Six Sigma

Case Studies

with Minitab®

Kishore K. Pochampally

Surendra M. Gupta

CRC Press Taylor & Francis Croup Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Croup, an informa business

Contents

Preface xi Acknowledgments xiii About the Authors xv

Section I Background

1. Introduction to Six Sigma Quality 3 1.1 Definitions ,..3

1.1.1 Defects Per Million Opportunities (DPMO) 5 1.1.1.1 DPMO Example 1 5 1.1.1.2 DPMO Example 2 7 1.1.1.3 DPMO Example 3 9

1.2 DMAIC Approach 10 1.3 Book Outline 11 References 13

2. Quality Analysis and Improvement Tools/Techniques Used in This Book 15 2.1 Confidence Interval Estimation 15 2.2 Hypothesis Testing 15 2.3 Chi-Square Analysis 16 2.4 Process Capability Analysis 16 2.5 Binary Logistic Regression 17 2.6 Item Analysis 17 2.7 Cluster Analysis 17 2.8 Mixture Design and Analysis of Experiments 17 2.9 Multivariate Analysis 18 2.10 Pareto Chart 18 2.11 Cause-and-Effect Diagram 18 2.12 Gage Repeatability and Reproducibility Analysis 18 2.13 Taguchi Design and Analysis of Experiments 19 2.14 Factorial Design and Analysis of Experiments 19 2.15 Statistical Control Charts 19 2.16 Normality Test 19 2.17 Analysis of Variance (ANOVA) 20 References 20

viii Contents

Section II Six Sigma Case Studies

3. Confidence Intervals to Assess Variation in Fat Content at a Fast-Food Restaurant 23 3.1 Define Phase 23 3.2 Measure Phase 23 3.3 Analyze Phase 31 3.4 Improve Phase 31 3.5 Control Phase 42

4. Hypothesis Testing for Quality Control at a Manufacturing Company 43 4.1 Define Phase 43 4.2 Measure and Analyze Phases 44

4.2.1 Test 1 44 4.2.2 Test 2 46 4.2.3 Test 3 47

4.3 Improve and Control Phases 60

5. Chi-Square Analysis to Verify Quality of Candy Packets 61 5.1 Define Phase 61 5.2 Measure Phase 61 5.3 Analyze Phase 62 5.4 Improve and Control Phases 79

6. Process Capability Analysis at a Manufacturing Company 81 6.1 Define Phase 82 6.2 Measure Phase 82 6.3 Analyze Phase 84 6.4 Improve Phase 88 6.5 Control Phase 103

7. Binary Logistic Regression to Predict Customer Satisfaction at a Restaurant 105 7.1 Define Phase 105 7.2 Measure Phase 106 7.3 Analyze Phase 106 7.4 Improve and Control Phases 118

Contents ix

8. Item Analysis and Cluster Analysis to Gather "Voice of the Customer" (VOC) Data from Employees at a Service Firm 119 8.1 Define Phase 119 8.2 Measure Phase 120 8.3 Analyze Phase 124 8.4 Improve and Control Phases 135

9. Mixture Designs to Optimize Pollution Level and Temperature of Fuels 137 9.1 Define Phase 137 9.2 Measure Phase 137 9.3 Analyze Phase 137 9.4 Design and Verify Phases 160

10. Multivariate Analysis to Reduce Patient Waiting Time at a Medical Center 161 10.1 Define Phase 161 10.2 Measure Phase 161 10.3 Analyze Phase 163 10.4 Improve Phase 183 10.5 Control Phase 183

11. Pareto Chart and Fishbone Diagram to Minimize Recyclable Waste Disposal in a Town 185 11.1 Define and Measure Phases 185 11.2 Analyze Phase 187 11.3 Improve and Control Phases 194

12. Measurement System Analysis at a Medical Equipment Manufacturer 195 12.1 Define Phase 195 12.2 Measure Phase 195 12.3 Analyze, Improve, and Control Phases 213

13. Taguchi Design to Improve Customer Satisfaction of an Airline Company 215 13.1 Define Phase 215 13.2 Measure Phase 218 13.3 Analyze Phase 219 13.4 Improve and Control Phases 229

X Contents

14. Factorial Design of Experiments to Optimize a Chemical Process 231 14.1 Define Phase 231 14.2 Measure Phase 231 14.3 Analyze Phase 237 14.4 Improve and Control Phases 255

15. Chi-Square Test to Verify Source Association with Parts Purchased and Products Produced 257 15.1 Define Phase 257 15.2 Measure Phase 257 15.3 Analyze Phase 258 15.4 Improve and Control Phases 272

Appendix 273

Index 293