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# INTRODUCING SIX SIGMA: A FRAMEWORK FOR QUALITY MANAGEMENT

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• 1. INTRODUCING SIX SIGMA: A FRAMEWORK FOR QUALITY MANAGEMENT Wenny Chandra Industrial Engineering Department Maranatha Christian University T N Goh Industrial & Systems Engineering Department National University of Singapore Abstract This paper introduces Six Sigma as a quality improvement initiative and quality management framework. The motivation for Six Sigma, as well as the scheme for its implementation, is outlined. Although the strength of Six Sigma lies in its deployment of statistical tools and the application of statistical thinking, the features of Six Sigma are highlighted in a non-technical language in this paper. Some comments are also given on the requisites for the effectiveness of Six Sigma at the organizational level. Key words: sigma, Six Sigma, sigma level, dpmo, DMAIC, DMADV 1. Introduction 2. The Six Sigma rationale “Sigma” or σ is a Greek letter that denotes Six Sigma arises from the need to improve the standard deviation of a random variable. quality. The main cause of quality problems Traditionally, Six Sigma is a number used to is basically variation. To improve quality, represent the range of values of a population variation must be measured, reduced, and with a normal distribution -- as prevented. For example, in a process where mathematically, 99.73% of all values can be the output y, say the time it takes to process expected to fall within a range that extends a customer order for shipment, is desired to from three sigma less to three sigma higher be not more than a specification limit of L, than the population mean. In the last fifteen the best way to ensure that there is no out- years, “Six Sigma” has been known as an of-specification performance, i.e. initiative for quality improvement and more exceedingly long time taken, is to make sure broadly, business excellence. In this paper, that there is a sufficient buffer between an overview is taken of the Six Sigma process mean and L. To achieve this, the framework as it is known today, and some variation or sigma of y must be reduced. suggestions are given as to how the Six With measurements taken from the process Sigma paradigm could be improved upon to in question, the mean and standard deviation enhance the viability and competitiveness of (“sigma”) of the data are first calculated, an organization.
• 2. indicating the long-term performance level When the concept is applied to physical and the variation. items, level of performance is often cited as defective parts per million pieces tested or If it happens that there is a gap of six inspected. When applied to non-physical standard deviations between process mean items, it is presented in terms of defects per and specification limit L, then the process in million opportunities, or dpmo. This means question is said to be performing at “Six that the performance of either a physical Sigma” level. Subsequently, given time, as a manufacturing process or a transaction result of process deterioration and drifts, this process can be gauged by some sigma level. buffer could be reduced to as low as 4.5 The more consistent a manufacturing or sigma. With such a worst-case scenario, the transaction process, the smaller is the value out-of-specification or defective rate is 3.4 of standard deviation or sigma value, and parts per million (ppm) individual values of hence the higher the sigma level. Figure 1 y. Thus Six Sigma level quality is depicts the direct conversion between dpmo commonly understood to correspond to 3.4 value and sigma level. The Six Sigma ppm; other levels of undesirable methodology thus develops on this basis, performance can be correspondingly specifically in the format developed by represented by a sigma figure, e.g. 233 ppm Mikel Harry and his colleagues (Harry and for a 5 sigma process, 6,210 ppm for a 4 Schroeder, 1999; Hahn et al, 2000) sigma process, and so on. “Sigma level” then becomes a means of calibrating process performance. dpmo 1,000,000 100,000 10,000 1,000 100 10 1 1 2 3 4 5 6 7 Sigma Level Figure 1. Conversion between dpmo and Sigma Level 2
• 3. 3. Six Sigma methodology quality during the design process, getting it “right at the first time”. DMADV is also Six Sigma makes use of a collection of known as Design for Six Sigma (DFSS) quality management and statistical tools to because its objective is to design a product construct a framework for process or process that is Six Sigma able (Hahn et improvement. The objective is to enhance al, 2000). the sigma level of performance measures that reflect the needs of the customer. Such In DMADV, during the Define phase, the measures are generally referred to as tone is set for the entire design project by “critical to quality” or CTQ. CTQ is establishing of goals, charter, and improved via a systematic approach of infrastructure. In the Measure phase, key Define-Measure-Analyze-Improve-Control, customer, their critical needs and the CTQ or DMAIC, taken on a project-by-project requirements are determined. In the Analyze basis. In the Define phase, the problem is phase, a design is selected from several captured, and customer impact and potential alternatives, followed by the development of benefits of the project are assessed. In the design requirements against which a Measure phase, CTQs of the product or detailed design is to be optimized. service are identified, measurement capability is assured, current performance as The Design phase builds the detailed design well as improvement goals are determined. produced in the Analyze phase to deliver an In the Analyze phase, root causes of defects optimum functional design that also meets are uncovered, and key process variables the manufacturing and service requirements. that may be linked to defects are identified. To ensure that the new design can be manufactured and supported in the field The most critical phase of Six Sigma within the required quality, reliability and application is Improve, when the influences cost parameters, the Verify phase is carried of key process variables on the CTQs are out. (De Feo & Bar-El, 2002). quantified, acceptable limits of these variables are identified, and the process modified to reduce CTQ defect levels. 4. Statistics-driven improvement Finally, in the Control phase, actions are taken to sustain the improved level of It bears repetition that the impact of Six performance and ensure long term gains. Sigma stems from a customer focus and Tools ranging from Quality Function conscious improvement of CTQ. The Deployment, Failure Mode and Effects effectiveness of Six Sigma is rooted in its Analysis, Design of Experiments, Robust judicious application of statistical tools for Design, Mistake-Proofing and Statistical information gathering, analysis, and Process Control are deployed in a interpretation. Six Sigma translates an purposeful and integrated manner (Harry operational problem into a statistical and Schroeder, 2000). problem, makes use of proven mathematical tools to solve it, and translates the results When a product or a process is not in back to practical actions. In particular, tools existence but needs to be developed, the of statistical Design of Experiments (DOE) approach of Define-Measure-Analyze- lie at the core of all improvement efforts as Design-Verify, or DMADV methodology is they offer the most efficient and effective used. Except for the first three letters in the means of discovering input-output relations acronyms, DMADV and DMAIC have in any new or existing environment. DOE different aims. DMAIC is used to react to provides the needed knowledge linking or fix unwanted situations, while DMADV process parameters to CTQs, thus is used to prevent problems by building establishing the critical relationship between 3
• 4. key process input and output measurements Champions, Master Black Belts, Black and making optimization of key process Belts, Green Belts, and so on, each parameters possible even for complex designation reflecting the level of processes. competence with respect to level of DMAIC knowledge and practice. The statistical nature of Six Sigma is reflected in a statement by Mikel Harry and The hierarchy of the Six Sigma personnel Richard Schroeder, describing it as “a can be visualized as an inverted pyramid in disciplined method of using extremely Figure 2. At the bottom of the pyramid, rigorous data gathering and statistical executive leadership is needed to support analysis to pinpoint sources of errors and and balance the structure. Commitment at ways of eliminating them” (Harry and this level lays the foundation for success; Schroeder, 2000). Minitab, a popular however, at every level, Six Sigma initiative software package, defines Six Sigma as “an is meant to be driven towards customer information-driven methodology for satisfaction. reducing waste, increasing customer satisfaction and improving processes, with a Champions are functioning at a strategic focus on financially measurable results”. level. They are expected to create the vision Further technical details can be found in and define the path for Six Sigma works such as Pyzdek (2001a). With implementation across the organization. improved processes and products, They choose the appropriate people for the organizations using Six Sigma are expected projects and oversee that these people to bring value to customers and strengthen remain focused on the problem. They are their competitive edge. also the middlemen to convey worthwhile improvement opportunities to the top-level management so that resources needed are 5. Six Sigma applications allocated. Champions are typically the senior executives heading a major functional Generally, Six Sigma in an organization is a department or a strategic business unit or top-down initiative carried out by a SBU. hierarchy of trained personnel designated as Customer Green Belts Black Belts Master Black Belts Champions Executive Leadership Figure 2. Six Sigma personnel hierarchy (Harry & Schroeder, 2000) 4
• 5. Master Black Belts are the partners of the from actual data, hard techniques, and Champions. They must understand the big purposeful changes. It does not rely on business picture and provide assistance in some past quality management practices identification of the projects. But they are such as slogans, pep talks, will power, also the leader of major programs, and take accreditation, audit, certification or awards. on a role to develop and deliver training to Six Sigma may thus be characterized by the other Six Sigma personnel throughout the common “5W and 1H”, summarized via six organization. They devote 100% of their Ss: time for Six Sigma projects and most of it by using “soft skill” ⎯ organizing and WHY Six Sigma? Satisfaction of dealing with people. customers Like the Master Black Belts, Black Belts are WHO does it? Structured top-down also full time advocates of Six Sigma initiative quality initiatives. After the projects are defined by the Champions and Master Black WHAT is it? Statistical thinking Belts, the Black Belts take the problem using data to combat further through the next phases of Six Sigma variation in processes methodology ⎯ Measure, Analyze, Improve and Control. Consequently Black WHERE is it? Standardized Belts are more technically oriented; they framework of must be familiar and be able to determine “DMAIC” the most effective Six Sigma tools to attack the problem. Nevertheless, leadership HOW is it done? Software packages quality must also be present, because they such as Minitab for must manage the project and provide information analysis training to Green Belts. WHEN is it done? Sustained effort via Green Belts are employees trained in Six projects Sigma who spend a portion of their time completing projects, but maintain their regular work role and responsibilities. Working with Black Belts, they experience the practical application of Six Sigma tools 6. Key success factors for Six Sigma as they apply to particular projects. implementation The logical sequence and integration of There are several factors that make Six various quality improvement tools can lead Sigma an increasingly popular quality to results far exceeding what is possible initiative, even more so than the past TQM, with isolated application of single tools. Six ISO, Zero Defect, and so on. These factors Sigma is deployed in terms of projects, each are also the determinants as to whether a Six with clear objectives, time frame and Sigma program will lead to significant results, with tangible gains expressed improvements: financially where possible. Upper management involvement and The prevalence of statistical thinking in Six commitment Sigma leads to decision-making based on This is not just the attitude of endorsing facts rather than arbitrary opinions or the projects, but a much more active preferences. Consequently, as a rule, Six involvement in seeing the bigger Sigma brings about improvements arising 5