I frequently hear discussions among engineers, managers, and higher-ups concerning process capability, an alphabet soup of indexes and three-letter designations. The indexes are bandied about as though a single number communicates knowledge, understanding, and certainty. My experience is that this is simply not the case, and I have come to the depressing conclusion that most people are confused as to how capability indexes should be used and what they truly mean.

“Say, what’s the capability of that part?” I cringe when I hear this question and its related variations. I shake my head and wonder about the future of our planet when I hear someone answer quickly with a single-number response.

When overhearing capability discussions, I have found that critical issues are simply not a part of the conversation. Issues that seem obvious to me, such as to which process the capability index refers, are often ignored. When evaluating capability, one simply cannot separate part capability from process capability. That is, a part’s capability index is dependent upon the machine, or process, from which the part was made. I find it curious that the very thing that SPC helps to control—the process—is rarely, if ever, considered when discussing capability. So, before even beginning a discussion on capability, you must consider a minimum of three distinct and very import items:

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