Understanding Process Capability Indices

1. Introduction A process capability index is a numerical summary that compares the behavior of a product or process characteristic to engineering specifications. These measures are also often called capability or performance indices or ratios; we use capability index as the generic term. A capability index relates the voice of the customer (specification limits) to the voice of the process. A large value of the index indicates that the current process is capable of producing parts that, in all likelihood, will meet or exceed the customer's requirements. A capability index is convenient because it reduces complex information about the process to a single number. Capability indices have several applications, though the use of the indices is driven mostly by monitoring requirements specified by customers. Many customers ask their suppliers to record capability indices for all special product characteristics on a regular basis. The indices are used to communicate how well the process has performed. For stable or predictable processes, it is assumed that these indices also indicate expected future performance. Suppliers may also use capability indices for different characteristics to establish priorities for improvement activities. Similarly, the effect of a process change can be assessed by comparing capability indices calculated before and after the change. Despite the widespread use of capability indices in industry, and some good review articles, such as Gunter (1989abcd), there is much confusion and misunderstanding regarding their interpretation and appropriate use. This problem is magnified because many quality programs, such as, for example, the automotive industry standard QS-9000 mandate the use of capability indices, but state the capability requirements in an overly complex and confusing way. The