The Quality Improvement Model

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TheQualityImprovementModel
Define Process
Select Measures
Collect Interpret Data
Is Process Stable?
IsProcessStable?
No
Investigate Fix Special Causes
Purpose Determine the stability of key measures
of the product.
Yes
IsProcessCapable?
No
Improve Process Capability
Yes
Use SPC to Maintain Current Process
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Types of VariationCommon Causes

Run Chart

• Causes that are inherent in the
• process over time, and affect all
• outcomes of the process.
• Ever-present
• Create small, random fluctuations in the process
• Lots of them
• The sum of their effects creates the expected
  variability
• Predictable

Quality Characteristic
Time
3
Types of VariationSpecial Causes

• Causes that are not present in the
• process all the time, but arise
• because of specific circumstances.
• Not always present in the process
• Can create large process disturbances, or
  sustained shifts
• Relatively few in number
• Pull the process beyond the expected level of
  variability
• Unpredictable

Run Chart
Quality Characteristic
Time
Control charts help identify the presence of
special causes.
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Control Chart Components

• Run chart of the data
• Center Line (CL)
• A line at the average of the data or target of
  the process
• Upper Control Limit (UCL)
• A line at the upper limit of expected variability
• Lower Control Limit (LCL)
• A line at the lower limit of expected variability

The control limits are based on data collected
from the process.
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Rules for Separating Common Special Causes
Two commonly used signals of special causes
are Rule 1 Any point above the Upper Control
Limit (UCL) or below the Lower Control Limit
(LCL) Rule 2 8 points in a row on the same
side of the center line (CL)
Note Additional rules do exist.
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Evaluation of Individuals Data
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Estimating Sigma (Standard Deviation)
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Problem Individuals Chart Calculations
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Individuals Chart Calculations
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Minitab Creating Individuals Control Charts

• Open Minitab Software and the Line 1.MTW
  Worksheet.
• Create an Individuals Control Chart following
  the commands in the notes.
• Your output should look like the charts below

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Minitab Creating Individuals Control Charts

• Open Problem 14.MTW located in the Minitab
  Datasets folder.
• Use Minitab to create an Individuals and Moving
  Range control chart for X in column C1.
• What do you notice about the X and MR Charts?
• What should you do to establish control limits?

12
Minitab Creating Individuals Control Charts With
Range Edited Limits

• As you noticed the moving range control chart for
  problem 14 has two ranges that are above the
  upper control limit.
• So, a special cause source of variation is
  included in the limits calculation.
• The special cause needs to be removed and the
  limits re-calculated.
• Row 24 is the data point causing the moving
  ranges to be out of the limits.
• Use the brush tool to select and update the
  control chart or create a new column without data
  point 24 (Remember to use the Backspace).
• These were taught in the Introduction to Minitab
  Course which is a pre-requisite for the SPC
  course.
• Brush tool Page 13 Introduction to Minitab book

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Minitab Creating Individuals Control Charts With
Range Edited Limits

• The chart below shows the updated control charts
  without point 24.

• The updated moving range chart shows more moving
  ranges outside of limits.
• These are caused by rows 7 and 29.
• Remove the data points and update the control
  charts again.

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Minitab Creating Individuals Control Charts With
Range Edited Limits

• The second updated Moving Range chart does not
  have any points outside of limits

• The objective is to calculate control limits that
  represent common cause sources of variation only.
• However, stop editing data points once 10 to 20
  of the data has been edited.
• If the initial data has this many special causes,
  the limits will identify plenty special causes in
  the future for you to work on.

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Minitab Obtain Statistics to Calculate Limits
for Continuous Process Monitoring
Select I-MR Options in the I-MR dialog box Choose
Storage and select Means and Standard deviations
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Minitab Obtain Statistics to Calculate Control
Limits Continued
Two new columns are created Mean1 Average of
your data STDE1 Sigma_C
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Process Stability

• Stable Process
• A process in which the key measures of the output
  from the process show no signs of special causes.
  Variation is a result of common causes only.
• Unstable Process
• A process in which the key measures of the output
  from the process show signs of special causes in
  addition to common causes. Variation is a result
  of both common and special causes.

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Process Stability
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A Stable Process
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A Stable Process Sigma S Sigma C
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Un-Stable Process
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Un-Stable Process Sigma S Not Equal to Sigma C
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Advantages of Stable Processes Are
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Polymer Manufacturing Data
Note b is a measure of yellowness
Histogram does not show whether the process is
stable!
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Whats Wrong With Putting SpecificationLimits on
Control Charts?
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Histograms Control Charts

• Histograms
• Plot past data
• Cannot tell if process is stable
• Only useful for prediction if the process is
  stable

• Control Charts
• Real-time evaluation
• Help identify presence of special causes
• Assess past and present stability of process

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Pump Maintenance Data
IsProcessStable?
Number of Failures
UCL11.4
Avg4.8
LCLNone
2
4
6
8
10
12
14
16
18
20
22
24
Week
Are there any signals of special causes? Circle
them.
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Driving to Work Data
The next 5 observations are 47, 46, 43, 52, 45.
Plot them. Are there any signals of special
causes? Circle them.
29
Purchase Order Data
Are there any signals of special causes? Circle
them.
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Shipping Data
p (fraction nonconforming)
Week
Are there any signals of special causes? Circle
them.
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Quality ImprovementTypes of Control Charts
Counting Measures p charts np charts
c charts u charts
When you have a problem to solve, you want to
choose the right tool.
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ExercisesCircle any signals of special causes
you find in the following control charts.
Example 1
Example 2
Monthly Sales
Time
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Skill Check - Continued
Example 3
Example 4

Number of Lost-Time Accidents

Time
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Exercises
1.) Your Catapult Team should complete page 9 of
the Catapult Process handout. 2.) Be ready to
present your results in PowerPoint Limit
yourselves to 10 minutes for this exercise.