ABSTRACT
The six-sigma approach is a systematic application of business and statistical concepts and techniques for the purpose of reducing process variations and preventing deficiencies in a product. Six Sigma is both a technique and a philosophy based on the desire to eliminate waste and improve performance as much as is technically possible. Motorola introduced this innovative development to quality in the mid-1980s, in its quest to reduce defects of manufactured electronic products. The main goal of Six Sigma is to improve the performance of processes to the point where the rate of defect is 3.4 per million or less. The concept was designed for use in a high-volume manufacturing or service environment. A defect could be any of the following:
A faulty part Incorrect customer bill Turnaround time for x-ray
The name Six Sigma is derived from the statistical concept of standard deviation, usually denoted by the Greek letter sigma (s). The variation in a process or in the output of that process is typically measured in terms of the number of standard deviations from the mean (Figure 27-1). The following are examples of Sigma values and the corresponding Defects Per Million Opportunities:1
It is clear that the numbers used by Motorola in its Six Sigma program are significantly different from those derived from statistical knowledge. For example, at Three Sigma, there will be 2,700 failures out of one million opportunities. This represents a success rate of 99.73%. At Six Sigma, the prediction is that 0.002 failures will occur out of one million opportunities; that is a success rate of 99.9999998%. However, the most often cited number of failures per million opportunities at Six Sigma is 3.4. This is a significant difference. Statistics predicts one failure in 500 million opportunities at Six Sigma. Motorola uses 3.4 defects out of one million, or 1,700 times more failures that what is predicted by statistics. The question is, which is correct?
