Defining Process CapabilityFigure 3

Defining Process CapabilityFigure 3.13 shows the relationship between a process distribution and the upper and lower specifications for the lab process turnaround time under two conditions. In Figure 3.13(a), the process is capable because the extremes of the process distribution fall within the upper and lower specifications. In Figure 3.13(b), the process is not capable because the lab process produces too many reports with long turnaround times.Figure 3.13 shows clearly why managers are so concerned with reducing process variability. The less variability—represented by lower standard deviations—the less frequently bad output is produced. Figure 3.14 shows what reducing variability implies for a process distribution that is a normal probability distribution. The firm with two-sigma performance (the specification limits equal the process distribution mean {2 standard deviations) produces 4.56 percent defects, or 45,600 defects per million. The firm with four-sigma performance produces only 0.0063 percent defects, or 63 defects per million. Finally, the firm with six-sigma performance produces only 0.0000002 percent defects, or 0.002 defects per million.