Statistical Quality Control. Bhisham C. Gupta
target="_blank" rel="nofollow" href="#ulink_68201391-ab94-5b11-a1e2-d5588d9098c0">Figure 1.3, our comparison points change from the shaded region under the distribution tails to the center of the distribution. A practical decision then requires that we consider how far off the intended target the observed process behavior is as compared with the statistical difference identified in Figure 1.3. Note that differentiating between a practical and statistical difference is a business or financial decision. When making a practical versus a statistical decision, we may well be able to detect a statistical difference; however, it may not be cost‐effective or financially worth making the process improvement being considered.
1.5 Conclusion
In this chapter, we have given a general overview of quality improvement and its management. For more details on these topics, we refer you to the works of Philip B. Cosby, W. Edwards Deming, Joseph M. Juran, and Armand V. Feigenbaum (see the Bibliography). Also, the Six Sigma methodology, which we introduce in the next chapter, is a step forward to help achieve and manage quality improvement, since understanding the idea of Six Sigma means customer requirements must be met. In the remainder of this book, we discuss statistical techniques and SPC tools that are essential to implementing the Six Sigma methodology for improving process performance.
2 Basic Concepts of the Six Sigma Methodology
2.1 Introduction
Far from being just another quality fad, Six Sigma has continued to grow in popularity and influence since its creation at Motorola in 1986. Six Sigma techniques have been adopted by a wide range of manufacturing firms and have also translated successfully into other sectors, including retail, hospitality, financial services, high tech, transportation, government, and healthcare. According to the American Society for Quality, as of 2009, 82 of the largest 100 companies in the US had deployed Six Sigma. Fifty‐three percent of Fortune 500 companies have adopted Six Sigma practices, saving an estimated $427 billion over the past 20 years [1]. In a broad sense, it can be said that if a company has customers and a process, it can benefit from implementing Six Sigma.
2.2 What Is Six Sigma?
In statistics, the lowercase Greek letter sigma (σ) represents the population standard deviation, which is a measure of variation or spread. In the quality field, we aim to reduce the process standard deviation to achieve more consistent outcomes. Whether we are measuring the dimension of a metal flange, inner diameter of a pipe, burst strength of a package, monthly labor costs for a division, or repair time of a subassembly, a repeatable process is the desired state.
If we were limited to a single‐sentence elevator speech to describe Six Sigma, a reasonable definition might be “Six Sigma is a quality approach that strives to reduce variation and decrease defects, which results in increased customer satisfaction and improved bottom‐line results.” If we dig a little deeper, however, it becomes clear that there are at least three different definitions of the term Six Sigma. Six Sigma can be correctly classified as a management philosophy; it is also defined as a systematic approach to problem‐solving. Last, Six Sigma is a term used for a statistical standard of quality. Let’s explore each of these definitions in more detail.
2.2.1 Six Sigma as a Management Philosophy
Six Sigma is a management philosophy that emphasizes reducing variation, driving down defect rates, and improving customer satisfaction. Specifically, the tenets of the philosophy include:
Enterprise‐wide deployment of Six Sigma to improve processes
Implementation driven from the top down
Process improvements achieved through projects completed by teamsFigure 2.1 Six Sigma project selection.
Project benefits linked directly to the organization’s bottom line
Rigorous application of data analysis and statistical tools
An extremely high standard for quality
In this management framework, quality is no longer relegated to the quality department, nor is it reserved only for the shop floor. Each facility, each department, and each employee plays a part in improving quality.
In larger companies with an established Six Sigma program, a high‐level steering committee chooses Six Sigma projects that align with and help advance the company’s strategic goals. These strategic goals are derived from customer requirements and upper management’s overall business strategy. The relationship among these elements is illustrated in Figure 2.1. These projects are completed by cross‐functional teams, and the benefits are reported in terms of defect reduction and, especially, dollar savings.
The Six Sigma philosophy also places an emphasis on measurement. Decisions are based on data, not on company folk wisdom or gut feel. In addition, there is a relentless emphasis on reducing variation, driven in large part by an extremely high standard of quality in which virtually no defects are produced.
2.2.2 Six Sigma as a Systemic Approach to Problem Solving
The second definition of Six Sigma refers to a systematic approach to problem‐solving. The emphasis in Six Sigma is on solving process problems. A process is a series of steps designed to produce products and/or services.
As shown in Figure 2.2, the inputs to a process may include materials, people, or information. These inputs flow through the steps of the process, which produces a product or service as a final output. The concept can be applied to any industry or function. A manufacturing process may take raw materials and transform them into a finished product. In a front‐office process, invoices may go through process steps that then create vendor payments. A physician may order a series of tests, which finally leads to a diagnosis and treatment.
The Six Sigma methodology is driven by team projects that have a clearly defined purpose and specific goals. The projects concentrate on improving processes and have relatively short durations, with the majority completed within two to nine months. Project goals generally focus on reducing variation in a process and consequently reducing defects. Teams work together to accomplish project goals by following problem‐solving approach or five defined phases: Define, Measure, Analyze, Improve, and Control, (DMAIC, pronounced “duh may’ ik”), as shown in Figure 2.3.
Figure 2.2 Flow chart of a process.
Figure 2.3 The DMAIC cycle.
In the Define phase, a team sifts through customer feedback and product performance metrics to craft project problem and goal statements and establish baseline measures. Labor, material, and capital resources required by the project are identified, and a rough timeline for project milestones is created. This information is collected into a project charter that is approved by upper management.
During the Measure phase, the team identifies important metrics