The four common capability and performance indexes collectively contain all of the summary information about process predictability, process conformity, and process aim that can be expressed numerically. As a result, any additional capability measures that your software may provide can only repackage what is already known. This article will contrast and compare the so-called third-generation capability ratios with the traditional capability indexes.

The capability indexes

The capability ratio, Cp, is an index that addresses the question, “Is there enough elbow room for my process within the specifications?” It answers this question by comparing the total space available within the specifications with the space required by the process:

And this ratio is an index number because a value of 1.00 is the borderline between positive and negative answers to the question posed. Here, values greater than 1.00 indicate that the process has adequate elbow room.

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