The Definitions of Six Sigma
Six sigma integrates well with all of the quality programs and trends of the last few decades. The purpose of this section is to outline conceptually where the six sigma program connects in the quality hierarchy and some of the quality tools that are in common use today. Specific mathematical background and formulations are discussed in detail in later chapters.
Six sigma is a condition of the generalized formula for process capability, which is defined as the ability of a process to turn out a good product. It is a relationship of product specifications to manufacturing variability, measured in terms of Cp or Cpk, or expressed as a numerical index. Six sigma is equivalent to Cp = 2 or Cpk = 1.5 (more on that in the next chapter). The classical definition of the capability of the process or Cp is:
Six sigma is achieved when the product specifications are at ±6d (a is the symbol for standard deviation) of the manufacturing process corresponding to Cp =： 2 (or Cpk = 1.5, discussed in Chapter 2)
Six sigma or Cp is an excellent indicator of the capability of a process，which can be expressed numerically. This numerical expression can be translated into a defect level using normal distribution statistical assumptions. It is a useful tool for manufacturing process comparisons, as well as a common language of design and manufacturing personnel during the development phase of a product. The design project team and their managers can use it to set new product quality goals. It can be used to assess the quality of internal manufacturing plants anywhere in the world or to measure the capability of a supplier. Companies can use it to communicate a particular contractual level of quality for their supply chain.