Gatlin Education Six Sigma Green Belt - OVERVIEW Why Do Six Sigma - Page 4

3.4 PPM Defect Rate You often hear the quoted defect rate of a Six Sigma process as 3.4 parts per million (ppm), or 3.4 defects per million opportunities. The reason the quoted defect rate is not the 2 parts per billion we saw on the last Slide is that it is assumed that the process may drift over a long period of time. Even if we use a control chart to detect process shifts, we may fail to immediately detect the shift, or the product may operate differently in the field than what we observe in our production or development activities. When Motorola developed the Six Sigma techniques, they estimated that the process could shift by as much as one and a half sigma, so that the closest specification may actually be at the four and half sigma level. We could use statistics to calculate the probability of exceeding the limits at 3.4 parts per million opportunities. At this point, you may wonder: What if our process isn't like the one Motorola described? As we will see, the concepts are applicable across a broad range of processes and organizations. The one and a half sigma shift shouldn�t be much of a concern either. What is important is that we speak a consistent language for convenient comparisons, and we have a useful means of measuring real process improvement. �Defects� will not be our prime focus. Rather, we will concentrate on profitability and customer satisfaction.