The process capability index measur

The process capability index measures how well the process is centered as well as whether the variability is acceptable. As a general rule, most values of any process distribution fall within {3 standard deviations of the mean. Consequently, {3 standard deviations are used as the benchmark. Because the process capability index is concerned with how well the process distribution is centered relative to the specifications, it checks to see if the process average is at least three standard deviations from the upper and lower specifications. We take the minimum of the two ratios because it gives the worst-case situation.The process capability index must be compared to a critical value to judge whether a process is capable. Firms striving to achieve three-sigma performance use a critical value for the ratio of 1.0. A firm targeting four-sigma performance will use 1.33 (or 4/3), a firm targeting five-sigma performance will use 1.67 (or 5/3), and a firm striving for six-sigma performance will use 2.00 (or 6/3). Processes producing services or products with less than three-sigma performance will have Cpk values less than 1.0.If a process passes the process capability index test, we can declare the process is capable. Suppose a firm desires its processes to produce at the level of four-sigma performance. If Cpk is greater than or equal to the critical value of 1.33, we can say the process is capable. If Cpk is less than the critical value, either the process average is too close to one of the tolerance limits and is generating defective output, or the process variability is too large. To find out whether the variability is the culprit, we need another test.