Statistical Quality Control
Before studying this chapter you should know or, if necessary, review 1. 2. Quality as a competitive priority, Chapter 2, page 00. Total quality management (TQM) concepts, Chapter 5, pages 00 – 00.
LEARNING OBJECTIVES After studying this chapter you should be able to 1 2 3 4 5 6 7 8 9
Describe categories of statistical quality control (SQC). Explain the use of descriptive statistics in measuring quality characteristics. Identify and describe causes of variation. Describe the use of control charts. Identify the differences between x-bar, R-, p-, and c-charts. Explain the meaning of process capability and the process capability index. Explain the term Six Sigma. Explain the process of acceptance sampling and describe the use of operating characteristic (OC) curves. Describe the challenges inherent in measuring quality in service organizations.
What Is Statistical Quality Control? 172 Links to Practice: Intel Corporation 173 Sources of Variation: Common and Assignable Causes 174 Descriptive Statistics 174 Statistical Process Control Methods 176 Control Charts for Variables 178 Control Charts for Attributes 184 C-Charts 188 Process Capability 190 Links to Practice: Motorola, Inc. 196
Acceptance Sampling 196 Implications for Managers 203 Statistical Quality Control in Services 204 Links to Practice: The Ritz-Carlton Hotel Company, L.L.C.; Nordstrom, Inc. 205 Links to Practice: Marriott International, Inc. 205 OM Across the Organization 206 Inside OM 206 Case: Scharadin Hotels 216 Case: Delta Plastics, Inc. (B) 217
172 • CHAPTER 6
STATISTICAL QUALITY CONTROL
e have all had the experience of purchasing a product only to discover that it is defective in some way or does not function the way it was designed to. This could be a new backpack with a broken zipper or an “out of the box” malfunctioning computer printer. Many of us have struggled to assemble a product the manufacturer has indicated would need only “minor” assembly, only to ﬁnd that a piece of the product is missing or defective. As consumers, we expect the products we purchase to function as intended. However, producers of products know that it is not always possible to inspect every product and every aspect of the production process at all times. The challenge is to design ways to maximize the ability to monitor the quality of products being produced and eliminate defects. One way to ensure a quality product is to build quality into the process. Consider Steinway & Sons, the premier maker of pianos used in concert halls all over the world. Steinway has been making pianos since the 1880s. Since that time the company’s manufacturing process has not changed signiﬁcantly. It takes the company nine months to a year to produce a piano by fashioning some 12,000-hand crafted parts, carefully measuring and monitoring every part of the process. While many of Steinway’s competitors have moved to mass production, where pianos can be assembled in 20 days, Steinway has maintained a strategy of quality deﬁned by skill and craftsmanship. Steinway’s production process is focused on meticulous process precision and extremely high product consistency. This has contributed to making its name synonymous with top quality.
WHAT IS STATISTICAL QUALITY CONTROL?
In Chapter 5 we learned that total quality management (TQM) addresses organizational quality from managerial and philosophical viewpoints. TQM focuses on customer-driven quality standards, managerial leadership, continuous improvement, quality built into product and process design, quality identiﬁed problems at the source, and quality made everyone’s responsibility. However, talking about solving quality problems is not enough. We need speciﬁc tools that can help us make the right quality decisions. These tools come from the area of statistics and are used to help identify quality problems in the production process as well as in the...