Process Capability

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Process capability studies determine whether a process is unstable, investigate any sources of instability, determine their causes, and take action to resolve such sources of instability. After all sources of instability have been resolved in a process, the natural behavior of the process is called its process capability. Process capability compares the output of a process (called “Voice of the Process”) with the customer’s specification limits for the outputs (called “Voice of the Customer”). A process must be stable (have an established process capability) before it can be improved. Consequently, a process capability study must be successfully completed before a process improvement study can have any chance for success.

Process improvement studies follow the Deming cycle of Plan, Do, Study, Act. First, managers construct a plan to decrease the difference between customer needs (Voice of the Customer) and process performance (Voice of the Process). Recall, that a plan is an intention to move from an existing method or flowchart to a revised method or flowchart by incorporating one or more change concepts. Second, they test the revised flowchart’s (Plan) viability using a planned experiment (Do). Third, they collect data and study the results of the planned experiment to determine if the plan (revised flowchart) will decrease the difference between customer needs and process performance (Study). Fourth, if the data collected about the revised flowchart show if the plan will achieve its objective(s). Finally, the revised flowchart is standardized through "best practices" and training (Act); and the managers responsible for the plan return to the Plan phase of the Deming cycle to find further revisions to the flowchart that will further reduce the difference between customer needs and process performance. If the data collected about the plan show that the plan will not achieve its objective(s), the managers responsible for the plan return to the Plan phase of the Deming cycle to find a different revision to the flowchart that will reduce the difference between customer needs and process performance. Hence, the Deming cycle follows a never-ending path of process and quality improvement.

This chapter is divided into four sections: specifications, process capability studies, process improvement studies, and quality improvement stories. The quality improvement story is an effective format for quality management practitioners to present process capability and process improvement studies to management.

11.2 Specifications (Voice of the Customer) and Created Dimensions

Specifications fall into two broad categories: performance specifications and technical specifications.

11.2.1 Performance Specifications

Performance specifications address a customer’s needs or wants. An example of a performance specification can be seen in restaurants rated by the Red Michelin Guide. The customers of these restaurants set their performance specifications as “a perfect dining experience.” Perfection is measured in terms of the synergistic experience created by the interaction of food, service, ambience and price. The Red Michelin Guide rates restaurants on a one to three star scale. Only the best restaurants in the world receive Michelin stars. A restaurant receives one Michelin star for consistently serving very good food in a good setting, but it is not considered worthy of a special traveling effort. A restaurant receives two Michelin stars for consistently serving excellent food, including specialties and wines of choice in a great setting. The restaurant is worth a detour from one’s existing travel itinerary. A restaurant receives three Michelin stars for serving excellent food and great wine, with impeccable and elegant service and ambience. The restaurant is one of the best restaurants in the world and is worth a special trip. All starred restaurants have a high average level of quality with very little variation...
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