Chap 10 Quality Control

 

10-1

Quality Control 

Chapter 10

Quality Control

Operations Management, Seventh Edition, by William J. Stevenson Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw Hill/Irwin  

10-2

Quality Control 

Phases of Quality Assurance Figure 10-1 Inspection before/after 

Corrective action during

Quality built into the

production

production

process

Acceptance sampling

The least progressive

Process control

Continuous improvement

The most progressive

Operations Management, Seventh Edition, by William J. Stevenson Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw Hill/Irwin  

10-3

Quality Control 

Inspection Figure 10-2

• How Much/How Often • Where/When • Centralized vs. On-site Inputs

Acceptance sampling

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.

McGraw-Hill/Irwin  

10-4

Quality Control 

Inspection Costs Figure 10-3

      t      s       o        C 

Total Cost Cost of  inspection Cost of  passing defectives

Optimal Amount of Inspection Operations Management, Seventh Edition, by William J. Stevenson Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

10-5

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

Operations Management, Seventh Edition, by William J. Stevenson Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

10-6 

Quality Control 

Examples of Inspection Points Table 10-1 Type of  business

Inspection points

Characteristics

Fast Food

Cashier  Counter area

Accuracy Appearance, productivity

Eating area Building Kitchen

Cleanliness Appearance Health regulations

Park Parkin ing g lot lot

Safe, well lighted

Accounting Building Main desk

Accuracy, timeliness Appearance, safety Waiting times

Hote Ho tel/ l/mo mote tell

Superm Su permarke arkett Cashiers Cashiers Deliveries

Accuracy, courtesy Quality, quantity Operations Management, Seventh Edition, by William J. Stevenson Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin

Copyright © 2002 by The McGraw Hill Companies, Inc. All rights reserved.

 

10-7 

•

Quality Control 

Statistical Process Control : Statistical evaluation of the output of a process during production

Operations Management, Seventh Edition, by William J. Stevenson

Copyright © 2002 by The McGraw Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

10-8

Quality Control 

Statistical Process Control • The Control Process  – Define  – Measure  – Compare to a standard  – Evaluate  – Take corrective action  – Evaluate corrective action

Operations Management, Seventh Edition, by William J. Stevenson

Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

10-9

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

Operations Management, Seventh Edition, by William J. Stevenson

Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

10-10

Quality Control 

Sampling Distribution Figure 10-4 Sampling distribution

Process distribution

Mean

Operations Management, Seventh Edition, by William J. Stevenson

Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

10-11

Quality Control 

Normal Distribution Figure 10-5

σ

=

Standard deviation

3σ

−2 σ

Mean

+ 2σ

+ 3σ

95.44% 99.74%

Operations Management, Seventh Edition, by William J. Stevenson

Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

10-12

Quality Control 

Control Limits Sampling distribution

Figure 10-6

Process distribution

Mean Lower  control limit

Upper  control limit

Operations Management, Seventh Edition, by William J. Stevenson

Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

10-13

Quality Control 

Type I Error  Figure 10-7

α  /2

α  /2

Mean Probability of Type I error  α

=

LC L

UCL

Operations Management, Seventh Edition, by William J. Stevenson

Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

10-14

Quality Control 

Control Chart Figure 10-8 Out of  control

Abnormal variation due to assignable sources

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 

Operations Management, Seventh Edition, by William J. Stevenson

Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

10-15

Quality Control 

Observations from Sample Distribution Figure 10-9 UCL

LCL 1

2

3

Sample number 

Operations Management, Seventh Edition, by William J. Stevenson

4

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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 Operations Management, Seventh Edition, by William J. Stevenson

Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

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10-17  Quality Control 

Mean and Range Charts Figure 10-10B Sampling Distribution

(process variability is increasi

UCL

Does not reveal increase

x-Chart LC L UCL

R-chart LC L

Reveals increase

Operations Management, Seventh Edition, by William J. Stevenson

Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

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

Operations Management, Seventh Edition, by William J. Stevenson

Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

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

Operations Management, Seventh Edition, by William J. Stevenson

McGraw-Hill/Irwin

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,per dents, or distance errors per item Cracks or faults unit of Breaks or Tears per unit of area Bacteria or pollutants per unit of volume Calls, complaints, failures per unit of time

Operations Management, Seventh Edition, by William J. Stevenson

Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

10-21

Quality Control 

Counting Runs Figure 10-11

Counting Above/Below Median Runs

B

A

A

B

A

B

B

B

(7 runs)

A

A

B

Figure 10-12 Counting Up/Down Runs

U

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.

McGraw-Hill/Irwin  

10-22

Quality Control 

Process Capability • Tolerances

 – specifications

• Process variability  – Natural variability in a process

• Process capability  – Process variability relative to specification

Operations Management, Seventh Edition, by William J. Stevenson

Operations Management, Seventh Edition, by William J. Stevenson Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

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 specifications

Lower  Upper  Specification Specification

Process variability exceeds specifications

Operations Management, Seventh Edition, by William J. Stevenson Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

10-24

Quality Control 

Process Capability Ratio

specification width Process capability ratio, Cp =  process width Cp =

Upper specification – lower specification 6σ

Operations Management, Seventh Edition, by William J. Stevenson Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved.

McGraw-Hill/Irwin  

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