Operations management by stevenson 9th student slides chapter 10

Chapter 10
Quality Control

McGraw-Hill/Irwin

Copyright © 2009 by The McGraw-Hill Companies, Inc. All Rights Reserved.


Chapter 10: Learning Objectives
• You should be able to:
–
–
–
–
–
–

List and briefly explain the elements in the control proces
Explain how control charts are used to monitor a process
Explain the concepts underlying the use of control charts
Use and interpret control charts
Understand the use of run tests
Assess process capability

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Statistical Process Control (SPC)
• Quality control seeks
– Quality of Conformance
• A product or service conforms to specifications

• A tool used to help in this process:
– SPC
• Statistical evaluation of the output of a process
• Helps us to decide if a process is “in control” or if corrective
action is needed

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Process Variability
• All processes generate output that exhibits
some degree of variability
• The fundamental question:
– Is the variation within acceptable boundaries?

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Control Chart
Abnormal variation
due to assignable causes

Out of
control

UCL
Normal
variation
due to

chance

Mean

LCL
Abnormal variation

Each point on the control chart represents a sample of n
observations

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Control Charts for Variables
• Variables generate data that are measured
– Mean control charts
• Used to monitor the central tendency of a process.
– X bar charts

– Range control charts
• Used to monitor the process dispersion
– R charts

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Establishing Control Limits
k

k

x

 xi
i 1

k

where

R

i

R  i 1
k
where

x Average of sample means

R Average of sample ranges

x i mean of sample i
k number of samples

Ri Range of sample i

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X-Bar Chart: Control Limits
• Used to monitor the central tendency of a
process

x  chart Control Limits
UCLx  x  A2 R
LCLx  x  A2 R
where
A2 a control chart factor based on sample size, n

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Range Chart: Control Limits
• Used to monitor process dispersion

R Chart Control Limits
UCLR D4 R
LCLR D3 R
where
D3 a control chart factor based on sample size, n
D4 a control chart factor based on sample size, n
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Control Charts for Attributes
• Attributes generate data that are counted.
– 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

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Process Capability
• Once a process has been determined to be stable, it is necessary
to determine if the process is capable of producing output that is
within an acceptable range
– Tolerances or specifications
• Range of acceptable values established by engineering design or
customer requirements
– Process variability
• Natural or inherent variability in a process
– Process capability
• Process variability relative to specification

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Cp : Process Capability Ratio
UTL - LTL
Cp 
6
where
UTL upper tolerance (specifica tion) limit
LTL lower tolerance(specification) limit

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Cpk : Process Capability Index
• Used when a process is not centered at its
target, or nominal, value

C pk min C pu , C pl 
 UTL  x x  LTL 
min 
,

3 
 3

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Taguchi Loss Function

10-14


Operations Strategy
• Quality is a primary consideration for nearly all
customers
– Achieving and maintaining quality standards is of
strategic importance to all business organizations
• Product and service design

• Increase capability in order to move from extensive use
of control charts and inspection to achieve desired
quality outcomes

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