adding a 1.5 sigma shift to a centered process, as shown in Figure 2.6, we then use a normal distribution table to calculate the fraction of parts that fall outside the specification limits. Thus the 6σ defect rate is equal to 3.4 DPMO after the shift in the mean. Table 2.1 lists the DPMO rates for various sigma levels for centered and shifted processes.
Figure 2.5 For a normally distributed characteristic, centered at specification target, 6σ quality is achieved when the lower and upper specifications map to ±6σ.
Figure 2.6 Applying the 1.5σ shift to a centered 6σ process.
Table 2.1 Defects per million opportunities (DPMO) for various sigma levels; centered and 1.5σ shifted processes.
| DPMO | ||
|---|---|---|
| Sigma | Centered Process | 1.5 sigma shift |
| 6.0 | 0.00 | 3.4 |
| 5.0 | 0.57 | 232.6 |
| 4.0 | 63.34 | 6209.7 |
| 3.0 | 2699.80 | 66,811 |
| 2.0 | 45,500.26 | 308,770 |
| 1.0 | 317,310.51 | 697,672 |
2.2.4 Six Sigma Roles
As an enterprise‐wide system, it follows that Six Sigma deployment requires the efforts of a cross‐section of people filling various roles. These roles include Executive, Champion or Sponsor, Process Owner, and the Belt‐holders.
An Executive is vital to the success of a Six Sigma implementation. Since the program is driven from the top down, executives must show support through their communications and actions. Without key support, the program will fade.
A Champion or Sponsor is a top‐level manager familiar with Six Sigma principles who works with a project team. The champion’s role is to support the team by providing necessary resources and removing roadblocks when they occur. A champion does not attend every team meeting but will check in at major project phases and serve as a go‐to when the team needs top management support.
A Process Owner is a manager who is responsible for all aspects of a process and who also has the authority to make changes to the process. The process owner is a key member of a Six Sigma project team and often serves as a team leader.
If you are new to Six Sigma, you might be a bit confused by quality being described using a karate metaphor. It is admittedly a bit strange. Mikel Harry at Motorola is credited for coining the “belt” term. As the story goes, a plant manager told Harry that the Six Sigma tools he was applying were “kicking the hell out of variation” [4]. In Harry’s mind, the comment conjured an image of a ninja who could expertly wield tools to make the data reveal what it knows about a process. Hence the “belt” nomenclature was born.
Within the Six Sigma hierarchy, there are several levels of Belt‐holders. Depending on the size and complexity of the organization, there may be White, Yellow, Green, and Black Belts, as well as Master Black Belts.
White Belts, who often are also Executives in the organization, are not directly involved with improvement projects but have an awareness of Six Sigma concepts and help support enterprise‐wide deployment.
Yellow Belts serve as members of Six Sigma project teams. They are familiar with Six Sigma principles and can apply basic tools.
Green Belts tend to work on projects on a part‐time basis while maintaining their current positions in the company. A Green Belt may lead a small project team or assist a Black Belt on a larger project. Green Belts know statistical techniques and can apply a large array of non‐statistical problem‐solving tools.
At larger companies, Black Belts work full‐time in their Six Sigma roles. They lead Six Sigma projects and train and mentor Green Belts. A Black Belt possesses sophisticated knowledge of statistics and data analysis tools, as well as team dynamics.
Finally, Master Black Belts work full time managing, mentoring, and training Black Belts. A Master Black Belt may also work with upper management and the steering committee to plan Six Sigma deployment efforts and to select and prioritize Six Sigma projects.
2.3 Is Six Sigma New?
The origin of quality standards can be traced back to the medieval guild system in which craftsman placed their marks on only those products that met their standards. Quality tools such as statistical process control (SPC) charts date back to Walter Shewhart in 1924, and both W. Edwards Deming and Joseph M. Juran introduced quality thinking and methods in Japan after World War II. Japanese industry continued to innovate and develop quality systems and tools, with major contributions from Ishikawa, Shingo, and Taguchi, among others. In the last 70 years, there has been no shortage of quality programs touted as definitive solutions, including Total Quality Management, Quality Circles, and Zero Defects. Given the long history of quality, then, is Six Sigma new?
On the one hand, it can be argued that Six Sigma is simply a repackaging of existing quality know‐how. It is true that Six Sigma uses existing quality tools such as the Basic seven tools: cause and effect diagrams, check sheets, control charts, histograms, Pareto charts, scatter diagrams, and stratification; and the New seven tools: affinity diagrams, arrow diagrams, interrelationship diagrams, matrix diagrams, prioritization matrices, process decision program charts, and tree diagrams. (See Section 2.4.1.) Six Sigma’s focus on team problem‐solving is taken from Japanese practices such as quality circles. Both Deming and Juran advocated data‐based decision‐making and emphasized the vital role of management in successful quality implementation.
Several features of the Six Sigma approach are unique, however. Because Six Sigma requires data‐driven decisions, it has led to the widespread use of sophisticated statistical techniques. Analysis tools once only known to high‐level industrial statisticians, such as linear regression, chi‐squared tests, and designed experiments, are now routinely employed in Six Sigma projects by non‐statisticians. The project approach, in which
