This document discusses quality control concepts including inspection, statistical process control using control charts, process capability, and improving process capability. It defines key terms like inspection, acceptance sampling, control charts for variables and attributes, process capability ratio Cp, process capability index Cpk, and Taguchi's loss function. It explains where and how much to inspect in a process and provides examples of inspection points for different businesses.

1. 10-1 Quality Control
William J. Stevenson
Operations Management
8th edition

2. 10-2 Quality Control
CHAPTER
10
Quality Control
McGraw-Hill/Irwin
Operations Management, Eighth Edition, by William J. Stevenson
Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.

3. 10-3 Quality Control
Inspection
 Inspection is a process of checking the
product whether it meets specifications.
Inputs Transformation Outputs
Acceptance
sampling
Process
control
Acceptance
sampling

4. 10-4 Quality Control
Where to Inspect in the Process
 Raw materials and purchased parts
 Finished products
 Semi finished goods i.e., between operations
 Before a covering process

5. 10-5 Quality Control
How much to Inspect
 100 % inspection
 Sampling Inspection

6. 10-6 Quality Control
Cost
Optimal
Amount of Inspection
Inspection Costs
Cost of
inspection
Cost of
passing
defectives
Total Cost
Figure 10.3

7. 10-7 Quality Control
Examples of Inspection Points
Type of
business
Inspection
points
Characteristics
Fast Food Cashier
Counter area
Eating area
Building
Kitchen
Accuracy
Appearance, productivity
Cleanliness
Appearance
Health regulations
Hotel/motel Parking lot
Accounting
Building
Main desk
Safe, well lighted
Accuracy, timeliness
Appearance, safety
Waiting times
Supermarket Cashiers
Deliveries
Accuracy, courtesy
Quality, quantity
Table 10.1

8. 10-8 Quality Control
 Quality of Design: the extent or degree to which
design specifications confirms customer requirements.
 Quality of Conformance:
A product or service conforms to design specifications
 Statistical Process Control:
Statistical evaluation of the output of a process during
production.
 It involves collection and analysis of data of a process
to improve and to control quality.

9. 10-9 Quality Control
Process in statistical control
 Variations are inevitable. Two types of
variations
 Assignable variation: A variation whose source
can be identified . Such variations can be
identified, analyzed and eliminated
 Random variation: Natural variations in the
output of a process, created by countless minor
factors which cannot be identified and hence
cannot be eliminated.

10. 10-10 Quality Control
 In a process if all the assignable variations are identified
and eliminated and if the process is operating under
chance variations only, then the process is said to be
under state of statistical control.
 If the process is operating under assignable variations, the
process is said to be out of control.
 To know whether process is under control or out of
control, control charts are used.
 A control chart is time based graphical comparison of
actual quality with control limits to know whether it under
control. It is used to detect the presence of assignable
variations in the process

11. 10-11 Quality Control
Control chart
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
UCL
LCL
Sample number
Mean
Out of
control
Normal variation
due to chance
Abnormal variation
due to assignable sources
Abnormal variation
due to assignable sources

12. 10-12 Quality Control
Types of Control Charts
 1. Control charts for variables
a. Control charts for averages ( - Charts)
b. Control charts for Ranges ( R – Charts)
c. Control charts for Standard deviation ( - chart)
 Control charts for Attributes
a. Control chart for fraction defective (P -chart)
b. Control chart for number of defectives (nP -chart)
c. Control chart for fraction defects per sample (C - chart)
d. Control chart for fraction defects per item (U - chart)

13. 10-13 Quality Control
Mean and Range Charts
UCL
LCL
UCL
LCL
R-chart
x-Chart Detects shift
Detects variations
within sample
Figure 10.10A

14. 10-14 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
Attributes generate data that are counted.

15. 10-15 Quality Control
Use of p-Charts
 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
Table 10.3

16. 10-16 Quality Control
Use of c-Charts
 Use only when the number of occurrences per
unit of measure can be counted; non-
occurrences 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
Table 10.3

17. 10-17 Quality Control
Use of Control Charts
 At what point in the process to use control
charts (bottleneck operations)
 What size samples to take: between 5 -20 if
cst is more. If cost is less large sample size
 What type of control chart to use
 Variables
 Attributes

18. 10-18 Quality Control
 Tolerances or specifications
 Range of acceptable values established by
engineering design or customer requirements
 Process variability
 Natural variability in a process
 Process capability: ability of the process in
meeting the specifications.
Process Capability

19. 10-19 Quality Control
Process Capability
Lower
Specification
Upper
Specification
A. Process variability
matches specifications
Lower
Specification
Upper
Specification
B. Process variability
well within specifications
Lower
Specification
Upper
Specification
C. Process variability
exceeds specifications
Figure 10.15

20. 10-20 Quality Control
 whether process is capable can be judged on a ratio called process
capability ratio Cp . The process capability ratio, Cp is defined as:
 A firm targeting 3-sigma quality will use 1.00.
 A firm targeting five-sigma quality will use 1.67.
 A firm targeting 6 sigma qualities will use 2.0.
Process Capability Ratio :Cp

21. 10-21 Quality Control
The process capability index, Cpk
 The process capability index can be used to judge whether
process is capable of meeting the specifications are not
even when process average is not centered at the mid of
specifications.
 Its value more than 1, meets specifications.
 Industries use a value of 1.66

22. 10-22 Quality Control
Improving Process Capability
 Simplify: eliminate steps, reduce the number of
parts
 Standardize: use standard parts and standard
procedures
 Mistake-proof: Design the parts that can only be
assembled the correct way
 Upgrade equipment: Replace old worn out
equipment, take advantage of technological
advancements.
 Automate: Substitute automated process for
manual processing

23. 10-23 Quality Control
Taguchi Loss Function
 Genichi Taguchi (1980) proposed that any
deviation from target value incurs loss to society,
He measures quality in terms of deviation of
quality from its target value.
 He propose that the loss incurred is given the
square of deviation of quality from target value.
This is known as quality loss function

24. 10-24 Quality Control
Taguchi Loss Function
Cost
Target
Lower
spec
Upper
spec
Traditional
cost function
Taguchi
cost function
Figure 10.17

25. 10-25 Quality Control
Limitations of Capability Indexes
1. Process may not be stable
2. Process output may not be normally
distributed
3. Process not centered but Cp is used