The Quality Improvement Model

1
TheQualityImprovementModel
Define Process
Select Measures
Collect Interpret Data
Is Process Capable?
IsProcessStable?
Investigate Fix Special Causes
No
Purpose Determine the adequacy of the process
with respect to customer /management needs.
Yes
IsProcessCapable?
No
Improve Process Capability
Yes
Use SPC to Maintain Current Process
2
Capable Process

• A stable process that meets customer requirements.

Histogram
Control Chart
UCL
CL
LCL
Lower Spec
Upper Spec
Run Order
Target
Capability assessments for unstable processes,
may not be indicative of how the process is
actually performing.
3
Assessing Process Capability

• Counting Measures
• The average percent defectives.
• The average number of defects.
• Instrument Measures
• Comparing both the center of the process and the
  process variation

4
Capability Assessment for Counting Measures
Order Entry Process
Is this process adequate as is? Should it be
improved?
5
Capability Assessment for Instrument Measures
Is this process adequate as is? Should it be
improved?
6
Measures of Process Capability
6sc

lt 1.0

Process is not capable of meeting specs

gt 1.0

Process is capable of meeting specs
Problem We are assuming the process has a
target that is


in the center of the specification range, and
that


the process is in fact centered on that target.
3sc
Note a negative result is possible if the
process average is outside specifications

lt 1.0

Process is not capable of meeting specs

1.0

Process is marginally capable

gt 1.0

Process is capable of meeting specs
Benefits
Optimal values are attained by running exactly



between specs.


Can (must) be used for 1-sided specifications
Warning Capability assessments for unstable
processes, may not be indicative of how the
process is actually performing.
7
Process Capability

• How much material is out of spec?
• In the short term?
• In the long term?

8
Process Capability Ratios
Voice of The Process
Voice of The Customer
9
Process Capability - The Strategy
Centering The Process Is On Target
Spread Reduce The Variation
10
Process Capability Ratios
2 Key Metrics for Measuring Capability

X
-
LSL
USL
-
X
C
Min(
?
,
)
pk
3
3


11
Process Capability Ratios - Concept
Total Tolerance
C
?
p
Process Spread
12
Process Capability Example 1
LSL
USL
27 Inches
33 Inches
u 30 Inches 1
Cp __________
Sigma Level __________
13
Process Capability Example 2
LSL
USL
29 Inches
31 Inches
u 30 Inches 1
Cp __________
Sigma Level __________
14
Process Capability Example 3
LSL
USL
28 Inches
32 Inches
u 30 Inches .333
Cp __________
Sigma Level __________
15
Process Capability Example 4
LSL
USL
28 Inches
32 Inches
u 33 Inches .333
Cp __________
Sigma Level __________
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Process Capability Ratios

X
-
LSL
USL
-
X
C
Min(
?
,
)
pk
3
3


A metric to take into account process shift
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Process Capability Example 5
LSL
USL
28 Inches
32 Inches
u 33 Inches .333
CpU ___________
CpL ___________
Cpk ____________
Recall Cp2.0
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CP CPK Measure Short-term Capability
Is The Process In Control ? Is It Producing
Defects ?
A Short-term Capability study covers a relatively
short period of time (days, weeks) generally
consisting of 30 to 50 data points. The actual
number depends on the subject under study.
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Long Term Performance
Short term Capability
Is The Process In Control ? Is It Producing
Defects ?
A long-term capability study covers a relatively
long period of time (weeks, months) generally
consisting of 100-200 data points. Again, the
actual amount depends on the subject under study.
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A Further Look at Capability
Compare the estimates of the process deviations
from the short-term and long-term data
Descriptive Statistics Variable N
Mean StdDev short term 30
30.6 2.23 long term 180
33.8 4.44
What is the difference between the short-term and
the long-term data? What implication does this
have in doing capability studies?
21
The Dynamic Process
USL
LSL
Short-Term Capability
Short-Term Capability
Long-Term Capability
Over time, a process tends to shift by
approximately 1.5?
22
Measures of Process Performance
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Performance vs. Capability
These data show that the process, if well
controlled can perform much better than it
currently is
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Capability vs. Performance
Process Performance Total Variation including
shifts and drifts (Pp Ppk)
Capability Only random or short term
variability (Cp Cpk)
25
Process Performance Ratios

X
-
LSL
USL
-
X
P
Min(
?
,
)
pk
3
3


The P-family of indices are computationally the
same as the C-family of capability indices, but
use the observed long-term standard deviation.
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How Does Process Capability Relate toSix Sigma?
27
Six Sigma Goals (Defect Level)
The Quality goal of six sigma corresponds to
3.4ppm defect level.
For attributes data, the ppm level is calculated
by
For variables data, the ppm level is calculated
using the normal table.
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Minitab Process Capability Analysis
Is it in control ?
Is it in control ?
Is it Normal ?
How does the process variation compare to
the spec limits ?
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WARNING!!!

• Statistical Assumptions Made In Capability
  Studies
• 1. Data Comes From a Stable Process
• If not, work towards getting the process in
  control
• Dont despair, you can still make some
  assumptions about your process in the mean time
• 2. Data are Normally Distributed
• If not, transform it (ask the instructor)
• If Items 1 and 2 arent met, results will be
  misleading

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Minitab Process Capability Analysis

• Minitab has many tools that will help you in this
  area. We will find Process Capability under
• StatgtQuality Toolsgt
• Capability Sixpackgt
• Normal
• Open Line1.MTW in the
• Mintab Datasets Folder
• and go to the Capability Sixpack

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Minitab Process Capability Analysis
Enter the information shown below and select OK
32
Minitab Process Capability Analysis
Your output should look like this
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Example - Capability Study Results

• Notice that both Cp / Cpk and Pp / Ppk values are
  given
• The data shows that the Capability of the process
  (Cpk) is 1.32 and the Entitlement (Cp) is 1.35
• This shows that the process is on target. Why?
• The data also shows the Performance of the
  process (Ppk) is reasonably capable with a value
  of 1.26
• This indicates the process is in control. Why?
• But how good should a process get Next Slide?

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Process Performance and Capability

• The Ppk can closely approach the Cp when
• The Customer specifications truly reflect
  customer requirements
• The process in under statistical control
• The data approximate the normal distribution
• The process average is very close to target.
• The Cp is like a benchmark or entitlement
• Sigma capability is driven primarily by random
  error (Common Cause Sources of Variation)
• We would like Ppk to be very close to a Cpk of
  1.5
• This would be a process that is performing at a
  Six-Sigma level

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Test Method Capability
36
Total Variability Components
(Assumes process is stable)
Where
(For Unstable Processes)

• Can be estimated three ways
• Standard Reference Material Repeated Sampling
• Half-Blind Technique
• Gauge RR

(Estimated by subtracting test from total
variability)
37
Repeated Sampling (SRM) Technique
38
Half-Blind Technique
39
Gauge RR Technique
40
Measures of Test Method Capability
Eastman has traditionally used
Percent of variation due to test method
Six Sigma uses the following measures
Percent Gauge RR
Precision to Tolerance Ratio
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How Good is Good? The Debate is On
Eastman has traditionally used
Percent Test Variability Goal is 20 to 30
Six Sigma uses the following measures
Percent Gauge RR Less Than 30 P/T Rate Best
Case 10, Acceptable 30 Problem 30 Gauge RR
translates to 9 Percent Test Variability
(Unrealistic for a lot of tests in Chemical
Industry)
Note Initial analysis is showing that processes
can be monitored with SPC techniques even when
the percent test variability is as high as 50!
42
Resources Allocation?
43
Review

• Indices to measure processes capability
• Cp Goal is 2, Short Term Sigma used in Estimate
• Cpk Goal is 1.5, Short Term Sigma used in
  Estimate
• Pp Approach Cp, Long Term Sigma used in
  Estimate
• Ppk Approach Cpk, Long Term Sigma used in
  Estimate
• Note All of the above are equal for stable
  processes on target
• Indices to measure test method capability
• Percent Test Variation Goal is 20 to 30
• Percent Gauge RR Less than 30 (Percent Test lt
  9)
• 50 may be more reasonable
• P/T Ratio Best Case 10, Acceptable at 30

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Exercise
1.) Your Catapult Team should complete page 11 of
the Catapult Process handout. Limit
yourselves to 30 minutes for this exercise.