and 2 sigma and
2.1% between 2 and 3 sigma
Because the likelihood is so small in
this example, 0.1%, because the
process’s parameter value is greater
than 3 sigma from the mean, there
must be something unique causing it
This condition forms the first decision
rule of the control chart rules and all of
the rules are based on a similar
approach…the likelihood of occurrence
Bell-shaped or normally distributed curve
Observed Data
Point
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Rule 1 Rule 3 Rule 4
Rule 2
• Two of the three data points plot
outside and on the same side as one
of the 2-sigma control limits.
• Rule 2 primarily indicates a medium
level shift in process performance has
occurred. Look for things like: New
person doing the Job, wrong setup,
measurement error, process step
skipped, process not completed,
equipment failure.
Understanding Each Rule
Control Chart Rules
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Rule 8
Rule 7
Rule 6
Rule 5
• Six points in a row increasing or
decreasing.
• Rule 5 primarily indicates a time
dependent shift or trend in the
process performance has occurred.
• Look for things like: tooling wear,
temperature effects (heating,
cooling).
Understanding Each Rule
Control Chart Rules
This document is a partial preview. Full document download can be found on Flevy:
https://flevy.com/browse/document/control-chart-rules-western-electric-rules--5504

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• Eight points in a row outside one
sigma.
• Primarily indicates a mixture “pattern”
is affecting process performance.
• Look for things like: More than one
process present (e.g. shifts,
machines, raw materials).
Rule 8
Rule 7
Rule 6
Rule 5
Understanding Each Rule
Control Chart Rules
This document is a partial preview. Full document download can be found on Flevy:
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