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10-1
Quality Control
Chapter 10
Quality Control
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-2
Quality Control
Quality Control
• The process that evaluates output
relative to a standard and takes
corrective action when output fails to
meet the standard
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-3
Quality Control
Phases of Quality Assurance
Figure 10-1
Inspection
before/after
production
Acceptance
sampling
The least
progressive
McGraw-Hill/Irwin
Corrective
action during
production
Process
control
Quality built
into the
process
Continuous
improvement
The most
progressive
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-4
Quality Control
Inspection
Figure 10-2
• How Much/How Often
• Where/When
• Centralized vs. On-site
Inputs
Acceptance
sampling
McGraw-Hill/Irwin
Transformation
Process
control
Outputs
Acceptance
sampling
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-5
Quality Control
How Much/How Often
•
•
•
•
•
Value of the item
Cost of passing defects
Cost of inspection
Line disruptions due to inspections
Variability of the process
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-6
Quality Control
Inspection Costs
Cost
Figure 10-3
Total Cost
Cost of
inspection
Cost of
passing
defectives
Optimal
Amount of Inspection
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-7
Quality Control
Where to Inspect in the Process
• Raw materials and purchased parts
• Finished products
• Before a costly operation
• Before an irreversible process
• Before a covering process
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-8
Quality Control
Examples of Inspection Points
Table 10-1
Type of
b u s in e s s
In s p e c tio n
p o in ts
C h a ra c te ris tic s
F a st F o o d
C a sh ie r
C o u n te r a re a
E a tin g a re a
B u ild in g
K itch e n
A ccu ra cy
A p p e a ra n ce , p ro d u ctivity
C le a n lin e ss
A p p e a ra n ce
H e a lth re g u la tio n s
H o te l/m o te l
P a rkin g lo t
A cco u n tin g
B u ild in g
M a in d e sk
S a fe , w e ll lig h te d
A ccu ra cy, tim e lin e ss
A p p e a ra n ce , sa fe ty
W a itin g tim e s
S u p e rm a rke t C a sh ie rs
D e live rie s
McGraw-Hill/Irwin
A ccu ra cy, co u rte sy
Q u a lity, q u a n tity
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-9
Quality Control
Centralized vs. On-site
• Environmental requirements of the
inspection equipment
• Are the advantages of centralized
inspection enough to offset the
disruption caused by moving products
to the inspection point
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-10
Quality Control
• Statistical Process Control:
Statistical evaluation of the output of a
process during production
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-11
Quality Control
Statistical Process Control
• The Control Process
– Define the characteristic to be controlled
– Measure
– Compare to a standard
– Evaluate
– Take corrective action
– Evaluate corrective action
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-12
Quality Control
Statistical Process Control
• Variations and Control
– Random variation: Natural variations in the
output of process, created by countless
minor factors
– Assignable variation: A variation whose
source can be identified
• A process is in control when there are
no assignable variations, all
variations are random
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-13
Quality Control
Normal Distribution
Figure 10-5
Standard deviation


Mean
95.44%


99.74%
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-14
Quality Control
Control Chart
Figure 10-8
Abnormal variation
due to assignable sources
Out of
control
UCL
Mean
Normal variation
due to chance
LCL
Abnormal variation
due to assignable sources
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
Sample number
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-15
Quality Control
Observations from Sample Distribution
Figure 10-9
UCL
LCL
1
2
3
Sample number
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
4
10-16
Quality Control
Mean and Range Charts
Figure 10-10A
(process mean is
shifting upward)
Sampling
Distribution
UCL
Detects shift
x-Chart
LCL
UCL
R-chart
Does not
detect shift
LCL
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-17
Quality Control
Mean and Range Charts
Figure 10-10B
Sampling
Distribution
(process variability is increasing)
UCL
Does not
reveal increase
x-Chart
LC
L
UCL
R-chart
LC
L
McGraw-Hill/Irwin
Reveals increase
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-18
Quality Control
Control Chart for Attributes
• p-Chart - Control chart used to monitor
the proportion of defectives in a process
• c-Chart - Control chart used to monitor
the number of defects per unit
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-19
Quality Control
Use of p-Charts
Table 10-3
• When observations can be placed into
two categories.
– Good or bad
– Pass or fail
– Operate or don’t operate
• When the data consists of multiple
samples of several observations each
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-20
Quality Control
Use of c-Charts
Table 10-3
• Use only when the number of occurrences
per unit of measure can be counted; nonoccurrences cannot be counted.
–
–
–
–
–
Scratches, chips, dents, or errors per item
Cracks or faults per unit of distance
Breaks or Tears per unit of area
Bacteria or pollutants per unit of volume
Calls, complaints, failures per unit of time
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-21
Quality Control
Counting Runs
Figure 10-11
Counting Above/Below Median Runs
B A
A
B
A
B
B
B A
(7 runs)
A
B
Figure 10-12
Counting Up/Down Runs
U
McGraw-Hill/Irwin
U
D
U
(8 runs)
D
U
D U
U D
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-22
Quality Control
Process Capability
• Tolerances
– specifications
• Process variability
– Natural variability in a process
• Process capability
– Process variability relative to specification
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-23
Quality Control
Figure 10-14
Lower
Specification
Process Capability
Upper
Specification
Process variability matches
specifications
Lower
Specification
Upper
Specification
Process variability well within
Lower
Upper
specifications
Specification Specification
Process variability exceeds
specifications
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-24
Quality Control
Process Capability Ratio
specification width
Process capability ratio, Cp =
process width
Cp =
McGraw-Hill/Irwin
Upper specification – lower specification
6
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
10-25
Quality Control
3 Sigma and 6 Sigma Quality
Figure 10-15
Upper
specification
Lower
specification
1350 ppm
1350 ppm
1.7 ppm
1.7 ppm
Process
mean
+/- 3 Sigma
+/- 6 Sigma
McGraw-Hill/Irwin
Operations Management, Seventh Edition, by William J. Stevenson
Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.
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