Introduction to Control Charts
Supplement to “Using Control Charts in a
Healthcare Setting” case
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Variation
 Variety may be the “spice of life”, but in the context of product quality,
process performance, & service delivery, variation is a BAD thing
Bears Fans
Cheer!
Bears Fans
Groan!
Career
187 made
218 attempts
85.8% success rate
Robbie Gould Image - http://www.nflfootballpicks.org/authors/6/Vic-Busey?Page=9
Robbie Gould Stats - http://www.nfl.com/player/robbiegould/2506264/careerstats
Goal Post Image - http://www.partycheap.com/Jointed_Goal_Post_p/50474.htm
Bears Fans
Groan!
Customer
Specification
Limits
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All Processes are Variable
Special Cause Variation
•
•
Unpredictable
Intermittent
Good
Expected
Variation
Change
Common Cause Variation
•
•
Inherent
Random
New
Expected
Variation
But the key is…
“We must understand variation.”
W. Edwards Deming
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Variation & Customer Specifications
Expected
Variation
Customer
Specs
Expected
Variation
Customer
Specs
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Reacting to Variation
One day, Tiger went to the driving
range & hit the same way each time.
Not satisfied with the results, he tried
again the next day, but tried to improve
each shot based upon the last one.
What went wrong?
Tiger Woods Image #1 - http://moodfresher.blogspot.com/2012/02/tiger-woods-wallpapers.html
Tiger Woods Image #2 - http://www.topnews.in/light/people/tiger-woods
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Over-Reaction to Common Cause Variation?
 Examples
 An operator reacting in the opposite direction based on the previous result
 Reacting to a single customer complaint without understanding if it is
common to many customers
• A manager reacting to a single data point
 Setting the current period’s sales quota based on last periods overage or
underage
 Setting the next period’s budget as a percentage of the last period’s budget
 The stock market reacting to good news or bad news
 Unintended Consequences
 Existing workers training the new workers
• Symptom of no Standard Work (Lean concept)
 Over-reacting to Common Cause Variation leads to MORE variation,
not less!
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Understanding Variation
 Descriptive Statistics




Average
Percentage
Median
Range
 Visual Tools
 Histograms
• An approximation of the distribution’s shape




Box Plots
Scatter Diagrams
Run Charts
Control Charts
• SPC – Statistical Process Control
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Run Charts
 Displays observed data in a time sequence
 So what?
 3 different run charts with the same distribution
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Control Charts
 Run Charts w/Control limits
 Typically ± 3 standard deviations
 “Rules” for flagging special causes
Customer
Specs
Expected
Variation
Customer
Specs
Control Limits & Specification Limits
are NOT the same
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Some Control Chart Basics
 Typically, data is plotted as it happens
 However, a control chart can be created in an retrospective manner
 This approach requires some judgment when determining if there has been
a process shift
 There are standard control chart tests (see next page)
 If one of these tests (or conditions) are violated, it is flagged  which
is supposed to prompt an investigation as to “why” it happened
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Control Chart Tests
Attribute
Tests
Numbers of the Line
= Failed Test
Test
1
2
3
4
5
Plus
More Tests
for
Variable
Data
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7
8
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Control Chart Selection Guide
Source: Nancy R. Tague’s The Quality Toolbox, Second Edition, ASQ Quality Press, 2005.
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Control Chart Selection Guide (another view)
Variable Control Charts
Chart Type Characteristic Monitored
Individual X Individual Values
Location
Xbar
Sample Mean
Spread
R
MR
D
Sample Range
Moving Range
Sample Standard Deviation
Attribute Control Charts
Chart Type
p
np
u
c
Characteristic Monitored
for defectives (%) – sample size varies
for defectives (%) – sample size fixed
for defects – sample size fixed
for defects – sample size varies
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More Control Chart Basics
 What they can do…
 Identify a process that is out-of-control
 Provide statistical evidence when a process has actually changed (either
for better or for worse)
 What they cannot do…
 Identify why the process is out-of-control
• That is left for you to investigate
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Never Stop Asking “Why?”
Image - http://1.bp.blogspot.com/_IbpK-GhiWSc/TJTfX63zpVI/AAAAAAAACOI/na3ZgyUFd2M/s1600/1227573326bxGKsyD.jpg
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