Process Capability Ratio If a proce

Process Capability Ratio If a process fails the process capability index test, we need a quick test to see if the process variability is causing the problem. If a process is capable, it has a process distribution whose extreme values fall within the upper and lower specifications for a service or product. For example, if the process distribution is normal, 99.74 percent of the values fall within {3 standard deviations. In other words, the range of values of the quality measure generated by a process is approximately six standard deviations of the process distribution. Hence, if a process is capable at the three-sigma level, the difference between the upper and lower specification, called the tolerance width, must be greater than six standard deviations. The process capability ratio, Cp,is defined as Suppose management wants four-sigma capability in their processes, and a process just failed the process capability index test at that level. A Cp value of 1.33, say, implies that the variability of the process is at the level of four-sigma quality and that the process is capable of consistently producing outputs within specifications, assuming that the process is centered. Because Cp passed the test, but Cpk did not, we can assume that the problem is that the process is not centered adequately.